4. Physical Characteristics

This section contains the physical definition of the SCSI.  The connectors, 
cables, signals, terminators, and bus timing needed to implement SCSI are 
covered.

4.1 Physical Description.  SCSI devices are daisy-chained together using a 
common cable.  Both ends of the cable are terminated.  All signals are common 
between all SCSI devices.  Two driver/receiver alternatives are available:

  (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)

4.2 Cable Requirements.  An ideal impedance match with cable terminators 
implies a cable characteristic impedance of 132 ohms (singled-ended option) or 
122 ohms (differential option). In general, cables with this high of a 
characteristic impedance are not available; however, impedances that are 
somewhat lower are satisfactory.  A characteristic impedance of 100 ohms + 10% 
is recommended for unshielded flat or twisted pair ribbon cable.  A 
characteristic impedance greater than 90 ohms is preferred for shielded 
cables; however, most available cables have a somewhat lower characteristic 
impedance.  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.

  A minimum conductor size of 28 AWG shall be employed to minimize noise 
effects and ensure proper distribution of optional terminator power.
  4.2.1 Single-Ended Cable.  A 50-conductor flat cable or 25-signal twisted-
pair cable shall be used.  The maximum cable length shall be 6.0 meters.

  A stub length of no more than 0.1 meters is allowed off the mainline 
interconnection within any connected equipment.

  SCSI bus termination may be internal to the SCSI devices that are at the 
ends of the cable.

  4.2.2 Differential Cable.  A 50-conductor cable or 25-signal twisted-pair 
cable shall be used.  The maximum cable length shall be 25 meters.

  A stub length of no more than 0.2 meters is allowed off the mainline 
interconnection within any connected equipment.

  SCSI bus termination may be internal to the SCSI devices that are at the 
ends of the cable.

4.3 Connector Requirements.  Nonshielded connectors are specified.  The 
nonshielded connectors are typically used for in-cabinet applications.  
Appendix D defines recommended shielded connectors and their pin assignments.  
These 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.

  The nonshielded SCSI device connector (Figure 4-1) 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 nonshielded cable connector (Figure 4-2) 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.

  The unshielded connector pin assignments shall be as shown in Table 4-1 for 
single-ended drivers and as shown in Table 4-2 for differential drivers.
































































































                Figure 4-1a. Nonshielded SCSI Device Connector
==============================================================================
Dimensions    Millimeters    Inches
------------------------------------------------------------------------------
   D1          2.54          0.100 
   D2*        82.80          3.260 
   D3          2.54          0.100 
   D4          4.83          0.190 
   D5*         8.51          0.335 
   D6*        72.64          2.860 
   D7*        78.74          3.100 
   D8*        13.94          0.549 
   D9          4.19          0.165 
   D10         6.09          0.240 
   D11         6.60          0.260 
==============================================================================
NOTES:
  (1) Fifty Contacts on 2.54-mm (0.100-inch) spacing = 60.96 mm (2.40 inch).
  (2) Tolerances + 0.127 mm (0.005 inch) noncumulative.
  (3) Dimensions listed with asterisks (*) are shown for reference only.


                Figure 4-1b. Nonshielded SCSI Device Connector






(Editors note:  Figures 4-1a and 4-1b are to be combined into a single figure 
during the final editing.)















































































                   Figure 4-2a. Nonshielded Cable Connector
==============================================================================
Dimensions    Millimeters    Inches
------------------------------------------------------------------------------
   C1          2.5400        0.100
   C2         60.9600        2.400
   C3          2.5400        0.100
   C4          8.3570        0.329
   C5          3.3025        0.130
   C6         68.0720        2.680
   C7          6.0960        0.240
   C8*         8.1530        0.321
   C9*        13.4870        0.531
   C10*        3.8100        0.150
   C11*        1.2700        0.050
   C12*        6.0960        0.240
   C13        32.3850        1.275
   C14         3.3020        0.130
   C15         7.4930        0.295
   C16         2.6670        0.105 
   C17         1.6250        0.064
==============================================================================
NOTES:
  (1) Fifty contacts on 1.27-mm (0.05-inch)* staggered spacing = 62.23 mm 
(2.450 inch)*.
  (2) Tolerances + 0.127 mm (0.005 inch) noncumulative.
  (3) Dimensions listed with asterisks (*) are shown for reference only.

