Model 6220/6221 Reference Manual Status Structure 11-9
Return to Section 11 topics
Serial polling and SRQ
Any enabled event summary bit that goes from 0 to 1 will set bit B6 and generate
an SRQ (service request). In a test program, the status byte can be periodically
read to check if an SRQ has occurred and what caused it. If an SRQ occurs, the
program can, for example, branch to an appropriate subroutine that will service
the request.
Typically, SRQs are managed by the serial poll sequence of the Model 622x. If an
SRQ does not occur, bit B6 (RQS) of the status byte register will remain cleared
and the program will proceed normally after the serial poll is performed. If an SRQ
does occur, bit B6 of the status byte register will set and the program can branch
to a service subroutine when the SRQ is detected by the serial poll.
The serial poll automatically resets RQS of the status byte register. This allows
subsequent serial polls to monitor bit B6 for an SRQ occurrence generated by
other event types. After a serial poll, the same event can cause another SRQ,
even if the event register that caused the first SRQ has not been cleared.
The serial poll does not clear MSS. The MSS bit stays set until all status byte sum-
mary bits are reset.
SPE, SPD (serial polling)
The SPE, SPD general bus command is used to serial poll the Model 622x. Serial
polling obtains the serial poll byte (status byte). Typically, serial polling is used by
the controller to determine which of several instruments has requested service
with the SRQ line.
Status byte and service request commands
The commands to program and read the status byte register and service request
enable register are listed in Table 11-3. For details on programming and reading
registers, see “Programming enable registers,” on page 11-5 and “Reading regis-
ters,” on page 11-6.
To reset the bits of the service request enable register to 0, use 0 as the parame-
ter value for the *SRE command (i.e., *SRE 0).
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