Technical Information
Chapter 3
GPIB-PRL User Manual
3-2
© National Instruments Corp.
GPIB system performance may become unacceptable if other GPIB devices
are required to transfer data on the bus.
To alleviate this problem, the GPIB-PRL has been designed with a special
SRQ-ON-EMPTY feature which can be enabled by setting the internal
configuration switches. If maximum GPIB performance is required at all
times this feature can be enabled and the GPIB-PRL will request service
from the GPIB Controller when its buffer is empty (indicating it is ready for
more data). The Controller can then determine the status of the GPIB-PRL
buffer by performing a serial poll and analyzing the response byte. If the
Controller observes that the GPIB-PRL buffer is empty, it can then send
data (up to the buffer size) to the GPIB-PRL and send the Unlisten
command to the GPIB-PRL so the GPIB can be used by other devices.
If the GPIB-PRL is serial polled, its response byte will depend on the status
of the internal data buffer. If the buffer is empty, a 41 hex will be returned
as the status byte. If the buffer is not empty, a 0 will be returned.
The GPIB-PRL will accept data at the GPIB address specified by the
configuration switches. The GPIB-PRL also has the ability to be
configured as a listen-only device if all GPIB address switches are set to
ON. In this mode, no addressing is required to make the GPIB-PRL a GPIB
listener and it cannot be unaddressed to listen. All data transferred on the
GPIB will be accepted by the GPIB-PRL and output to the parallel port
(even if the data was intended for another device). This scheme could be
used to monitor all GPIB data bus activity.
Note:
When the GPIB-PRL is configured as a listen-only device, it may
not be serial polled.
If the GPIB-PRL receives the universal Device CLear (DCL) command or
its listen address and the Selected Device Clear (SDC) command, the
GPIB-PRL will clear its internal data buffer and reset to its power-up state.
The GPIB-PRL will also pulse the parallel port signal INIT* so that the
parallel device will be initialized.
In G mode, the GPIB-PRL may act only as a GPIB Listener. The
GPIB-PRL should not be passed control. If it is, the GPIB circuitry in the
GPIB-PRL will accept control and immediately assert ATN*. This is an
error condition which may lock up your system and, therefore, should be
avoided. Neither Trigger nor Go To Local has any effect on the GPIB-
PRL.
