Check your set-up!!! The most common cause for this type of behavior is that your configuration changed to the point where you've violated a bus phyisical requirement.
Quoting fro NI-488.2 help file----
"Configuration Requirements
"To achieve the high data transfer rate that the GPIB was designed
for, you must limit the number of devices on the bus and the physical distance
between devices. The following restrictions are typical:
- A maximum separation of 4 m between any two devices and an average
separation of 2 m over the entire bus.
- A maximum total cable length of 20 m.
- A maximum of 15 devices connected to each bus, with at least two-thirds
powered on.
For high-speed operation, the following restrictions apply:
- All devices in the system must be powered on.
- Cable lengths must be as short as possible with up to a maximum of 15 m of
cable for each system.
- There must be at least one equivalent device load per meter of cable.
If you want to exceed these limitations, you can use a bus
extender to increase the cable length or a bus expander to increase the number
of device loads. You can order bus extenders and expanders from National
Instruments."
"Should be" isn't "Is" -Jay