                   Figure 4-2b. Nonshielded Cable Connector






(Editors note:  Figures 4-2a and 4-2b are to be combined into a single figure 
during the final editing.)

(Pages 17-19 and 19.1-19.4 are deleted.  The information previously contained 
on these pages has been moved to Appendix D.)
















                                  Table 4-1
                         Single-Ended Pin Assignments

==============================================================================
                            Signal      Pin Number
------------------------------------------------------------------------------
                            -DB(0)           2
                            -DB(1)           4
                            -DB(2)           6
                            -DB(3)           8
                            -DB(4)          10
                            -DB(5)          12
                            -DB(6)          14
                            -DB(7)          16
                            -DB(P)          18
                            GROUND          20
                            GROUND          22
                            GROUND          24
                            TERMPWR         26
                            GROUND          28
                            GROUND          30
                            -ATN            32
                            GROUND          34
                            -BSY            36
                            -ACK            38
                            -RST            40
                            -MSG            42
                            -SEL            44
                            -C/D            46
                            -REQ            48
                            -I/O            50
==============================================================================
NOTES:
  (1) All odd pins except pin 25 shall be connected to ground.  Pin 25 should 
be left open.  Some products designed prior to the generation of this standard 
connected this pin to ground.
  (2) The minus sign next to the signals indicates active low.


















                                  Table 4-2
                         Differential Pin Assignments

==============================================================================
              Signal Name        Pin Number         Signal Name
------------------------------------------------------------------------------
              SHIELD GROUND      1         2        GROUND 
              +DB(0)             3         4        -DB(0) 
              +DB(1)             5         6        -DB(1) 
              +DB(2)             7         8        -DB(2) 
              +DB(3)             9        10        -DB(3) 
              +DB(4)            11        12        -DB(4) 
              +DB(5)            13        14        -DB(5) 
              +DB(6)            15        16        -DB(6) 
              +DB(7)            17        18        -DB(7) 
              +DB(P)            19        20        -DB(P) 
              DIFFSENS          21        22        GROUND 
              GROUND            23        24        GROUND 
              TERMPWR           25        26        TERMPWR
              GROUND            27        28        GROUND 
              +ATN              29        30        -ATN 
              GROUND            31        32        GROUND 
              +BSY              33        34        -BSY 
              +ACK              35        36        -ACK 
              +RST              37        38        -RST 
              +MSG              39        40        -MSG 
              +SEL              41        42        -SEL 
              +C/D              43        44        -C/D 
              +REQ              45        46        -REQ 
              +I/O              47        48        -I/O 
              GROUND            49        50        GROUND 
==============================================================================
NOTE:
  (1) SHIELD GROUND is optional on some cables.  (Implementors note:  Some 
shielded flat ribbon cables use pin 1 as a connection to the shield.)




















4.4 Electrical Description

NOTE:  For these measurements, SCSI bus termination is assumed to be external 
to the SCSI device.  An SCSI device may have the provision for allowing 
optional internal termination.

  4.4.1 Single-Ended Alternative.  All assigned signals shall be terminated 
with 220 ohms to +5 volts (nominal) and 330 ohms to ground at each end of the 
cable.  (See Figure 4-5.)  All signals shall use open-collector or three-state 
drivers.

