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Difference Between SRQ and RQS
What is the Difference Between
There are five hardware lines that manage flow of information across the bus. These lines are
(end or identify). The
is the line that any instrument on the bus can drive to request service from the controller. Once the
has been asserted it gets represented in the global status word,
bit being set.
is related to
but they are not interchangeable. The
bit is a bit in the status byte of a GPIB instrument. Whereas a GPIB bus only has one
line for any number of instruments, each instrument has its own individual status word with an
bit. If the status word for the instrument is updated so that the
bit is set, the instrument will then assert the
line on the GPIB bus. From a controller standpoint, you can see that the
line is set, but you do not know which instrument is actually requesting service. That is where a serial poll is useful: calling
on a specific device-level descriptor will return that device's status byte. If the status byte shows that the
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
event at a GPIB board level means that the
line has been asserted by someone on the GPIB. The
bit at the device level means not only that
was asserted, but that the controller polled the device and saw that its
bit was set and so that device is requesting service.
For more information, see the NI-488.2 User Manual.
Product Manuals:NI-488.2 User Manual.