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Chapter 4 Remote Interface Reference
The SCPI Status System
Using Service Request (SRQ) and Serial Poll
You must configure your computer to respond to the IEEE-488 service
request (SRQ) interrupt to use this capability. Use the Status Byte
enable register (*SRE command) to select which condition bits will assert
the IEEE-488 SRQ line. If bit 6 (RQS) transitions from a "0" to a "1",
then an IEEE-488 service request message is sent to your computer.
The computer may then poll the instruments on the interface bus to
identify which one is asserting the service request line (that is, the
instrument with bit 6 set in its Serial Poll response).
When a Serial Poll is issued, bit 6 (RQS) is cleared in the Serial Poll
response (no other bits are affected) and the service request line is
cleared. The "Master Summary Bit" in the *STB? response is not cleared.
To obtain the Serial Poll response, send an IEEE-488 Serial Poll message.
The instrument sends a one-byte binary response. Performing a Serial
Poll is handled automatically by the IEEE-488 bus interface hardware.
Unlike ASCII commands, and some other GPIB commands, a Serial
Poll is executed immediately and does not involve the instrument's main
processor. Therefore, the status indicated by a Serial Poll may not
necessarily indicate the effect of the most recent command. Use the
*OPC? command to guarantee that commands previously sent to the
instrument have completed before executing a Serial Poll.
Using *STB? to Read the Status Byte
The *STB? command is similar to a Serial Poll but it is processed like
any other ASCII instrument command. The *STB? command returns the
same result as a Serial Poll but bit 6 is not cleared as long as the enabled
conditions remain.
The *STB? command is not handled automatically by the IEEE-488 bus
interface hardware and will be executed only after previous commands
have completed. You cannot clear an SRQ using the *STB? command.
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