SCSI Bus Timing

Ref: draft proposed American National Standard (dpANS) for information
systems -SMALL COMPUTER SYSTEM INTERFACE - 2 (SCSI-2), March 9, 1990


....This standard is referred to herein as SCSI-2. The term SCSI is used
wherever
it is not necessary to distinguish between the two versions of SCSI.

[....]

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.






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