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4. Physical Characteristics
This section contains the physical definition of SCSI-2. The connectors,
cables, signals, terminators, and bus timing values needed to implement the
interface are covered.
4.1. Physical Description
SCSI devices are daisy-chained together using a common 50-conductor A cable
and, optionally, a 68-conductor B cable. Both ends of each cable are
terminated. All signals are common between all SCSI devices on the A cable.
In systems that employ the wide SCSI option, wide SCSI devices additionally
connect to the B cable. Various width SCSI devices may be mixed.
IMPLEMENTORS NOTE: X3T9.2 is documenting an alternate 16-bit single-cable
solution and an alternate 32-bit solution and expects to be able to remove
the B cable definition in a future version of SCSI.
Two driver/receiver alternatives are specified:
(1) Single-ended drivers and receivers, which allow a maximum cable length
of six meters (primarily for connection within a cabinet).
(2) Differential drivers and receivers, which allow a maximum cable length
of 25 meters (primarily for connection outside of a cabinet).
The single-ended and differential alternatives are mutually exclusive within
a system.
IMPLEMENTORS NOTE: Use of single-ended drivers and receivers with the fast
synchronous data transfer option is not recommended.
4.2. Cable Requirements
The characteristic impedance of the cable should be no less than 90 ohms and
no greater than 140 ohms. The characteristic impedance of the cable used when
implementing the fast synchronous data transfer option is defined in 4.2.3.
NOTE: There are successful single-ended implementations using cables with
with less than 90 ohms characteristic impedance. However, system integrity
in single-ended implementations is improved when the characteristic
impedance of the cable is greater than 90 ohms. Cable parameters other than
characteristic impedance are critical to system integrity. The X3T9.2 Task
Group is investigating alternative ways to specify cable parameters as a
part of a future version of SCSI.
A minimum conductor size of 0.08042 square mm (28 AWG) should be used to
minimize noise effects and ensure proper distribution of terminator power.
A smaller conductor size may be used for signals other than terminator power.
IMPLEMENTORS NOTES:
(1) To minimize discontinuities and signal reflections, cables of different
impedances should not be used in the same bus. Implementations may require
trade-offs in shielding effectiveness, cable length, the number of loads,
transfer rates, and cost to achieve satisfactory system operation.
(2) To minimize discontinuities due to local impedance variation, a flat
cable should be spaced at least 1.27 mm (0.050 in) from other cables, any
other conductor, or the cable itself when the cable is folded.
(3) Regulatory agencies may require use of larger wire size.
4.2.1. Single-Ended Cable
A 50-conductor flat cable or 25-signal twisted-pair cable shall be used for
the A cable. A 68-conductor flat cable or 34-signal twisted-pair cable shall
be used for the B cable if the wide SCSI option is implemented. The maximum
cumulative cable length shall be 6.0 meters. If twisted-pair cables are used,
then twisted pairs in the cable shall be wired to physically opposing contacts
in the connector.
A stub length of no more than 0.1 meters is allowed off the mainline
interconnection within any connected equipment or from any connected point.
IMPLEMENTORS NOTE: Stub clustering should be avoided. Stubs should be
spaced at least 0.3 meters apart.
SCSI bus termination shall be at each end of the cable and may be internal
to the SCSI devices that are at the ends of the cable.
4.2.2. Differential Cable
A 50-conductor flat cable or 25-signal twisted-pair cable shall be used for
the A cable. A 68-conductor flat cable or 34-signal twisted-pair cable shall
be used for the B cable if the wide SCSI option is implemented. The maximum
cumulative cable length shall be 25 meters. If twisted-pair cables are used,
then twisted pairs in the cable shall be wired to physically opposing contacts
in the connector.
A stub length of no more than 0.2 meters is allowed off the mainline
interconnection within any connected equipment or from any connected point.
SCSI bus termination shall be at each end of the cable and may be internal
to the SCSI devices that are at the ends of the cable.
IMPLEMENTORS NOTE: The use of twisted pair cable (either twisted-flat or
discrete wire twisted pairs) is strongly recommended. Without twisted
pairs, even at slow data rates and very short distances, crosstalk between
adjacent signals causes spurious pulses with differential signals.
4.2.3. Cable Requirements for Fast Synchronous Data Transfer
In systems which use the fast synchronous data transfer option (see 4.8),
the A and B cables should meet the conductor size recommendation in 4.2. The
cable should have an overall shield suitable for termination in a shielded
connector.
In such systems, the cables shall have the following electrical
characteristics:
Characteristic Impedance: 90 to 132 ohms
Signal Attenuation: 0.095 dB maximum per meter at 5 MHz
Pair-to-Pair Propagation Delay Delta: 0.20 ns maximum per meter
DC Resistance: 0.230 ohms maximum per meter at 20 degrees C
4.3. Connector Requirements
Two types of connectors are defined: nonshielded and shielded. The
nonshielded connectors are typically used for in-cabinet applications.
Shielded connectors are typically used for external applications where
electromagnetic compatibility (EMC) and electrostatic discharge (ESD)
protection may be required. Either type of connector may be used with the
single-ended or differential drivers.
4.3.1. Nonshielded Connector Requirements
Two nonshielded connector alternatives are specified for the A cable and one
nonshielded connector is specified for the B cable.
4.3.1.1. Nonshielded Connector Alternative 1 - A Cable
The alternative 1 nonshielded high-density SCSI device connector for the A
cable (Figure 4-1) shall be a 50-conductor connector consisting of two rows of
25 female contacts with adjacent contacts 1.27 mm (0.05 in) apart. The
nonmating portion of the connector is shown for reference only.
