Chapter 6
Programming in G Mode
© National Instruments Corp.
6-25
GPIB-SCSI-A User Manual
Serial Poll
The GPIB-SCSI-A can return status information to the GPIB Controller
through the serial poll response byte. The GPIB-SCSI-A maintains this
response byte throughout operation, regardless of the
srqen
configuration. The GPIB-SCSI-A can provide certain status information
when it is serial polled. This byte contains eight bits of information as
detailed in Figure 6-1.
7
6
5
4
3
2
1
0
bit
ERR
SRQ*
BSY*
RDY
DCR
MSG*
C/D*
I/O*
0 0 0
0 0 1
0 1 0
0 1 1
1 1 0
1 1 1
Data Out
Data In
Command
Status
Message Out
Message In
Figure 6-1. Serial Poll Status Byte
SCSI Bits
BSY*, MSG*, C/D*, and I/O* are SCSI signal indicators. MSG*, C/D*,
and I/O* indicate the current information transfer phase for the SCSI bus.
BSY* indicates that the SCSI bus is currently in use. MSG*, C/D*, and
I/O* are valid only if BSY* is active. For more information on the purpose
of these SCSI signals, refer to Appendix D, Operation of the SCSI.
DCR Bit
DCR indicates Data Channel ready. This bit is set when the Data Channel
initially becomes ready for information transfer. For GPIB writes of data to
a SCSI Target, this bit is set when the Target enters the Data Out phase.
For GPIB reads of data from a SCSI Target, this bit is set when the Target
enters the Data In phase.
