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Parallel SCSI Interface Product Manual, Rev. A 93
See Section 3.5 for an explanation of the differences between ST and DT data transfers.
The REQ/ACK OFFSET value is chosen to prevent overflow conditions in the port’s receive buffer and offset
counter. The REQ/ACK OFFSET values and which timing values shall be selected are defined below.
TRANSFER WIDTH EXPONENT
This field defines the transfer width to be used during DATA IN phases, and DATA OUT phases. The transfer
width that is established applies to both SCSI devices.
Valid transfer widths are:
a. 8 bits (m=00h)
b. and 16 bits (m=01h) if all the protocol options bits are zero.
c. The only valid transfer width is 16 bits (m=01h) if any of the protocol options bits are one.
TRANSFER PERIOD EXPONENT field values greater than 01h are reserved.
If any of the protocol options bits (other than QAS_REQ) are set to one, then only wide transfer agreements
are valid. If all the protocol options bits other than QAS_REQ are set to zero, wide transfer agreements and
narrow transfer agreements are valid.
The protocol options bits (IU_REQ, DT_REQ, and QAS_REQ) are used by the initiator to indicate the protocol
options to be enabled. The target uses the protocol options bits to indicate to the initiator if the requested pro
-
tocol options are enabled. The target shall not enable any options that were not enabled in the PPR message
received from the initiator.
An information units enable request bit (IU_REQ) of zero indicates that information unit transfers shall not be
used (i.e., data group transfers shall be enabled) when received from the initiator and that information unit
transfers are disabled when received from the target. An IU_REQ bit of one indicates that information unit
transfers shall be used when received from the initiator and that information unit transfers are enabled when
received from the target. If the IU_REQ bit is changed from the previous agreement (i.e., zero to one or one to
zero) as a result of a negotiation the target shall go to a BUS FREE phase on completion of the negotiation.
A DT enable request bit (DT_REQ) of zero indicates that DT DATA phases are to be disabled when received
from the initiator and that DT DATA phases are disabled when received from the target. A DT_REQ bit of one
indicates that DT DATA phases are to be enabled when received from the initiator and that DT DATA phases
are enabled when received from the target.
A QAS enable request bit (QAS_REQ) of zero indicates that QAS is to be disabled when received from the ini-
tiator and that QAS is disabled when received from the target. A QAS_REQ bit of one indicates that QAS is to
be enabled when received from the initiator and that QAS is enabled when received from the target.
Not all combinations of the protocol options bits are valid. Only the bit combinations defined in Table 34 shall
be allowed. All other combinations are reserved.
Value Description Timing values
00h
Specifies asynchronouse transfer agreement. The Transfer period
factor and protocol options other than QAS_REQ shall be ignored.
Asynch
01h-FEh
Synchronous or paced transfers with specified offset. Determined by transfer period
factor.
FFh
Synchronous or paced transfers with unlimited offset. Determined by transfer period
factor.