Spoll issue with Prologix and Boonton 4242

[jclederman]

I ran into the following unusual behavior and want to document it for the future. It's not clear to me exactly what's breaking, and I am not confident in my workaround.

I have a Prologix GPIB controller interfacing over GPIB with a Boonton 4242 RF Power Meter. When a query is sent to the Boonton (e.g. "*IDN?"), the message is received successfully and a response is prepared. The expected order of events following should be:

• The "++spoll " command is repeatedly sent to the Prologix box, returning the status byte of the device
• When the MAV bit of the status byte is set, a response is available
• The response is read from the device

Instead, the spoll command never shows the MAV bit as set. The query commands times out.

When the status byte is read directly, however, with "*STB?", the MAV byte is set, contradicting the spoll response. If I bypass the checking of the MAV bit via spoll, the response returns normally. As best I can tell, something is not working properly with the Prologix spoll command.

My workaround, shortly to be available in the "joshDev" branch, will be to add an argument either allowing the MAV bit checking to be disabled or switching to the "*STB?" command instead of spoll for MAV bit checking. I worry at the side effects of this, however.