The alternative 1 nonshielded high-density cable connector for the A cable
(Figure 4-2) shall be a 50-conductor connector consisting of two rows of 25
male contacts with adjacent contacts 1.27 mm (0.05 in) apart. The nonmating
portion of the connector is shown for reference only.
4.3.1.2. Nonshielded Connector Alternative 2 - A Cable
The alternative 2 nonshielded low-density SCSI device connector for the A
cable (Figure 4-3) shall be a 50-conductor connector consisting of two rows of
25 male pins with adjacent pins 2.54 mm (0.1 in) apart. A shroud and header
body should be used. The nonmating portion of the connector is shown for
reference only.
The alternative 2 nonshielded low-density cable connector for the A cable
(Figure 4-4) shall be a 50-conductor connector consisting of two rows of 25
female contacts with adjacent contacts 2.54 mm (0.1 in) apart. It is
recommended that keyed connectors be used.
4.3.1.3. Nonshielded Connector - B Cable
The nonshielded high-density SCSI device connector for the B cable (Figure
4-1) shall be a 68-conductor connector consisting of two rows of 34 female
contacts with adjacent contacts 1.27 mm (0.05 in) apart. The nonmating
portion of the connector is shown for reference only.
The nonshielded high-density cable connector for the B cable (Figure 4-2)
shall be a 68-conductor connector consisting of two rows of 34 male contacts
with adjacent contacts 1.27 mm (0.05 in) apart. The nonmating portion of the
connector is shown for reference only.
4.3.2. Shielded Connector Requirements
Two shielded connector alternatives are specified for the A cable and one
shielded connector is specified for the B cable. The connector shielding
system should provide a dc resistance of less than 10 milliohms from the
cable shield at its termination point to the SCSI device enclosure.
In order to support daisy-chain connections, SCSI devices that use shielded
connectors should provide two shielded device connectors on the device
enclosure. These two connectors may be wired "one-to-one" with a stub to the
SCSI device's drivers and receivers provided the maximum stub length is not
violated. Alternatively, two cables may be run from the two shielded
connectors to the drivers and receivers so that the maximum stub length is not
violated. The length of the cable within the device enclosure is included
when calculating the total cable length of the SCSI bus.
IMPLEMENTORS NOTE: SCSI-1 defined three shielded connector systems in an
appendix. The alternative 1 shielded connector of SCSI-1 has been replaced
by a high-density connector in this standard. The alternative 2 shielded
connector remains unchanged. The EUROCARD Boxes shielded connector system
of SCSI-1 has been deleted in this standard.
4.3.2.1. Shielded Connector Alternative 1 - A Cable
The shielded high-density SCSI device connector for the A cable (Figure 4-5)
is a 50-conductor connector consisting of two rows of 25 female contacts with
adjacent contacts 1.27 mm (0.05 in) apart. The nonmating portion of the
connector is shown for reference only.
The shielded high-density cable connector for the A cable (Figure 4-6) is a
50-conductor connector consisting of two rows of 25 male contacts with
adjacent contacts 1.27 mm (0.05 in) apart. The nonmating portion of the
connector is shown for reference only.
4.3.2.2. Shielded Connector Alternative 2 - A Cable
The shielded low-density device connector for the A cable (Figure 4-7) is a
50-conductor connector consisting of two rows of ribbon contacts spaced 2.16
mm (0.085 in) apart. The nonmating portion of the connector is shown for
reference only.
The shielded low-density cable connector for the A cable (Figure 4-8) is a
50-conductor connector consisting of two rows of ribbon contacts spaced 2.16
mm (0.085 in) apart. The nonmating portion of the connector is shown for
reference only.
4.3.2.3. Shielded Connector - B Cable
The shielded high-density SCSI device connector for the B cable (Figure 4-5)
is a 68-conductor connector consisting of two rows of 34 female contacts with
adjacent contacts 1.27 mm (0.05 in) apart. The nonmating portion of the
connector is shown for reference only.
The shielded high-density cable connector for the B cable (Figure 4-6) is a
68-conductor connector consisting of two rows of 34 male contacts with
adjacent contacts 1.27 mm (0.05 in) apart. The nonmating portion of the
connector is shown for reference only.
Figure 4-1: 50/68-Contact Nonshielded High-Density SCSI Device Connector
(A Cable/B Cable)
Figure 4-2: 50/68-Contact Nonshielded High-Density Cable Connector
(A Cable/B Cable)
Figure 4-3: 50-Contact Nonshielded Low-Density SCSI Device Connector
(A Cable)
Figure 4-4: 50-Contact Nonshielded Low-Density Cable Connector
(A Cable)
Figure 4-5: 50/68-Contact Shielded High-density SCSI Device Connector
(A Cable/B Cable)
Figure 4-6: 50/68-Contact Shielded High-density Cable Connector
(A Cable/B Cable)
Figure 4-7: 50-Contact Shielded Low-Density SCSI Device Connector
Figure 4-8: 50-Contact Shielded Low-Density Cable Connector
4.3.3. Connector Contact Assignments
The connector contact assignments are defined in Tables 4-1 through 4-5.
Table 4-1 defines which of the other four tables to use and which set of
contact assignments to use.
