Measurement Computing Digital HS - Manual (Page 34)

3.1.2.1 NDAC Holdoff Selection
The Digital488HS/32 stops any additional data transfers to the output port while the
oBusy handshake line is active, indicating the connected external device has not
accepted the previous data provided by the oDAV transition. It does this by not
releasing the NRFD IEEE 488 bus handshake line, thus indicating its “Not Ready for
Data” condition.
Even though the Digital488HS/32 is not ready, the IEEE 488 bus controller can
continue to process other bus commands and control other instruments. Occasionally,
such as when a trigger command is to be issued, it is desirable to prohibit the bus
controller from processing other bus commands until the output data have been
accepted by the external device.
In this example, the bus controller may be able to output data to the Digital488HS/32
and issue the trigger faster than the external device can accept the output port data. If
trigger must be synchronized with the data transfer, it would be necessary to serial
poll or parallel poll the Digital488HS/32 to determine if the data have been
transferred.
Aternatively, the Digital488HS/32 can stop the IEEE 488 bus controller until the data
have been accepted by the external device. It does so by leaving NDAC asserted until
the external device has accepted the output data. Since every data byte on the IEEE
488 bus must be accepted (for example, with an NDAC transition) the bus is not
allowed to continue until the Digital488HS/32 releases NDAC.
3.1.3 oClear Control Line
When the Digital488HS/32 detects a Device Clear (DCL) or a Selected Device Clear
(SDC) IEEE 488 bus command, it generates a 1µsec pulse on the oClear control line.
3.1.4 oTrigger Control Line
When the Digital488HS/32 detects the Group Execute Trigger (GET) IEEE 488 bus
command, it generates a 1µsec pulse on the oTrigger control line.