Measurement Computing Personal488 rev.3.0 - Manual (Page 141)

Personal488 for Windows 95/98/Me/NT/2000 03/16/01 Troubleshooting C-3
Common Problems and Solutions
Occasionally systems will encounter problems due to the interaction of several devices in the
system. These are among the most difficult problems to debug. You should connect an
Analyzer488 and let it run while the application is processing. Recording the bus transactions as
they occur and inspecting the transactions one at a time will usually allow you to diagnose these
types of problems rather quickly.
Often the problems encountered in a system are due to interactions between one device and the
controller. Here is a list of common symptoms and their suggested solutions:
“I get a time-out error whenever I try to send device-dependent commands to my instrument.”
The first thing you should check is the setting of IEEE addresses. Every device on the bus must
have a unique address between 0 and 30. When sending Device-Dependent Commands (DDCs)
to an instrument to change its state or operating mode, the device will first be addressed to listen,
then the data will be sent. If the device has TALK and LISTEN indicators on its front panel, you
can tell immediately if the address used by the controller matches the actual address of the
instrument. If the LISTEN indicator does not come on when sending commands to the device,
you are probably using the wrong address for that device.
As we mentioned in the tutorial section of this note, when the ATN line is asserted by the
controller all of the instruments on the bus will handshake with, and accept data from, the
controller. After the time-out is received, step through the transactions recorded by the
Analyzer488. If no instrument addressing commands such as Listen Address Group 16 (LAG16)
were recorded, your instrument is probably off or broken, or the cable is disconnected. Regardless
of the present state of the instrument, it should handshake (accept data) when the ATN line is
asserted. If the addressing commands were successfully recorded on the analyzer, step through
the transactions until the ATN line is unasserted. If there are no more recorded transactions, then
no instrument was placed in the Listen mode. The controller had no one to handshake with so it
“timed-out.” Your instrument is probably set to the wrong address.
“At certain points in my program, the system stops and I receive a time-out error.”
If portions of your program are operating correctly, then you can be certain that your addresses
are set correctly. If you encounter a time-out error in your program after other instrument tasks
have been completed successfully, you may have encountered an instrument-readiness problem.
IEEE interfaces and software like IOtech’s Personal488 operate very rapidly and can sometimes
out-run the instrument they are controlling. For most instruments, data requests are performed in
two steps: sending the necessary setup or inquiry commands via DDCs, then addressing the
device to Talk. It is possible to issue the necessary commands to request the data from the
instrument and then address it to Talk long before it is prepared to supply the requested data.
Many instruments will simply pause the bus until they have prepared the data to send. However,
other instruments react poorly by “hanging up.”
To check for this “outracing” condition, place the Analyzer488 into the Slow Handshake mode.
This will effectively slow the transaction speed of the bus to a rate set by the Analyzer488. If the
data request takes place successfully, it is probably an “outracing” condition.