Anritsu MT8801C - Reading and Clearing the STB Register

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5.2 Status Byte (STB) Register
5.2.3 Reading and clearing the STB register
Serial poll or the *STB? common query are used to read the contents of the STB
register. STB messages conforming to IEEE488.1 can be read by either method, but
the value sent to bit 6 (position) is different for each message.
The STB register can be cleared by using the *CLS command.
(1) Reading by serial poll (only when using the GPIB interface)
When using serial poll conforming to IEEE488.1, the device must return a 7-bit
status byte and an RQS message bit which conforms to IEEE488.1. According
to IEEE488.1, the RQS message indicates whether the device sent SRQ as true
or not. The value of the status byte is not changed by serial poll. The device
must set the RQS message to false immediately after being polled. As a result, if
the device is again polled before there is a new cause for a service request, the
RQS message is false.
(2) Reading by the *STB common query
The *STB? common query requires the device to send the contents of the STB
register and an integer format response message from the MSS (Master Sum-
mary Status) summary message. The response represents the total binary
weighted value of the STB register and the MSS summary message. STB regis-
ter bits 0 to 5 and 7 are weighted to 1, 2, 4, 8, 16, 32, and 128; and the MSS to 64,
respectively. Thus, excepting the fact that bit 6 represents the MSS summary
message instead of the RQS message, the response to *STB? is identical to that
for serial poll.