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Pad field data and pCRC field data are transferred using the same negotiated transfer period as the data field
data.
The target may continue to send REQs, up to the negotiated offset, for the next data group. The target shall not
transition REQ with P_CRCA asserted until the initiator has responded with all ACK transitions for the previous
data group.
When the initiator detects an assertion of the P_CRCA signal and the REQ signal is asserted (i.e., pad field
required), it shall then:
1. transfer data bytes for all outstanding REQs received prior to the REQ that had the P_CRCA signal
asserted;
2. drive the DB(15-0) signals to their pad values;
3. delay at least one transmit setup time;
4. negate the ACK signal;
5. hold the DB(15-0) signals valid for a minimum of one transmit hold time and hold the ACK signal
negated for a minimum of one transmit assertion period;
6. drive the DB(15-0) signals to their pCRC values;
7. delay at least one transmit setup time;
8. assert the ACK signal;
9. hold the DB(15-0) signals valid for a minimum of one transmit hold time and hold the ACK signal
asserted for a minimum of one transmit assertion period;
10.drive the DB(15-0) signals to their pCRC values;
11. delay at least one transmit setup time;
12.negate the ACK signal; and
13.hold the DB(15-0) signals valid for a minimum of one transmit hold time and hold the ACK signal
negated for a minimum of one transmit assertion period.
When the initiator detects an assertion of the P_CRCA signal and the REQ signal is negated (i.e., no pad field
required), it shall then:
1. transfer data bytes for all outstanding REQs received prior to the REQ that had the P_CRCA signal
asserted;
2. drive the DB(15-0) signals to their pCRC values;
3. delay at least one transmit setup time;
4. assert the ACK signal;
5. hold the DB(15-0) signals valid for a minimum of one transmit hold time and hold the ACK signal
asserted for a minimum of one transmit assertion period;
6. drive the DB(15-0) signals to their pCRC values;
7. delay at least one transmit setup time;
8. negate the ACK signal; and
9. hold the DB(15-0) signals valid for a minimum of one transmit hold time and hold the ACK signal
negated for a minimum of one transmit assertion period.
After either of the above sequences is complete, the target has ended a data group transfer.
As a result of a data group always being an even number of transfers, the REQ and ACK signals are negated
both before and after the transmission of the data group.
Parallel SCSI Interface Product Manual, Rev. A
67
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