G-4 GPIB 488.1 Protocol
NRFD hold-off
*OPC, *OPC?, and *WAI are still functional but are not needed for the 488.1 protocol.
When sending commands, the GPIB is automatically held off when it detects a terminator. The
hold-off is released when all the commands have finished executing, or if there is some parser
or command error. An exception is an initiate command, which releases the hold-off immedi-
ately and does not wait for all of the readings to be acquired. This immediate release of bus
hold-off is done to support GET, SDC, IFC, *TRG, *RCL, *RST, SYSTem:PRESet and
ABORt during data acquisition.
NDAC hold-off
NDAC is included with the GPIB 488.1 protocol mode to allow a single instrument to hold
off all others on the bus until it is finished executing a command. The following command con-
trols NDAC hold-off:
SYSTem:MEP:HOLDoff ON | OFF
The default is OFF, but NRFD hold-off will still be enabled and will prevent an instrument
from accepting further commands. See Figure G-1 for the complete IEEE-488 handshake
sequence.
DATA
DAV
SOURCE
SOURCE
VALID
ALL READY
ACCEPTOR
NRFD
NDAC
ALL ACCEPTED
ACCEPTOR
gure
-
EEE-488 handshake
sequence
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