Table 4-1: Cross-Reference to Connector Contact Assignments
==============================================================================
Driver/ Contact
Receiver Connector Assignment Contact
Connector Type Type Cable Figure Table Set
------------------------------------------------------------------------------
Nonshielded Alternative 1 Single-Ended A 4-1 & 4-2 4-2 2
Nonshielded Alternative 1 Single-Ended B 4-1 & 4-2 4-3
Nonshielded Alternative 1 Differential A 4-1 & 4-2 4-4 2
Nonshielded Alternative 1 Differential B 4-1 & 4-2 4-5
Nonshielded Alternative 2 Single-Ended A 4-3 & 4-4 4-2 1
Nonshielded Alternative 2 Differential A 4-3 & 4-4 4-4 1
Shielded Alternative 1 Single-Ended A 4-5 & 4-6 4-2 2
Shielded Alternative 1 Single-Ended B 4-5 & 4-6 4-3
Shielded Alternative 1 Differential A 4-5 & 4-6 4-4 2
Shielded Alternative 1 Differential B 4-5 & 4-6 4-5
Shielded Alternative 2 Single-Ended A 4-7 & 4-8 4-2 2
Shielded Alternative 2 Differential A 4-7 & 4-8 4-4 2
==============================================================================
Table 4-2: Single-Ended Contact Assignments - A Cable
==============================================================================
Connector Cable Connector
Signal Contact Number Conductor Contact Number Signal
Name Set 2 Set 1 Number Set 1 Set 2 Name
------------------------------------------------------------------------------
GROUND 1 1 1 | 2 2 26 -DB(0)
GROUND 2 3 3 | 4 4 27 -DB(1)
GROUND 3 5 5 | 6 6 28 -DB(2)
GROUND 4 7 7 | 8 8 29 -DB(3)
GROUND 5 9 9 | 10 10 30 -DB(4)
GROUND 6 11 11 | 12 12 31 -DB(5)
GROUND 7 13 13 | 14 14 32 -DB(6)
GROUND 8 15 15 | 16 16 33 -DB(7)
GROUND 9 17 17 | 18 18 34 -DB(P)
GROUND 10 19 19 | 20 20 35 GROUND
GROUND 11 21 21 | 22 22 36 GROUND
RESERVED 12 23 23 | 24 24 37 RESERVED
OPEN 13 25 25 | 26 26 38 TERMPWR
RESERVED 14 27 27 | 28 28 39 RESERVED
GROUND 15 29 29 | 30 30 40 GROUND
GROUND 16 31 31 | 32 32 41 -ATN
GROUND 17 33 33 | 34 34 42 GROUND
GROUND 18 35 35 | 36 36 43 -BSY
GROUND 19 37 37 | 38 38 44 -ACK
GROUND 20 39 39 | 40 40 45 -RST
GROUND 21 41 41 | 42 42 46 -MSG
GROUND 22 43 43 | 44 44 47 -SEL
GROUND 23 45 45 | 46 46 48 -C/D
GROUND 24 47 47 | 48 48 49 -REQ
GROUND 25 49 49 | 50 50 50 -I/O
==============================================================================
NOTES:
(1) The minus sign next to a signal indicates active low.
(2) The conductor number refers to the conductor position when using 0.050-
inch centerline flat ribbon cable with a low-density connector or when using
0.025-inch centerline flat ribbon cable with a high-density connector. Other
cable types may be used to implement equivalent contact assignments.
(3) Two sets of contact assignments are shown. Refer to Table 4-1 to
determine which set of contacts applies to each connector.
(4) See 4.4.4 for a definition of the RESERVED lines.
Table 4-3: Single-Ended Contact Assignments - B Cable
==============================================================================
Connector Cable Connector
Signal Contact Conductor Contact Signal
Name Number Number Number Name
------------------------------------------------------------------------------
GROUND 1 1 | 2 35 GROUND
GROUND 2 3 | 4 36 -DB(8)
GROUND 3 5 | 6 37 -DB(9)
GROUND 4 7 | 8 38 -DB(10)
GROUND 5 9 | 10 39 -DB(11)
GROUND 6 11 | 12 40 -DB(12)
GROUND 7 13 | 14 41 -DB(13)
GROUND 8 15 | 16 42 -DB(14)
GROUND 9 17 | 18 43 -DB(15)
GROUND 10 19 | 20 44 -DB(P1)
GROUND 11 21 | 22 45 -ACKB
GROUND 12 23 | 24 46 GROUND
GROUND 13 25 | 26 47 -REQB
GROUND 14 27 | 28 48 -DB(16)
GROUND 15 29 | 30 49 -DB(17)
GROUND 16 31 | 32 50 -DB(18)
TERMPWRB 17 33 | 34 51 TERMPWRB
TERMPWRB 18 35 | 36 52 TERMPWRB
GROUND 19 37 | 38 53 -DB(19)
GROUND 20 39 | 40 54 -DB(20)
GROUND 21 41 | 42 55 -DB(21)
GROUND 22 43 | 44 56 -DB(22)
GROUND 23 45 | 46 57 -DB(23)
GROUND 24 47 | 48 58 -DB(P2)
GROUND 25 49 | 50 59 -DB(24)
GROUND 26 51 | 52 60 -DB(25)
GROUND 27 53 | 54 61 -DB(26)
GROUND 28 55 | 56 62 -DB(27)
GROUND 29 57 | 58 63 -DB(28)
GROUND 30 59 | 60 64 -DB(29)
GROUND 31 61 | 62 65 -DB(30)
GROUND 32 63 | 64 66 -DB(31)
GROUND 33 65 | 66 67 -DB(P3)
GROUND 34 67 | 68 68 GROUND
==============================================================================
NOTES:
(1) The minus sign next to a signal indicates active low.
(2) The conductor number refers to the conductor position when using 0.025-
inch centerline flat ribbon cable. Other cable types may be used to implement
equivalent contact assignments.
IMPLEMENTORS NOTE: X3T9.2 is documenting an alternate 16-bit single-cable
solution and an alternate 32-bit solution and expects to be able to remove
the B cable definition in a future version of SCSI.
