National Instruments GPIB-BUF - Manual (Page 24)

Technical Information Chapte r 3
GPIB-BUF User Manual 3-2 © National Instruments Corp.
If the SRQ-on-empty feature is enabled, the GPIB-BUF asserts the Service
Request (SRQ*) signal on the GPIB IN port. The SRQ condition can be
used to signal the GPIB Controller that the GPIB-BUF is powered on and
ready to accept data.
Addressing
The five GPIB address switches (U38, switches 4 through 8) become the
GPIB primary address to which the GPIB IN port responds. These switches
are also used to determine the talk and listen addresses used by the
GPIB -BUF when addressing the target device attached to the GPIB OUT
port. Unless either the GPIB-BUF or the target device is operating in listen-
only mode, the target device must be set at the same GPIB address as the
GPIB-BUF.
If all the GPIB address switches are set to ON, the GPIB-BUF is configured
to operate in listen-only mode. In this mode, no addressing is required to
make the GPIB-BUF a GPIB Listener and it cannot be unaddressed to
listen. Therefore, all data transferred on the GPIB IN port is accepted by
the GPIB-BUF and output to the target device.
When operating in listen-only mode, the GPIB-BUF can neither return any
data from the target device nor respond to a serial poll request since it
cannot be addressed as a GPIB Talker. As a consequence, the SRQ feature
of the GPIB-BUF should not be enabled when operating in listen-only
mode.
When operating in listen-only mode, the GPIB-BUF sends all possible
listen addresses to the target device (via the GPIB OUT port) after it has
finished its power-on self-test. Since all listen addresses are sent out, the
target device need not be in listen-only mode. Instead, it can be at any valid
GPIB primary address. This mode can also be used to connect more than
one target device to the GPIB-BUF which allows multiple, simultaneous
copies of a document or drawing to be created.
Data Transfers
Upon being addressed to listen, the GPIB-BUF updates the front panel
address indicators by turning on the LISTEN indicator and turning off the
TALK indicator. The GPIB-BUF then begins to accept data from the GPIB
IN port using the onboard Direct Memory Access (DMA) Controller. The