Chapter 5
Programming Considerations
© National Instruments Corporation
5-23
GPIB-1014 User Manual
Serial Polls
Conducting a Serial Poll
The TLC, as CIC, can serial poll other devices as described in the IEEE 488 specification. From
the programming point of view, the TLC must first become Active Controller to send the
addressing and enabling commands to the device being polled, make itself a GPIB Listener by
issuing the Listen auxiliary command, and then go to standby with the Go To Standby auxiliary
command to read the status byte.
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 before asserting 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 STB any number of times. RQS, rsv, and PEND are 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.
Parallel Polls
Parallel Polls are used by the GPIB Active Controller to check the status of several devices
simultaneously. The meaning of the status returned by the devices being polled is
device-dependent. Two general ways in which Parallel Polls are useful are as follows:
•
When the GPIB Controller sees SRQ asserted in a system with several devices, it can quickly
determine which one needs to be serial polled, usually using only one Parallel Poll.
•
In systems in which the Controller response time requirement to service a device is low and
the number of devices is small, Parallel Polls can replace Serial Polls entirely, provided that
the Controller polls frequently.