matlabvisagpib

Matlab GPIB - how to read Message AVailable from Status Byte Register?

as the title says...

I want to check the SBR register of device connected over GPIB. I am interested in reading the MAV bit 4, which should be set if instrument has something it would like to send me.

The problem is, that in order to check the SBR, I inevitably have to send another query (*STB?), which clears the device output buffer by default. In other words, whenever I check if there is something to read, I remove it just by looking. Here is code to reproduce it, problem is at the last call to fscanf():

>> fid = gpib('agilent', 7, 26);
>> fopen(fid)
>> fprintf(fid, '*SRE 255; *SRE?') % Enable everything in SBR
>> fscanf(fid) % Returns +191 as expected (255 - 64 for MSS)

>> fprintf(fid, '*IDN?') % Make any query...
>> fprintf(fid, '*STB?') % Read SBR
   % The line above generates device Query Error (beep)

>> fscanf(fid) % << Returns +0 !!!

>> fclose(fid) % Just to prevent flames :]

I suspect, that there is some way to check the SBR without querying the device, but I could not find anything. MATLAB help for VISA drivers is silent on the topic of communicating directly with the driver or the bus.

I also tried to check for BusManagementStatus with no avail.

For reference, I am using MATLAB R2011b, with 32-bit Agilent VISA drivers, and the GPIB enabled device is Agilent E4980A LCR Meter. Thanks for any help.

Solution

  • OK, I think that I figured it out. Please correct me if I am wrong...

    First of all I need to anticipate the need to check for available message, because in my solution I won't be able to check for Error Queue, Master Summary, and other bits, that are set in SBR.

    Then, before my actual commands (that may produce some output), I need to mask the Service Request Enable Register (SRER) to only allow MAV bit. Thats done like so (following from example in question):

    >> fprintf(fid, '*SRE 16');
>> fprintf(fid, '...ACTUAL COMMANDS THAT ARE TO BE EXECUTED...');

    Now I can check, whether the device sends a service request, using the aforementioned BusManagementStatus command. The following command returns true iff there is MAV bit set in SBR.

    >> strcmp(fid.BusManagementStatus.ServiceRequest, 'on')

    The disadvantage is that there is no way to check for errors during my ACTUAL COMMANDS execution. If I do so, it might produce errors... :]