Table 4-4: Differential Contact Assignments - A Cable
==============================================================================
Connector Cable Connector
Signal Contact Number Conductor Contact Number Signal
Name Set 2 Set 1 Number Set 1 Set 2 Name
------------------------------------------------------------------------------
GROUND 1 1 1 | 2 2 26 GROUND
+DB(0) 2 3 3 | 4 4 27 -DB(0)
+DB(1) 3 5 5 | 6 6 28 -DB(1)
+DB(2) 4 7 7 | 8 8 29 -DB(2)
+DB(3) 5 9 9 | 10 10 30 -DB(3)
+DB(4) 6 11 11 | 12 12 31 -DB(4)
+DB(5) 7 13 13 | 14 14 32 -DB(5)
+DB(6) 8 15 15 | 16 16 33 -DB(6)
+DB(7) 9 17 17 | 18 18 34 -DB(7)
+DB(P) 10 19 19 | 20 20 35 -DB(P)
DIFFSENS 11 21 21 | 22 22 36 GROUND
RESERVED 12 23 23 | 24 24 37 RESERVED
TERMPWR 13 25 25 | 26 26 38 TERMPWR
RESERVED 14 27 27 | 28 28 39 RESERVED
+ATN 15 29 29 | 30 30 40 -ATN
GROUND 16 31 31 | 32 32 41 GROUND
+BSY 17 33 33 | 34 34 42 -BSY
+ACK 18 35 35 | 36 36 43 -ACK
+RST 19 37 37 | 38 38 44 -RST
+MSG 20 39 39 | 40 40 45 -MSG
+SEL 21 41 41 | 42 42 46 -SEL
+C/D 22 43 43 | 44 44 47 -C/D
+REQ 23 45 45 | 46 46 48 -REQ
+I/O 24 47 47 | 48 48 49 -I/O
GROUND 25 49 49 | 50 50 50 GROUND
==============================================================================
NOTES:
(1) The conductor number refers to the conductor position when using 0.050-
inch centerline flat ribbon cable with a low-density connector or when using
0.025-inch centerline flat ribbon cable with a high-density connector. Other
cable types may be used to implement equivalent contact assignments.
(2) Two sets of contact assignments are shown. Refer to Table 4-1 to
determine which set of contacts applies to each connector.
(3) See 4.4.4 for a definition of the RESERVED lines.
Table 4-5: Differential Contact Assignments - B Cable
==============================================================================
Connector Cable Connector
Signal Contact Conductor Contact Signal
Name Number Number Number Name
------------------------------------------------------------------------------
GROUND 1 1 | 2 35 GROUND
+DB(8) 2 3 | 4 36 -DB(8)
+DB(9) 3 5 | 6 37 -DB(9)
+DB(10) 4 7 | 8 38 -DB(10)
+DB(11) 5 9 | 10 39 -DB(11)
+DB(12) 6 11 | 12 40 -DB(12)
+DB(13) 7 13 | 14 41 -DB(13)
+DB(14) 8 15 | 16 42 -DB(14)
+DB(15) 9 17 | 18 43 -DB(15)
+DB(P1) 10 19 | 20 44 -DB(P1)
+ACKB 11 21 | 22 45 -ACKB
GROUND 12 23 | 24 46 DIFFSENS
+REQB 13 25 | 26 47 -REQB
+DB(16) 14 27 | 28 48 -DB(16)
+DB(17) 15 29 | 30 49 -DB(17)
+DB(18) 16 31 | 32 50 -DB(18)
TERMPWRB 17 33 | 34 51 TERMPWRB
TERMPWRB 18 35 | 36 52 TERMPWRB
+DB(19) 19 37 | 38 53 -DB(19)
+DB(20) 20 39 | 40 54 -DB(20)
+DB(21) 21 41 | 42 55 -DB(21)
+DB(22) 22 43 | 44 56 -DB(22)
+DB(23) 23 45 | 46 57 -DB(23)
+DB(P2) 24 47 | 48 58 -DB(P2)
+DB(24) 25 49 | 50 59 -DB(24)
+DB(25) 26 51 | 52 60 -DB(25)
+DB(26) 27 53 | 54 61 -DB(26)
+DB(27) 28 55 | 56 62 -DB(27)
+DB(28) 29 57 | 58 63 -DB(28)
+DB(29) 30 59 | 60 64 -DB(29)
+DB(30) 31 61 | 62 65 -DB(30)
+DB(31) 32 63 | 64 66 -DB(31)
+DB(P3) 33 65 | 66 67 -DB(P3)
GROUND 34 67 | 68 68 GROUND
==============================================================================
NOTE: The conductor number refers to the conductor position when using 0.025-
inch centerline flat ribbon cable. Other cable types may be used to implement
equivalent contact assignments.
IMPLEMENTORS NOTE: X3T9.2 is documenting an alternate 16-bit single-cable
solution and an alternate 32-bit solution and expects to be able to remove
the B cable definition in a future version of SCSI.
4.4. Electrical Description
For the measurements in this section, SCSI bus termination is assumed to be
external to the SCSI device. See 4.4.1 for the terminating requirements for
the RESERVED lines. SCSI devices may have the provision for allowing optional
internal termination.
4.4.1. Single-Ended Alternative
All signals not defined as RESERVED, GROUND, or TERMPWR shall be terminated
at both ends of the cable. The implementor may choose one of the following
two methods to terminate each end (see Figure 4-9):
(1) The termination of each signal shall consist of 220 ohms (+_5%) to the
TERMPWR line and 330 ohms (+_5%) to ground. Using resistors with +_1% tolerance
improves noise margins.
