Agilent Technologies 34401A manual How to Use the Message Available Bit MAV

Chapter 4 Remote Interface Reference

The SCPI Status Model

How to Use the Message Available Bit (MAV)

You can use the status byte “message available” bit (bit 4) to determine when data becomes available to read into your bus controller. The multimeter sets bit 4 when the first reading trigger occurs (which can be TRIGger:SOURce:IMMediate). The multimeter subsequently clears bit 4 only after all messages have been read from the output buffer.

The message available (MAV) bit can only indicate when the first reading is available following a READ? command. This can be helpful if you do not know when a trigger event such as BUS or EXTernal will occur.

The MAV bit is set only after all specified measurements have completed
when using the INITiate command followed by FETCh?. Readings are
placed in the multimeter’s internal memory when using INITiate.
Sending the FETCh? command transfers readings (stored in internal
memory by the INITiate command) to the multimeter’s output buffer.
Therefore, the MAV bit can only be set after all measurements have	4
been completed.

Using *OPC to Signal When Data is in the Output Buffer

Generally, it is best to use the “operation complete” bit (bit 0) in the standard event register to signal when a command sequence is completed. This bit is set in the register after an *OPC command has been executed. If you send *OPC after a command which loads a message in the multimeter’s output buffer (either reading data or query data), you can use the operation complete bit to determine when the message is available. However, if too many messages are generated before the *OPC command executes (sequentially), the output buffer will fill and the multimeter will stop taking readings.

139