Technical data

Using the STATus System 27
Agilent N8201A Performance Downconverter Synthetic Instrument Module, 250 kHz to 26.5 GHz 545
4
Enable the controller to respond to service requests.
When the condition changes, the instrument sets its RQS bit and the GPIB SRQ line. The
controller is informed of the change as soon as it occurs. As a result, the time the controller
would otherwise have used to monitor the condition can be used to perform other tasks.
Your program determines how the controller responds to the SRQ.
Generating a Service Request
To use the SRQ method, you must understand how service requests are generated. Bit 6 of
the status byte register is the request service (RQS) bit. The *SRE command is used to
configure the RQS bit to report changes in instrument status. When such a change occurs,
the RQS bit is set. It is cleared when the status byte register is queried using *SRE? (with
a serial poll.) It can be queried without erasing the contents with *STB?.
When a register set causes a summary bit in the status byte to change from 0 to 1, the
instrument can initiate the service request (SRQ) process. However, the process is only
initiated if both of the following conditions are true:
• The corresponding bit of the service request enable register is also set to 1.
• The instrument does not have a service request pending. (A service request is
considered to be pending between the time the instrument’s SRQ process is initiated
and the time the controller reads the status byte register.)
The SRQ process sets the GPIB SRQ line true. It also sets the status byte’s request service
(RQS) bit to 1. Both actions are necessary to inform the controller that the instrument
requires service. Setting the SRQ line only informs the controller that some device on the
bus requires service. Setting the RQS bit allows the controller to determine which
instrument requires service.
If your program enables the controller to detect and respond to service requests, it should
instruct the controller to perform a serial poll when the GPIB SRQ line is set true. Each
device on the bus returns the contents of its status byte register in response to this poll.
The device whose RQS bit is set to 1 is the device that requested service.
When you read the instrument’s status byte register with a serial poll, the RQS bit is reset to
0. Other bits in the register are not affected.
If the status register is configured to SRQ on end-of-measurement and the measurement is
in continuous mode, then restarting a measurement (INIT command) can cause the
measuring bit to pulse low. This causes an SRQ when you have not actually reached the
"end-of-measurement" condition. To avoid this:
1 Set INITiate:CONTinuous off.
2 Set/enable the status registers.
3 Restart the measurement (send INIT).