National Instruments NI-488.2 Serial Polling

Chapter 7 GPIB Programming Techniques

Some applications need to know the order in which certain messages are sent by the

Controller. To monitor the ordering of these messages, your application program must

enable the EVENT bit, using ibconfig (option IbcEventQueue ). When the

EVENT bit is enabled, the DCAS and DTAS bits are no longer activated. Instead, all

DCAS and DTAS messages are stored in a queue, in the order that they are received.

The event queue also stores interface clear (IFC) messages. When the queue contains

some information, the NI-488.2 software sets the EVENT bit in ibsta. When the

application program detects EVENT, it can call the function ibevent to retrieve the

first event that occurred. Retrieving events from the queue ensures that the application

can respond to device clear, device trigger, and interface clear messages in the correct

order.

Another type of event that might be important in a Talker/Listener application is the

serial poll. A Talker/Listener application can call ibrsv with a serial poll response byte

when it needs to request service from the Controller. If the application needs to know

when the Controller has read the serial poll response byte, it can enable the SPOLL bit in

ibsta using ibconfig , option IbcSPollBit. The NI-488.2 software sets the

SPOLL bit when the Controller serial polls the board.

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.

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 open device on the

bus 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.