(2) The termination of each signal shall meet these requirements:
(a) The terminators shall each supply a characteristic impedance between
100 and 132 ohms.
(b) The terminators shall be powered by the TERMPWR line and may receive
additional power from other sources but shall not require such additional
power for proper operation (see 4.4.3).
(c) The current available to any signal line driver shall not exceed
48 milliamps when the driver asserts the line and pulls it to 0.5 volts dc.
Only 44.8 mA of this current shall be available from the two terminators.
(d) The voltage on all released signal lines shall be at least 2.5 volts
dc when the TERMPWR line is within specified values (see 4.4.3).
(e) These conditions shall be met with any legal configuration of targets
and initiators as long as at least one device is supplying TERMPWR.
The first termination method above is the same as in SCSI-1. The second
termination method is recommended for better signal quality.
4.4.1.1. Output Characteristics
All signals shall use open-collector or three-state drivers. Each signal
driven by an SCSI device shall have the following output characteristics when
measured at the SCSI device's connector:
VOL (Low-level output voltage) = 0.0 to 0.5 volts dc at 48 mA sinking
(signal assertion)
VOH (High-level output voltage) = 2.5 to 5.25 volts dc (signal negation)
4.4.1.2. Input Characteristics
SCSI devices with power on shall meet the following electrical
characteristics on each signal (including both receivers and passive drivers):
VIL (Low-level input voltage) = 0.0 to 0.8 volts dc (signal true)
VIH (High-level input voltage) = 2.0 to 5.25 volts dc (signal false)
IIL (Low-level input current) = -0.4 to 0.0 mA at VI = 0.5 volts dc
IIH (High-level input current) = 0.0 to 0.1 mA at VI = 2.7 volts dc
Minimum input hysteresis = 0.2 volts dc
Maximum input capacitance = 25 pF (measured at the device connector closest
to the stub, if any, within the device)
It is recommended that SCSI devices with power off also meet the above IIL
and IIH electrical characteristics on each signal.
To achieve maximum noise immunity and to assure proper operation with
complex cable configurations, it is recommended that the nominal switching
threshold be approximately 1.4 volts.
4.4.2. Differential Alternative
All signals consist of two lines denoted +SIGNAL and -SIGNAL. A signal is
true when +SIGNAL is more positive than -SIGNAL, and a signal is false when
-SIGNAL is more positive than +SIGNAL. All assigned signals of the A and B
cables described in 4.6 shall be terminated at each end of the cable with a
terminator network as shown in Figure 4-10. Resistor tolerances in the
terminator network shall be +_5% or less.
The DIFFSENS signal of the connector is used as an active high enable for
the differential drivers. If a single-ended device or terminator is
inadvertently connected, this signal is grounded, disabling the differential
drivers (see Figure 4-11).
The characteristic impedance of differential terminators is 122 ohms.
4.4.2.1. Output Characteristics
Each signal driven by an SCSI device shall have the following output
characteristics when measured at the SCSI device's connector:
VOL (Low-level output voltage) = 1.7 V maximum at IOL (Low-level output
current) = 55 mA.
VOH (High-level output voltage) = 2.7 V minimum at IOH (High-level output
current) = -55 mA.
VOD (Differential output voltage) = 1.0 V minimum with common-mode voltage
ranges from -7 to +12 volts dc.
VOL and VOH shall be as measured between the output terminal and the SCSI
device's logic ground reference.
The output characteristics shall additionally conform to ISO 8482.
4.4.2.2. Input Characteristics
SCSI devices shall meet the following electrical characteristics on each
signal (including both receivers and passive drivers):
II (Input current on either input) = +_ 2.0 mA maximum.
Maximum input capacitance = 25 pF.
The II requirement shall be met with the input voltage varying between -7
and +12 volts dc, with power on or off, and with the hysteresis equaling 35
millivolts, minimum.
The input characteristics shall additionally conform to ISO 8482.
4.4.3. Terminator Power
SCSI initiators shall supply terminator power to the TERMPWR contact(s) and,
if it implements the wide SCSI option, to the TERMPWRB contacts. This power
shall be supplied through a diode or similar semiconductor that prevents
backflow of power to the SCSI device. Targets and SCSI devices that become
temporary initiators (e.g., targets which implement the COPY command or
asynchronous event notification) are not required to supply terminator power.
Any SCSI device may supply terminator power. Interface error rates are lower
if the termination voltage is maintained at the extreme ends of the cable.
All terminators independent of location shall be powered from the TERMPWR
and TERMPWRB contact(s). The use of keyed connectors is recommended in SCSI
devices that provide terminator power to prevent accidental grounding or
the incorrect connection of terminator power.
IMPLEMENTORS NOTE: Regulatory agencies may require limiting maximum (short
circuit) current to the terminator power lines. Recommended current
limiting is 1.5 amperes for TERMPWR and 2 amperes for TERMPWRB. For systems
utilizing multiple initiators, the initiators may be configured with option
straps or current limiting devices. Maximum available current should not
exceed 5 amperes.
SCSI devices shall sink no more than 1.0 mA from TERMPWR and no more than
1.0 mA from TERMPWRB except to power an optional internal terminator.
