Status Structure 13-9
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 your test program, you can periodically read the status byte 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 Model 6514. If an SRQ does not
occur, bit B6 (RQS) of the status byte register will remain cleared, and the program will simply
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 summary bits
are reset.
SPE, SPD (serial polling)
The SPE, SPD general bus command is used to serial poll Model 6514. 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 regis-
ter are listed in Table 13-3. For details on programming and reading registers, see “Programming
enable registers” and “Reading registers”.
NOTE To reset the bits of the service request enable register to 0, use 0 as the parameter
value for the *SRE command (i.e. *SRE 0).
Table 13-3
Common commands — status byte and service request enable registers
Command Description Default
*STB? Read status byte register.
*SRE <NDN> or <NRf> Program the service request enable register:
<NDN> = #Bxx…x Binary format (each x = 1 or 0)
= #Hx Hexadecimal format (x = 0 to FF)
= #Qx Octal format (x = 0 to 377)
<NRf> = 0 to 255 Decimal format
(Note)
*SRE? Read the service request enable register
Note: *CLS and STATus:PRESet have no effect on the service request enable register.
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