D-4 Models 2303 and 2303B GPIB 488.1 Protocol
• When a query is sent, either the data must be read back or a Device Clear (DCL) or
Interface Clear (IFC) must be performed to reset the query.
• When sending a command or query, do not attempt to read data from the power supply
until the terminator has been sent (usually Line Feed with EOI). Otherwise, a DCL or
IFC must be sent to reset the input parser.
• When receiving data, all data, up to and including the terminator (LF with EOI), must
be accepted. Otherwise, a DCL or IFC must be sent to reset the output task.
• Empty command strings (terminator only) should not be sent.
Using SCPI-based programs
In general, an existing SCPI-based program will run properly and faster in the 488.1 proto-
col as long as it meets the previous guidelines and limitations.
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.
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 power supply 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.
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 que-
ries that take a long time to execute.
General operation notes
• The remote blinking R on the front panel still operates since it is critical to fundamental
GPIB operation.
• If the unit is in REMote, the GTL command may not put the power supply 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 feature
is useful when reading back long ASCII strings.
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