Single-ended SCSI devices providing terminator power on cable A shall have
the following characteristics:
VTerm = 4.25 to 5.25 volts dc
900 mA minimum source drive capability
Differential SCSI devices providing terminator power on cable A shall have
the following characteristics:
VTerm = 4.0 to 5.25 volts dc
600 mA minimum source drive capability
Single-ended SCSI devices providing terminator power on cable B shall have
the following characteristics:
VTerm = 4.25 to 5.25 volts dc
1500 mA minimum source drive capability
Differential SCSI devices providing terminator power on cable B shall have
the following characteristics:
VTerm = 4.0 to 5.25 volts dc
1000 mA minimum source drive capability
IMPLEMENTORS NOTE: It is recommended that the terminator power lines be
decoupled at each terminator with at least a 2.2 microfarad high-frequency
capacitor to improve signal quality.
Alternative 1 Termination
Alternative 2 Termination
Figure 4-9: Termination for Single-Ended Devices
Figure 4-10: Termination for Differential Devices
Figure 4-11: Differential Driver Protection Circuit
4.4.4. RESERVED Lines
The lines labeled RESERVED in the A cable contact assignment tables (Table
4-2 and Table 4-4) shall be connected to ground in the bus terminator
assemblies or in the end devices on the SCSI cable. The RESERVED lines should
be open in the other SCSI devices, but may be connected to ground.
4.5. SCSI Bus
Communication on the SCSI bus is allowed between only two SCSI devices at
any given time. There is a maximum of eight SCSI devices. Each SCSI device
has an SCSI ID bit assigned as shown in Figure 4-12. Three sample system
configurations are shown in Figure 4-13.
When two SCSI devices communicate on the SCSI bus, one acts as an initiator
and the other acts as a target. The initiator originates an operation and the
target performs the operation. An SCSI device usually has a fixed role as an
initiator or target, but some devices may be able to assume either role.
An initiator may address up to eight peripheral devices that are connected
to a target. The target may be physically housed within the peripheral device
in which case the peripheral device is referred to as an embedded SCSI device.
DB(7) DB(6) DB(5) DB(4) DB(3) DB(2) DB(1) DB(0) <-- DATA BUS
| | | | | | | |
| | | | | | | SCSI ID = 0
| | | | | | |
| | | | | | SCSI ID = 1
| | | | | |
| | | | | SCSI ID = 2
| | | | |
| | | | SCSI ID = 3
| | | |
| | | SCSI ID = 4
| | |
| | SCSI ID = 5
| |
| SCSI ID = 6
|
SCSI ID = 7
Figure 4-12: SCSI ID Bits
Figure 4-13: Sample SCSI Configurations
Up to eight SCSI devices can be supported on the SCSI bus. They can be any
combination of initiators and targets provided there is at least one of each.
Certain SCSI bus functions are assigned to the initiator and certain SCSI
bus functions are assigned to the target. The initiator may arbitrate for the
SCSI bus and select a particular target. The target may request the transfer
of COMMAND, DATA, STATUS, or other information on the DATA BUS, and in some
cases it may arbitrate for the SCSI bus and reselect an initiator for the
purpose of continuing an operation.
Information transfers on the DATA BUS are asynchronous and follow a defined
REQ/ACK handshake protocol. One byte of information may be transferred with
each handshake on the A cable and, if the wide data transfer option is
implemented, one or three bytes of information may be transferred with each
handshake on the B cable. An option is defined for synchronous data transfer.
4.6. SCSI Bus Signals
There are a total of 18 signals on the A cable and 29 signals on the B
cable. A total of 11 signals are used for control and 36 are used for data
(messages, commands, status, and data), including parity. These signals are
described as follows:
BSY (BUSY). An "OR-tied" signal that indicates that the bus is being used.
SEL (SELECT). An "OR-tied" signal used by an initiator to select a target or
by a target to reselect an initiator.
IMPLEMENTORS NOTE: The SEL signal was not defined as "OR-tied" in SCSI-1.
It has been defined as "OR-tied" in SCSI-2 in anticipation of needing
another "OR-tied" signal for future standardization. This does not cause an
operational problem in mixing SCSI-1 and SCSI-2 devices.
C/D (CONTROL/DATA). A signal driven by a target that indicates whether
CONTROL or DATA information is on the DATA BUS. True indicates CONTROL.
I/O (INPUT/OUTPUT). A signal driven by a target that controls the direction
of data movement on the DATA BUS with respect to an initiator. True indicates
input to the initiator. This signal is also used to distinguish between
SELECTION and RESELECTION phases.
MSG (MESSAGE). A signal driven by a target during the MESSAGE phase.
REQ (REQUEST). A signal driven by a target on the A cable to indicate a
request for a REQ/ACK data transfer handshake.
REQB (REQUEST). A signal driven by a target on the B cable to indicate a
request for a REQB/ACKB data transfer handshake.
ACK (ACKNOWLEDGE). A signal driven by an initiator on the A cable to indicate
an acknowledgment for a REQ/ACK data transfer handshake.
ACKB (ACKNOWLEDGE). A signal driven by an initiator on the B cable to
indicate an acknowledgment for a REQB/ACKB data transfer handshake.
ATN (ATTENTION). A signal driven by an initiator to indicate the ATTENTION
condition.
RST (RESET). An "OR-tied" signal that indicates the RESET condition.
DB(7-0,P) (DATA BUS). Eight data-bit signals, plus a parity-bit signal that
form a DATA BUS. DB(7) is the most significant bit and has the highest
priority during the ARBITRATION phase. Bit number, significance, and priority
decrease downward to DB(0). A data bit is defined as one when the signal
value is true and is defined as zero when the signal value is false. Data
parity DB(P) shall be odd. Parity is undefined during the ARBITRATION phase.
DB(31-8,P1,P2,P3) (DATA BUS). Twenty-four data-bit signals, plus three
parity-bit signals that form an extension to the DATA BUS. DB(P1,P2,P3) are
parity bits for DB(15-8), DB(23-16), and DB(31-24) respectively. A data bit
is defined as one when the signal value is true and is defined as zero when
the signal value is false. Data parity DB(Px) shall be odd.
