Multiple Messages; Instrument Response To Ieee-488 Interface Messages - Tektronix PFG 5505 Instruction Manual

Programming

Multiple Messages

T he Input Buffer has finite capacity and thus a single
message may be long enough to fill it. In this case,
a portion of the message is processed before the
instrument accepts additional input. During command
processing, the instrument holds off additional data
(by asserting N RFD) until space is available in the
buffer. When space is available, the instrument can
accept a second message before the first has been
processed. However, it holds off additional messages
with N RFD until it completes processing the first.
After the instrument executes a query-output com­
mand in a message, it holds the response in its
Output Buffer until the controller makes the instrument
a talker. If the instrument receives a new message
before all of the output from the previous message
is read, it clears the Output Buffer before executing
the new message. T his prevents the controller from
getting unwanted data from old messages.
One other situation may cause the instrument to
delete output. T he execution of a long message might
cause both the Input and Output Buffers to become
full. When this occurs, the instrument cannot finish
executing the message because it is waiting for the
controller to read the data it has generated; but the
controller cannot read the data because it is waiting
to finish sending its message. Because the instrument
Input Buffer is full and it is holding off the rest of the
controller's message with N RFD, the system is hung
up with the controller and instrument waiting for each
other. When the instrument detects this condition, it
generates an error, asserts SRQ and deletes the data
in the Output Buffer. T his action allows the controller
to transmit the rest of the message, and informs the
controller that the message was executed and that
the output was deleted.
A T M 5000 instrument can be made a talker without
having received a message that specifies the output.
In this case, an acquisition instrument (a counter or
a multimeter) returns a measurement if one is ready.
If no measurement is ready, it returns a single byte
message with all bits equal to 1 (with message
terminator). Non-acquisition T M 5000 instruments will
return only this message.
3-28
Instrument Response to IEEE-488
Interface Messages
Interface messages and the effects of those messages
on the instrument interface functions are defined in
IEEE Standard 488.1-1987. Abbreviations from the
standard are used in this discussion, which describes
the effects of interface messages on instrument opera­
tion. Where appropriate, the GPIB code is listed, in
decimal.
UNL - Unlisten (63 w ith ATN))
UNT - Untalk (95 w ith ATN)
When the UNL command is received, the instrument
listener function goes to its idle state (unaddressed).
In the idle state, the instrument will not accept
instrument commands from the I EEE-488 bus.
T he talker function goes to its idle state when the
instrument receives the UNT command. In this state,
the instrument cannot supply output data via the bus.
T he addressed indicator is off when both the talker
and listener functions are idle. If the instrument is
either talk-addressed or listen-addressed, the indicator
is on.
IFC - lnterface Clear (Bus pin 9)
T his uniline message has the same effect as both
the UNT and UNL messages. T he front panel AD­
DRESSED indicator is off.
DCL - Device Clear (20 w ith ATN)
T he Device Clear message reinitializes communication
between the instrument and controller. In response
to DCL, the instrument clears any input and output
messages and any unexecuted settings in the Pending
Settings Buffer. Also cleared are any errors or events
waiting to be reported, except the power-on event.
If the SRQ line is asserted for any reason other than
power-on when D CL is received, SRQ is unasserted.
SOC - Selected Device Clear (4 with ATN)
T his message performs the same function as DCL;
however, only instruments that are listen-addressed
respond to SDC.
GET -Group Execute Trigger (8 with ATN)
T he instrument responds to < GET > only if it is
listen-addressed and the instrument device trigger
function has been enabled by the Device Trigger
· command (DT ) . T he < GET > message is ignored
and an SRQ generated if the OT function is disabled
( DT OFF), the instrument is in the local state, or if
a message is being processed when < GET > is
received.
PF G 5105/5505 Instruction Manual