Calibrator
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Service Request (SRQ) Line
IEEE-488 Service Request (SRQ) is an IEEE-488.1 (GPIB) bus control line that the Calibrator asserts
to notify the controller that it requires some type of service. Many instruments can be on the bus, but
they all share a single SRQ line. To determine which instrument set SRQ, the Controller normally does
a serial poll of each instrument. The Calibrator asserts SRQ whenever the RQS bit in its Status Byte
Register is 1. This bit informs the controller that the Calibrator was the source of the SRQ.
RS-232/Ethernet Remote operations that use the RS-232 or Ethernet interface emulate the IEEE-488
SRQ line by sending the SRQSTR string over the serial interface when the RQS/MSS bit is set in the
Status Byte Register. See the SRQSTR? command description in Remote Commands.
The Calibrator clears SRQ and RQS whenever the controller/host performs a serial poll, sends *CLS,
or whenever the MSS bit is cleared. The MSS bit is cleared only when ESB, MAV, EAV, and ISCB are
0, or they are disabled by their associated enable bits in the SRE register being set to 0.
USBTMC The Calibrator uses the interrupt-in endpoint to request service (by sending a SRQ message
to the controller) and to communicate the status byte register value. The SRQ condition communicates
automatically whenever the RQS bit is set in the Status Byte Register.
Table 17. Status Byte Register (STB) and Service Request Enable (SRE)
76543210
0
RQS
ESB MAV EAV ISCB 0 0
MSS
RQS Requesting service. The RQS bit is set to 1 whenever bits ESB, MAV, EAV, or ISCB
change from 0 to 1 and are enabled (1) in the SRE. When RQS is 1, the Calibrator asserts
the SRQ control line on the IEEE-488 interface or sends a SRQ request message when it
uses USBTMC. You can do a serial poll and read this bit to see if the Calibrator is the
source of an SRQ.
MSS Master summary status. Set to 1 whenever bits ESB, MAV, EAV, or ISCB are 1 and
enabled (1) in the SRE. This bit can be read with the *STB? command in RS-232/
Ethernet remote control in place of doing a serial poll.
ESB Set to 1 when one or more enabled ESR bits are 1.
MAV Message available. The MAV bit is set to 1 whenever data is available in the IEEE-488
interface output buffer of the Calibrator.
EAV Error available. An error has occurred and an error is available to be read from the error
queue.Use the ERR? query.
ISCB One or more enable ISCR bits are 1.
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