     4.4.1.1 Output Characteristics.  Each signal driven by an SCSI device 
shall have the following output characteristics when measured at the SCSI 
device's connector:

  Signal assertion = 0.0 volts dc to 0.4 volts dc 
  Minimum driver output capability = 48 milliamps (sinking) at 0.5 volts dc
  Signal negation = 2.5 volts dc to 5.25 volts dc

     4.4.1.2 Input Characteristics.  Each signal received by an SCSI device 
shall have the following input characteristics when measured at the SCSI 
device's connector:

  Signal true = 0.0 volts dc to 0.8 volts dc
  Maximum total input load = -0.4 milliamps at 0.4 volts dc
  Signal false = 2.0 volts dc to 5.25 volts dc
  Minimum input hysteresis = 0.2 volts dc

  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 shall be terminated at each end of the cable as shown in 
Figure 4-6.

NOTE:  As an option, the DIFFSENS signal of the connector is reserved for an 
active high enable for the differential drivers.  If a single-ended device or 
terminator is inadvertently connected, this signal is grounded, disabling the 
drivers.  (See Figure 4-7.)

     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) = 2.0 V maximum at IOL (Low-level output 
                                      current) = 55 milliamps.
   VOH (High-level output voltage) = 3.0 V minimum at IOH (High-level output 
                                       current) = -55 milliamps.
   VOD (Differential voltage) = 1.0 V minimum with common-mode voltage ranges 
                                  from -7 volts dc 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 EIA RS-485-1983.

     4.4.2.2 Input Characteristics.  Each signal received by an SCSI device 
shall have the following input characteristics when measured at the SCSI 
device's connector:

  II (Input current on either input) = + 2.0 milliamps maximum.

NOTE:  These characteristics include both receivers and passive drivers.

  This requirement shall be met with the input voltage varying between 
-7 volts dc and +12 volts dc, with power on or off, and with the hysteresis 
equaling 35 millivolts, minimum.

  The input characteristics shall additionally conform to EIA RS-485-1983.

  4.4.3 Terminator Power (Optional).  Single-ended SCSI devices providing 
terminator power (TERMPWR) shall have the following characteristics:

  VTerm =  4.0 volts dc to 5.25 volts dc
           800 milliamps minimum source drive capability
           1.0 milliamp maximum sink capability (except for the purposes of 
             providing power to an internal terminator) with 1.0 amp 
             recommended current limiting (e.g., a fuse).

  Differential SCSI devices providing terminator power (TERMPWR) shall have 
the following characteristics:

  VTerm =  4.0 volts dc to 5.25 volts dc
           600 milliamps minimum source drive capability
           1.0 milliamp maximum sink capability (except for the purposes of 
             providing power to an internal terminator) with 1.0 amp 
             recommended current limiting (e.g., a fuse).

  The use of keyed connectors is recommended in SCSI devices that provide 
terminator power to prevent accidental grounding or misconnection of 
terminator power.

  SCSI devices that supply terminator power shall do so through a diode or 
similar semiconductor that prevents the backflow of power to the SCSI device.

































               Figure 4-5. Termination for Single-Ended Devices





















               Figure 4-6. Termination for Differential Devices















        Figure 4-7. Differential Driver Protection Circuit (Optional)
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 4-8.

  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.  An option allows the addressing of up to 2,048 peripheral 
devices per target using extended messages.  Three sample system 
configurations are shown in Figure 4-9.



     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 4-8. SCSI ID Bits









































































                    Figure 4-9. Sample SCSI Configurations
  Up to eight SCSI devices can be supported on the SCSI bus.  They can be any 
combination of initiators and targets.

  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.  An option is defined for synchronous data transfer.

4.6 SCSI Bus Signals.  There are a total of eighteen signals.  Nine are used 
for control and nine are used for data.  (Data signals include the parity 
signal option).  These signals are described as follows:

BSY (BUSY).  An "OR-tied" signal that indicates that the bus is being used.

SEL (SELECT).  A signal used by an initiator to select a target or by a target 
to reselect an initiator.

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 to indicate a request for a 
REQ/ACK data transfer handshake.

