Seagate Ultra 160 - Manual (Page 109)

Parallel SCSI Interface Product Manual, Rev. A 95
be changed. Each SCSI device when transmitting data shall respect the negotiated limits set by the other's
PARALLEL PROTOCOL REQUEST message, but it is permitted to transfer data with larger periods, smaller
synchronous REQ/ACK offsets, or both. The completion of an exchange of PARALLEL PROTOCOL
REQUEST messages implies an agreement as shown in Table 35.
If the target does not support the selected protocol option it shall clear as many bits as required to set the pro-
tocol option field to a legal value that it does support.
Table 35: PARALLEL PROTOCOL REQUEST messages implied agreement
If there is an unrecoverable parity error on the initial PARALLEL PROTOCOL REQUEST message (see Sec-
tion 4.3.12), the initiator shall retain its previous data transfer mode and protocol options. If there is an unex-
pected bus free on the initial PARALLEL PROTOCOL REQUEST message, the initiator shall retain its previous
data transfer mode and protocol options.
4.3.12.1 PARALLEL PROTOCOL REQUEST
If the initiator recognizes that Parallel Protocol Request negotiation is required, it creates an attention condition
and sends a PARALLEL PROTOCOL REQUEST message to begin the negotiating process. After successfully
completing the MESSAGE OUT phase, the target shall respond with a PARALLEL PROTOCOL REQUEST
message or a MESSAGE REJECT message.
Target’s Parallel Protocol
Request response Implied agreement
Non-zero REQ/ACK OFFSET Synchronous transfer (i.e., each SCSI device transmits data with a
period equal to or greater than and a REQ/ACK OFFSET equal to or
less than the negotiated values received in the target’s PPR message).
REQ/ACK OFFSET equal to zero Asynchronous transfer
Transfer width exponent equal
to one
16-bit data
Transfer width equal to zero Eight-bit data
Protocol options equal to 0h and
transfer period factor equal to 9h Eight-bit/asynchronous data transfer with protocol options field set to 0h
IU_REQ, DT_REQ, and QAS_REQ
equal to zero ST DATA IN and ST DATA OUT phases to transfer data
DT_REQ equal to one DT DATA IN and DT DATA OUT phases to transfer data
IU_REQ and DT_REQ equal to one DT DATA IN and DT DATA OUT phases with information units
IU_REQ, DT_REQ, and QAS_REQ
equal to one DT DATA IN and DT DATA OUT phases with information units and use
QAS for arbitration
MESSAGE REJECT message The initiator shall set eight-bit/asynchronous data transfer with protocol
options field set to 0h.
Parity error (on responding
message) Eight-bit/asynchronous data transfer with protocol options field set to
0h
Unexpected bus free (as a result of
the responding message) Eight-bit/asynchronous data transfer with protocol options field set to
0h
No response Eight-bit/asynchronous data transfer with protocol options field set to
0h