Seagate Ultra 320 - Manual (Page 30)

16 Parallel SCSI Interface Product Manual, Rev. A )
During the SELECTION PHASE, RESELECTION PHASE, ST DATA PHASE, COMMAND PHASE, MESSAGE
PHASE, and STATUS PHASE, this signal is referred to as DB(P_CRCA) and is sourced by the SCSI device
port driving the Data Bus. The DB(P_CRCA) signal is associated with the DB(7-0) signals and is used to detect
the presence of an odd number of bit errors within the byte. The DB(P_CRCA) bit is driven such that the num-
ber of logical ones in the byte plus the parity bit is odd.
Data group transfers are enabled (see Section 4.3.12) when this signal is referred to as P_CRCA and is
sourced by the target to control whether a data group field is a pad field, pCRC field, or data field (see Section
2.11.1). When asserted, the data group field shall be pad or pCRC fields that shall not be transferred to the
application client. When negated, the data group field shall be a data field that shall be transferred to the appli-
cation client.
During DT DATA phases when information unit transfers are enabled, this signal is referred to as P_CRCA and
sourced by the SCSI target. Depending on the negotiated condition of read streaming and write flow control,
the SCSI initiator and target usage for P_CRCA is different. When information unit transfers are enabled, the
SCSI target and initiator shall use the P_CRCA signal as indicated in Table 2.
P1 (Parity 1)—A signal normally sourced by the SCSI device driving the Data Bus. The P1 signal is associated
with the DB(15–8) signals and is used to detect the presence of an odd number of bit errors within the byte The
P1 bit is driven such that the number of logical ones in the byte plus the P1 bit is odd.
During the ST DATA PHASE with transfer length set for 8-bit transfers, COMMAND PHASE, MESSAGE
PHASE, and STATUS phase, the P1 signal shall not be driven by any SCSI device.
During the SELECTION phase and the RESELECTION phase on a 16-bit wide bus segment the P1 signal
shall be sourced by the SCSI device driving the DATA BUS.
When data group transfers are enabled (see Section 4.3.12), the P1 signal shall be continuously negated by
the SCSI device driving the DB(15-0) signals and shall be ignored by the SCSI device receiving the DB(15-0)
signals during DT DATA phases.
Table 2: P_CRC signal usage requirements
Write flow
control Read
streaming
DT Data phase
SCSI initiator
response to
P_CRCA
SCSI target usage of P_CRCA
Disabled Disabled All Ignore Continuously negated.
Enabled Disabled DT DATA IN Ignore Continuously negated.
DT DATA OUT Monitor Asserts to indicate when the current SPI data stream
information unit is the last SPI data stream information
unit of the current write stream.
Disabled Enabled DT DATA IN Monitor Asserts to indicate when the current SPI data stream
information unit is the last SPI data stream information
unit of the current read stream.
DT DATA OUT Ignore Continuously negated.
Enabled Enabled DT DATA IN Monitor Asserts to indicate when the current SPI data stream
information unit is the last SPI data stream information
unit of the current read stream.
DT DATA OUT Monitor Asserts to indicate when the current SPI data stream
information unit is the last SPI data stream information
unit of the current read stream.
A SCSI device is not required to use read streaming even if it is enabled
A SCSI device is not required to use write flow control even if it is enabled