Tandberg Data SLR7-140 PROGRAMMER Manual page 94

Transfer Period
REQ/ACK Offset
Responding Device SDTR response:
1) Non-zero REQ/ACK offset
2) REQ/ACK offset equal to zero
3) MESSAGE REJECT message
Tandberg Data
The Transfer Period is the minimum time allowed between
leading edges of successive REQ pulses and of successive
ACK pulses, set
successful reception of data.
The minimum transfer period is 50 nanoseconds, m = 0Ch
(12). The maximum transfer period is 544 nano seconds, m
= 88h (136).
When a synchronous data transfer period of less than 200
ns is negotiated, " fast synchronous data transfer"
used. This will result in different values for some
SCSI-bus Timing Values, see [2] and [5].
When a synchronous data transfer period of less than 100
ns is negotiated, Ultra SCSI (Fast-20) data transfer is
used. This will result in different values for some
SCSI-bus Timing Values, see [18].
The REQ/ACK Offset is the maximum number of REQ pulses
allowed to be outstanding before the corresponding ACK
pulse is received at the Drive. The value is limited by
the size of the device's reception buffer. A REQ/ACK
off- set of zero indicates asynchronous data transfer
mode. The maximum REQ/ACK offset of the Drive is 15.
The default transfer mode is entered at power on, after
a TARGET RESET message or a hard reset condition.
The Initiator sets its values to permit it to receive
data successfully. If the Drive can also receive data
successfully with
periods or larger REQ/ACK offsets or both), it returns
the same values in its SDTR message. If it requires a
larger transfer period, a smaller REQ/ACK offset, or
both in order
substitutes values
returning unchanged
changed.
The successful
messages implies an agreement as follows:
Implied Agreement:
Each device transmits data with a transfer period equal to or
greater than and a REQ/ACK offset equal to or less than the
values received in the other device's SDTR message.
Asynchronous transfer
Asynchronous transfer
If the initiator
required, it asserts the ATN signal and sends a SDTR
message to begin
successfully completing the MESSAGE OUT phase, the Drive
will respond with
abnormal condition prevents the Drive from returning an
appropriate response,
asynchronous data
between the two devices.
6-9 Tandberg SLR Product Line SCSI Functional Specifications
by the device
these values
to receive data
in its SDTR
any value
completion of
recognizes
the negotiating
the proper
both devices
transfer mode
Message System
specifications
(or smaller transfer
successfully,
message as required,
not required to
an exchange of
that negotiation
process.
SDTR message.
shall go
for data transfers
for
is
it
be
SDTR
is
After
If an
to