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Chapter 6
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If your setup includes more than one Controller or if your GPIB interface
board is not configured as the System Controller, use the CIC protocol
method. To use the protocol, issue the ibconfig function or use the
ibconf configuration utility to activate the CIC protocol. If the interface
board is not CIC and you make a device-level call with the CIC protocol
enabled, the following sequence occurs:
1.
The GPIB interface board asserts the SRQ line.
2.
The current CIC serial polls the board.
3.
The interface board returns a response byte of hex 42.
4.
The current CIC passes control to the GPIB board.
If the current CIC does not pass control, the NI-488.2M driver returns the
ECIC error code to your application. This error can occur if the current CIC
does not understand the CIC protocol. If this happens, you could send a
device-specific command requesting control for the GPIB board, then you
could use a board-level ibwait command to wait for CIC.
Serial Polling
You can use serial polling to obtain specific information from GPIB devices
when they request service. When the GPIB SRQ line is asserted, it signals the
Controller that a service request is pending. The Controller must then determine
which device asserted the SRQ line and respond accordingly. The most common
method for SRQ detection and servicing is the serial poll. This section describes
how you can set up your application to detect and respond to service requests
from GPIB devices.
Service Requests from IEEE 488 Devices
IEEE 488 devices request service from the GPIB Controller by asserting the
GPIB SRQ line. When the Controller acknowledges the SRQ, it serial polls
each device on the bus in order to determine which device requested service.
Any device requesting service returns a status byte with bit 6 set and then
unasserts the SRQ line. Devices not requesting service return a status byte with
bit 6 cleared. Manufacturers of IEEE 488 devices use lower order bits to
communicate the reason for the service request or to summarize the state of the
device.
© National Instruments Corp.
GPIB Programming Techniques
6-3
NI-488.2M SRM for OS/2
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