4.6.1. Signal Values
Signals may assume true or false values. There are two methods of driving
these signals. In both cases, the signal shall be actively driven true, or
asserted. In the case of OR-tied drivers, the driver does not drive the
signal to the false state, rather the bias circuitry of the bus terminators
pulls the signal false whenever it is released by the drivers at every SCSI
device. If any driver is asserted, then the signal is true. In the case of
non-OR-tied drivers, the signal may be actively driven false. In this
standard, wherever the term negated is used, it means that the signal may be
actively driven false, or may be simply released (in which case the bias
circuitry pulls it false), at the option of the implementor. The advantage to
actively driving signals false during information transfer is that the
transition from true to false occurs more quickly and the noise margin is much
higher than if the signal is simply released. This facilitates reliable data
transfer at high rates, especially at the longer cable lengths used with
differential drivers.
4.6.2. OR-Tied Signals
The BSY, SEL, and RST signals shall be OR-tied only. In the ordinary
operation of the bus, the BSY and RST signals may be simultaneously driven
true by several drivers. No signals other than BSY, RST, and DB(P) are
simultaneously driven by two or more drivers, and any signal other than BSY,
SEL, and RST may employ OR-tied or non-OR-tied drivers. DB(P) shall not be
driven false during the ARBITRATION phase but may be driven false in other
phases. There is no operational problem in mixing OR-tied and non-OR-tied
drivers on signals other than BSY and RST.
4.6.3. Signal Sources
Table 4-6 indicates which type of SCSI device is allowed to source each
signal. No attempt is made to show if the source is driving asserted, driving
negated, or is passive. All SCSI device drivers that are not active sources
shall be in the passive state. The RST signal may be asserted by any SCSI
device at any time.
Table 4-6: Signal Sources
==============================================================================
A Cable Signals B Cable Signals
------------------------------- ------------------------
C/D,
I/O,
MSG, ACK, DB(7-0) DB(31-8),
Bus Phase BSY SEL REQ ATN DB(P) REQB ACKB DB(P1,P2,P3)
----------- ---- ---- ---- ---- ------- ---- ---- ------------
BUS FREE None None None None None None None None
ARBITRATION All Win None None S ID None None None
SELECTION I&T Init None Init Init None None None
RESELECTION I&T Targ Targ Init Targ None None None
COMMAND Targ None Targ Init Init None None None
DATA IN Targ None Targ Init Targ Targ Init Targ
DATA OUT Targ None Targ Init Init Targ Init Init
STATUS Targ None Targ Init Targ None None None
MESSAGE IN Targ None Targ Init Targ None None None
MESSAGE OUT Targ None Targ Init Init None None None
==============================================================================
All: The signal shall be driven by all SCSI devices that are actively
arbitrating.
S ID: A unique data bit (the SCSI ID) shall be driven by each SCSI device
that is actively arbitrating; the other seven data bits shall be
released (i.e., not driven) by this SCSI device. The parity bit
(DB(P)) may be released or driven to the true state, but shall never
be driven to the false state during this phase.
I&T: The signal shall be driven by the initiator, target, or both, as
specified in the SELECTION phase and RESELECTION phase.
Init: If driven, this signal shall be driven only by the active initiator.
None: The signal shall be released; that is, not be driven by any SCSI
device. The bias circuitry of the bus terminators pulls the signal to
the false state.
Win: The signal shall be driven by the one SCSI device that wins
arbitration.
Targ: If the signal is driven, it shall be driven only by the active target.
4.7. SCSI Bus Timing
Unless otherwise indicated, the delay-time measurements for each SCSI
device, shown in Table 4-7, shall be calculated from signal conditions
existing at that SCSI device's own SCSI bus connection. Thus, these
measurements (except cable skew delay) can be made without considering delays
in the cable. The timing characteristics of each signal are described in the
following paragraphs.
Table 4-7: SCSI Bus Timing Values
==============================================================
Arbitration Delay . . . . . . . . 2.4 microseconds
Assertion Period . . . . . . . . 90 nanoseconds
Bus Clear Delay . . . . . . . . . 800 nanoseconds
Bus Free Delay . . . . . . . . . 800 nanoseconds
Bus Set Delay . . . . . . . . . . 1.8 microseconds
Bus Settle Delay . . . . . . . . 400 nanoseconds
Cable Skew Delay . . . . . . . . 10 nanoseconds
Data Release Delay . . . . . . . 400 nanoseconds
Deskew Delay . . . . . . . . . . 45 nanoseconds
Disconnection Delay . . . . . . . 200 microseconds
Hold Time . . . . . . . . . . . . 45 nanoseconds
Negation Period . . . . . . . . . 90 nanoseconds
Power-On to Selection Time . . . 10 seconds recommended
Reset to Selection Time . . . . . 250 milliseconds recommended
Reset Hold Time . . . . . . . . . 25 microseconds
Selection Abort Time . . . . . . 200 microseconds
Selection Time-out Delay . . . . 250 milliseconds recommended
Transfer Period . . . . . . . . . set during an SDTR message
Fast Assertion Period . . . . . . 30 nanoseconds
Fast Cable Skew Delay . . . . . . 5 nanoseconds
Fast Deskew Delay . . . . . . . . 20 nanoseconds
Fast Hold Time . . . . . . . . . 10 nanoseconds
Fast Negation Period . . . . . . 30 nanoseconds
==============================================================
4.7.1. Arbitration Delay
The minimum time an SCSI device shall wait from asserting BSY for
arbitration until the DATA BUS can be examined to see if arbitration has been
won. There is no maximum time.