ACK (ACKNOWLEDGE).  A signal driven by an initiator to indicate an 
acknowledgment for a REQ/ACK 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) is odd.  The use of parity is a system option (i.e., a 
system is configured so that all SCSI devices on a bus generate parity and 
have parity detection enabled, or all SCSI devices have parity detection 
disabled or not implemented).  Parity is not valid during the ARBITRATION 
phase.

  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, or 
negated.  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 drive signals false is that the transition from true 
to false occurs more quickly, and noise margins may be somewhat improved; this 
may permit somewhat faster data transfer. 

  4.6.2 OR-Tied Signals.  The BSY and RST signals shall be OR-tied only.  In 
the ordinary operation of the bus, these signals are 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 
and RST may employ OR-tied or non-OR-tied drivers.  DB(P) shall not be driven 
false during the ARBITRATION phase.  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 4-3 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.  Note that the RST 
signal may be sourced by any SCSI device at any time.



























                                  Table 4-3
                                Signal Sources

==============================================================================
                                             Signals
                    ----------------------------------------------------------
                                          C/D, I/O,
Bus Phase           BSY       SEL         MSG, REQ     ACK/ATN       DB(7-0,P)
------------------------------------------------------------------------------
BUS FREE            None      None        None         None          None
ARBITRATION         All       Winner      None         None          SCSI ID
SELECTION           I&T       Initiator   None         Initiator     Initiator
RESELECTION         I&T       Target      Target       Initiator     Target
COMMAND             Target    None        Target       Initiator     Initiator
DATA IN             Target    None        Target       Initiator     Target
DATA OUT            Target    None        Target       Initiator     Initiator
STATUS              Target    None        Target       Initiator     Target
MESSAGE IN          Target    None        Target       Initiator     Target
MESSAGE OUT         Target    None        Target       Initiator     Initiator
==============================================================================

All:          The signal shall be driven by all SCSI  devices that are 
              actively arbitrating.

SCSI 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 undriven 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.

Initiator:    If this signal is driven, it 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.

Winner:       The signal shall be driven by the one SCSI device that wins 
              arbitration.

Target:       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 4.7.1 through 4.7.14, 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.

  4.7.1 Arbitration Delay (2.2 microseconds).  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 (90 nanoseconds).  The minimum time that a target 
shall assert REQ while using synchronous data transfers.  Also, the minimum 
time that an initiator shall assert ACK while using synchronous data 
transfers.

  4.7.3 Bus Clear Delay (800 nanoseconds).  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.

NOTE:  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 (800 nanoseconds).  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 (1.8 microseconds).  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 (400 nanoseconds).  The 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 (10 nanoseconds).  The maximum difference in 
propagation time allowed between any two SCSI bus signals when measured 
between any two SCSI devices.

  4.7.8 Data Release Delay (400 nanoseconds).  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 (45 nanoseconds).  The minimum time required for deskew 
of certain signals.

  4.7.10 Hold Time (45 nanoseconds).  The minimum time added between the 
assertion of REQ or ACK and the changing of the data lines to provide hold 
time in the initiator or target, respectively, while using synchronous data 
transfers.

  4.7.11 Negation Period (90 nanoseconds).  The minimum time that a target 
shall negate REQ while using synchronous data transfers.  Also, the minimum 
time that an initiator shall negate ACK while using synchronous data 
transfers.

  4.7.12 Reset Hold Time (25 microseconds).  The minimum time for which RST is 
asserted.  There is no maximum time.

  4.7.13 Selection Abort Time (200 microseconds).  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 timeout 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 
timeout period; see Sections 5.1.3.5 and 5.1.4.2 for a complete description.

  4.7.14 Selection Timeout Delay (250 milliseconds, recommended).  The minimum 
time that an initiator (or target) should wait for a BSY response during the 
SELECTION (or RESELECTION) phase before starting the timeout procedure.  Note 
that this is only a recommended time period.  The specifications for the 
peripheral devices shall be consulted for the actual timing requirements.

  4.7.15 Transfer Period (set during a MESSAGE phase).  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 Sections 5.1.5.2 and 5.5.5.)



























