Measurement Computing Personal488 rev.3.0 - Manual (Page 142)

C-4 Troubleshooting 03/16/01 Personal488 for Windows 95/98/Me/NT/2000
“My instrument seems unaffected by the commands I send to it.”
If you have already made certain that you are sending the commands to the right instrument
address, you may have left off a crucial piece of information that instructs the instrument to
process the commands.
IEEE systems usually use data delimiters called terminators. A Talker will inform a Listener that
the data string has come to an end by appending a predefined terminator to the end of its data
string. Although terminators are issued solely by the talking device, the listening device(s) must
know what terminator to expect. Usually IEEE instruments will issue a carriage return (CR) and a
line feed (LF) as their terminator. Some instruments will not process the incoming command
string until they detect the proper terminator. You should step through the transactions captured
by the Analyzer488 to verify the transmission of the terminator, then make certain that it agrees
with the terminator expected by your instrument.
Some instruments have a DDC which instructs the instrument to process all of the previously
received commands. This EXECUTE command (typically a character like ‘X’) allows a
programmer to send several commands to an instrument in any order over any length of time and
then execute them all simultaneously within the instrument. If the EXECUTE DDC is not sent,
the state of the instrument will not change. It will react as if the commands were never received.
“When I ask for data, nothing is returned.”
This could be an address or terminator problem like the ones discussed above. See the previous
sections to diagnose these problems.
Not all instruments are ready to supply data whenever you ask. Some instruments have nothing to
say until they are commanded to acquire or generate data. Some data acquisition instruments have
triggering features which allow the instrument to collect and transmit data only after a specified
event has occurred. A typical multimeter might have a default trigger of TRIGGER ON TALK
which would enable the multimeter to take a reading every time the controller addressed it to
Talk. If the same multimeter was set to TRIGGER ON GET, no reading would be available until
the controller issued a Group Execute Trigger.
If the device has no data to give, the Analyzer488 will show that the controller has been
addressed to Listen and the device was addressed to Talk and then the process stopped. The
handshake indicators show that Not Ready For Data (NRFD) was unasserted by the controller but
the instrument never asserted Data Valid (DAV). Make certain that your device has data to
transmit before you ask it for some.
IOtech’s Driver488 has the capability of assigning a time-out value to the system. If an
instrument does not respond within the specified time-out, the process is aborted. In some
instances, an instrument may be unable to respond within the specified time-out period and the
time-out period will have to be increased.