Tandberg Data
Message System
6-9
Tandberg SLR Product Line SCSI Functional Specifications
Transfer Period
The Transfer Period is the minimum time allowed between
leading edges of successive REQ pulses and of successive
ACK pulses, set by the device specifications for
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” is
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].
REQ/ACK Offset
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 these values (or smaller transfer
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 to receive data successfully, it
substitutes values in its SDTR message as required,
returning unchanged any value not required to be
changed.
The successful completion of an exchange of SDTR
messages implies an agreement as follows:
Responding Device SDTR response:
Implied Agreement:
1) Non-zero REQ/ACK offset
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.
2) REQ/ACK offset equal to zero
Asynchronous transfer
3) MESSAGE REJECT message
Asynchronous transfer
If the initiator recognizes that negotiation is
required, it asserts the ATN signal and sends a SDTR
message to begin the negotiating process. After
successfully completing the MESSAGE OUT phase, the Drive
will respond with the proper SDTR message. If an
abnormal condition prevents the Drive from returning an
appropriate response, both devices shall go to
asynchronous data transfer mode for data transfers
between the two devices.
