Seagate Ultra 320, Ultra 160 manual Parallel Protocol Request message format

4.3.12PARALLEL PROTOCOL REQUEST

PARALLEL PROTOCOL REQUEST messages (see Table 33) are used to negotiate a synchronous data trans- fer agreement, a wide data transfer agreement, and set the protocol options between two SCSI devices.

Table 33: PARALLEL PROTOCOL REQUEST message format

Bit	7	6	5	4	3	2	1	0
Byte
0			EXTENDED MESSAGE (01H)
1			EXTENDED MESSAGE LENGTH (06H)
2			PARALLEL PROTOCOL REQUEST (04H)
3			TRANSFER PERIOD FACTOR
4				RESERVED
5				REQ/ACK OFFEST
6			TRANSFER WIDTH EXPONENT (M)
7[1]	PCOMP	RTI	RD_STRM	WR_FLOW	HOLD_MCS	QAS_REQ	DT_REQ	IU_REQ

[1]For details on protocol option bits See Table 18, Section 2.12.8.

TRANSFER PERIOD FACTOR

00h-007h		Reserved. Faster timings may be allowed by future SCSI parallel interface standards.
08h		Transfer period equals 6.25 ns. Fast-160 data is latched every 6.25 ns.
09h		Transfer period equals 12.5 ns. Fast-80 data is latched every 12.5 ns.
0Ah		Transfer period equals 25 ns. Fast-40 data is latched every 25 ns or 30.3 ns.
0Bh		Transfer period equals 30.3 ns. Fast-40 data is latched every 25 ns or 30.3 ns.
0Ch		Transfer period equals 50 ns. Fast-20 data is latched using a transfer period of less than or equal to 96 ns and
		greater than or equal to 50 ns.
ODh - 18h		Transfer period equals the period factor x 4. Fast-20 data is latched using a transfer period of less than or
		equal to 96 ns and greater than orequal to 50 ns.
19h	- 31h	Transfer period equals the period factor x 4. Fast-10 data is latched using a transfer period of less than or
		equal to 196 ns and greater than or equal to 100 ns.
32h	- FFh	Transfer period equals the period factor x 4. Fast-5 data is latched using a transfer period of less than
		or equal 1,020 ns and greater than or equal to 200 ns.

REQ/ACK OFFSET

This field determines the maximum number of REQs allowed to be outstanding before a correesponding ACK is received at the target during synchronous or paced transfers.

For ST synchronous transfers, the REQ/ACK OFFSET is the number of REQ transitions that may be sent by the target in advance of the number of ACK transitions received from the initiator.

For paced transfers in DT DATA IN phase, the REQ/ACK OFFSET is the number of data valid state REQ assertions that may be sent by the target in advance of ACK assertions received from the initiator.

For paced transfers in DT DATA OUT phase, the REQ/ACK OFFSET is the number of REQ assertions that may be sent by the target in advance of the number of data valid state ACK assertions received from the initia- tor.

92 Parallel SCSI Interface Product Manual, Rev. A