4.7.2. Assertion Period
The minimum time that a target shall assert REQ (or REQB) while using
synchronous data transfers. Also, the minimum time that an initiator shall
assert ACK (or ACKB) while using synchronous data transfers. REQB and ACKB
timings only apply to optional wide data transfers.
4.7.3. Bus Clear Delay
The maximum time for an SCSI device to stop driving all bus signals after:
(1) The BUS FREE phase is detected (BSY and SEL both false for a bus settle
delay)
(2) SEL is received from another SCSI device during the ARBITRATION phase
(3) The transition of RST to true.
For the first condition above, the maximum time for an SCSI device to clear
the bus is 1200 nanoseconds from BSY and SEL first becoming both false. If an
SCSI device requires more than a bus settle delay to detect BUS FREE phase, it
shall clear the bus within a bus clear delay minus the excess time.
4.7.4. Bus Free Delay
The minimum time that an SCSI device shall wait from its detection of the
BUS FREE phase (BSY and SEL both false for a bus settle delay) until its
assertion of BSY when going to the ARBITRATION phase.
4.7.5. Bus Set Delay
The maximum time for an SCSI device to assert BSY and its SCSI ID bit on the
DATA BUS after it detects BUS FREE phase (BSY and SEL both false for a bus
settle delay) for the purpose of entering the ARBITRATION phase.
4.7.6. Bus Settle Delay
The minimum time to wait for the bus to settle after changing certain
control signals as called out in the protocol definitions.
4.7.7. Cable Skew Delay
The maximum difference in propagation time allowed between any two SCSI bus
signals measured between any two SCSI devices.
4.7.8. Data Release Delay
The maximum time for an initiator to release the DATA BUS signals following
the transition of the I/O signal from false to true.
4.7.9. Deskew Delay
The minimum time required for deskew of certain signals.
4.7.10. Disconnection Delay
The minimum time that a target shall wait after releasing BSY before
participating in an ARBITRATION phase when honoring a DISCONNECT message from
the initiator.
4.7.11. Hold Time
The minimum time added between the assertion of REQ (or REQB) or ACK (or
ACKB) and the changing of the data lines to provide hold time in the initiator
or target while using synchronous data transfers. REQB and ACKB timings only
apply to optional wide data transfers.
4.7.12. Negation Period
The minimum time that a target shall negate REQ (or REQB) while using
synchronous data transfers. Also, the minimum time that an initiator shall
negate ACK (or ACKB) while using synchronous data transfers. REQB and ACKB
timings only apply to optional wide data transfers.
4.7.13. Power-On to Selection Time
The recommended maximum time from power application until an SCSI target is
able to respond with appropriate status and sense data to the TEST UNIT READY,
INQUIRY, and REQUEST SENSE commands.
4.7.14. Reset to Selection Time
The recommended maximum time after a hard RESET condition until an SCSI
target is able to respond with appropriate status and sense data to the TEST
UNIT READY, INQUIRY, and REQUEST SENSE commands.
4.7.15. Reset Hold Time
The minimum time for which RST is asserted. There is no maximum time.
4.7.16. Selection Abort Time
The maximum time that a target (or initiator) shall take from its most
recent detection of being selected (or reselected) until asserting a BSY
response. This time-out is required to ensure that a target (or initiator)
does not assert BSY after a SELECTION (or RESELECTION) phase has been aborted.
This is not the selection time-out period; see 5.1.3.1 and 5.1.4.2 for a
complete description.
4.7.17. Selection Time-out Delay
The minimum time that an initiator (or target) should wait for a BSY
response during the SELECTION (or RESELECTION) phase before starting the time-
out procedure. Note that this is only a recommended time period.
4.7.18. Transfer Period
The Transfer Period specifies the minimum time allowed between the leading
edges of successive REQ pulses and of successive ACK pulses while using
synchronous data transfers. (See 5.1.5.2 and 5.6.21.)
4.8. Fast Synchronous Transfer Option
When devices negotiate a synchronous data transfer period of less than 200
ns they are said to be using "fast synchronous data transfers". Devices which
negotiate a synchronous data transfer period greater than 200 ns use timing
parameters specified in 4.7. When a fast synchronous data transfer period is
negotiated, those specific times redefined in this section are used; those not
redefined remain the same. The minimum synchronous data transfer period is
100 ns.
4.8.1. Fast Assertion Period
This value is the minimum time that a target shall assert REQ (or REQB)
while using fast synchronous data transfers. Also, the minimum time that an
initiator shall assert ACK (or ACKB) while using fast synchronous data
transfers. REQB and ACKB timings only apply to optional wide data transfers.
4.8.2. Fast Cable Skew Delay
This value is the maximum difference in propagation time allowed between any
two SCSI bus signals measured between any two SCSI devices while using fast
synchronous data transfers.
4.8.3. Fast Deskew Delay
This value is the minimum time required for deskew of certain signals while
using fast synchronous data transfers.
4.8.4. Fast Hold Time
This value is the minimum time added between the assertion of REQ (or REQB)
or ACK (or ACKB) and the changing of the data lines to provide hold time in
the initiator or target, respectively, while using fast synchronous data
transfers. REQB and ACKB timings only apply to optional wide data transfers.
4.8.5. Fast Negation Period
This value is the minimum time that a target shall negate REQ (or REQB)
while using fast synchronous data transfers. Also, the minimum time that an
initiator shall negate ACK (or ACKB) while using fast synchronous data
transfers. REQB and ACKB timings only apply to optional wide data transfers.
These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.