Responding To A Serial Poll - National Instruments Corporation GPIB-1014P User Manual

Section Five Programming Considerations

Responding to a Serial Poll

The CIC can conduct Serial Polls to determine which device is asserting the GPIB SRQ signal to
request service.
Before requesting the service, the recommended practice is to wait until the PEND bit in the SPSR
is zero, indicating that the TLC is not presently in the middle of a Serial Poll (SPAS=0). If
PEND=0, write the desired Status Byte (STB) into the SPMR with the rsv bit set. At that time,
PEND sets and remains set until the Serial Poll completes.
Once rsv is set, the TLC waits until any current Serial Poll is complete and then asserts the GPIB
SRQ signal. In response to that signal, the CIC starts the poll addressing the TLC to talk. When the
CIC unasserts ATN, the TLC unasserts SRQ and transfers the STB message onto the GPIB data
bus with DIO7 (the RQS signal) asserted.
While the Serial Poll is in progress (SPAS=1), the CIC normally reads the STB only once;
however, it can read it any number of times provided that it asserts ATN between each one byte
read. RQS is set only during the first read. After the first read, rsv also is cleared. PEND is
cleared when the CIC asserts ATN to terminate the poll.
The GPIB EOI line is asserted along with the status byte (that is, the END message is sent) during
the serial poll if bit B1 of the AUXRB is set.
© National Instruments Corporation 5-9 GPIB-1014P User Manual