Measurement Computing Digital HS User Manual
Page 25
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DIS
IEEE ADDRESS
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NDAC HOLDOFF ENABLE
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IEEE ADDRESS
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16-Bit Output Mode Selected
(default)
8-Bit Output Mode Selected
Output Port Mode Selection Switch
2.3.3 NDAC Holdoff Enable/Disable
When Digital488HS/32 presents output data, it asserts the oDAV line to inform the
external device that data are available. The external device then responds by asserting
the oBusy signal to indicate that it has accepted the data. At this point, the
Digital488HS/32 unasserts oDAV and waits for the external device to unassert oBusy
in preparation for new output.
The Digital488HS/32 uses the IEEE 488 bus handshake lines to make sure data are
not sent to it faster than the external device can accept the data. Normally, the
Digital488HS/32 uses the Not Ready For Data (NRFD) data lines for this purpose.
This allows the IEEE 488 bus to continue with other activity, even though the
external device has not yet accepted the data.
When NDAC holdoff is enabled, the Digital488HS/32 uses the IEEE 488 NDAC line
to stop the IEEE 488 bus until the external device has accepted the data (as indicated
by its assertion of the oBusy line). In this way, the controller is guaranteed the data it
has sent to the Digital488HS/32 has been accepted by the external device.
When the oBusy handshake line is active, the Digital488HS/32 stops any additional
data transfers to the output port. However, the IEEE 488 bus controller can continue
to process other bus commands and control other instruments. In some cases, this
may not be desired. The switch labeled NDAC HOLDOFF ENABLE/DIS allows the