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Difference Between SRQ and RQS



Hardware: GPIB

Problem:
What is the Difference Between SRQ and RQS?

Solution:
There are five hardware lines that manage flow of information across the bus. These lines are ATN (attention), IFC (interface clear), REN (remote enable), SRQ (service request), EOI (end or identify). The SRQ is the line that any instrument on the bus can drive to request service from the controller. Once the SRQ has been asserted it gets represented in the global status word, ibsta as the SRQI bit being set.

RQS is related to SRQ but they are not interchangeable. The RQS bit is a bit in the status byte of a GPIB instrument. Whereas a GPIB bus only has one SRQ line for any number of instruments, each instrument has its own individual status word with an RQS bit. If the status word for the instrument is updated so that the RQS bit is set, the instrument will then assert the SRQ line on the GPIB bus. From a controller standpoint, you can see that the SRQ line is set, but you do not know which instrument is actually requesting service. That is where a serial poll is useful: calling IBRSP on a specific device-level descriptor will return that device's status byte. If the status byte shows that the RQS bit (bit 6 if the bits are numbered 0-7) is set, the controller knows that this device was the one requesting service.

In the NI-488.2 API, you can wait for either of these two events. The SRQI event at a GPIB board level means that the SRQ line has been asserted by someone on the GPIB. The RQS bit at the device level means not only that SRQ was asserted, but that the controller polled the device and saw that its RQS bit was set and so that device is requesting service.

For more information, see the NI-488.2 User Manual.

Related Links:
Product Manuals:NI-488.2 User Manual.

Attachments:





Report Date: 09/30/2004
Last Updated: 08/04/2017
Document ID: 3DTHGIRY

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