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G-4 GPIB 488.1 Protocol Model 2520 User’s Manual
Bus 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 imme-
diate release of bus hold-off is done to support GET, SDC, IFC, *TRG, *RCL, *RST,
SYSTem:PRESet and ABORt during data acquisition.
Trigger-on-talk
Trigger-on-talk functionality has been added for the 488.1 protocol. If a query has not
been received by the instrument, the Model 2520 will automatically assume a READ?
command has been sent when it is addressed to talk. This technique increases GPIB speed
by decreasing the transmission and parser times for the command.
Trigger-on-talk is extremely useful in the single-shot reading mode (*RST default) and is
the main reason for a >2x speed improvement over the SCPI protocol. Remember that the
outputs must be on (:OUTput[1]:STATe ON) before you can take readings.
The TRIG:SOUR BUS and TRIG:COUN INF commands are not supported by READ?
with the 488.1 protocol selected. If you send one of these commands, a DCL or IFC may
be required to reset the GPIB.
Message available
The MAV (message available) bit in the Serial Poll byte will be set when the query is fin-
ished being processed, not when there is data available in the output buffer (as with the
SCPI protocol). For the 488.1 protocol, output data will not be formatted until the first
request for data is received. This delay may cause unexpected time-outs when using SRQ
on MAV for queries that take a long time to execute.
General operation notes
• The TALK, LSTN, and SRQ annunciators are not functional in the 488.1 protocol.
This speeds up data throughput greatly. The REM annunciator still operates since it
is critical to fundamental GPIB operation.
• If the unit is in REMote, the GTL command may not put the Model 2520 into the
local mode. Only the front panel LOCAL key is guaranteed to operate, if not in
local lockout (LLO). GTL will still disable LLO.
• IEEE-488 bus commands and features (GET, IFC, SDC, DCL, LLO, Serial Poll,
and SRQ) are still fully supported.
• Multiple TALKs on the same query are supported as in the SCPI protocol. This fea-
ture is useful when reading back long ASCII strings.