Keithley 6487 - NRFD Hold-Off; NDAC Hold-Off

E-6 IEEE-488 and SCPI Conformance Information Model 6487 Reference Manual
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
immediately 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 hold-off is included with the GPIB 488.1 protocol mode to allow a single instru-
ment to hold off all others on the bus until it is finished executing a command. The follow-
ing command controls 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 E-1 for the complete IEEE-488 handshake
sequence.
Figure E-1
IEEE-488 handshake sequence
D ATA
DAV
SOURCE
SOURCE
VALID
ALL READY
ACCEPTOR
NRFD
NDAC
ALL ACCEPTED
ACCEPTOR