National Instruments NI-488.2 - Manual (Page 88)

Chapter 8 NI-488.2 Programming Techniques
© National Instruments Corporation 8-13 NI-488.2 User Manual
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 open device on the bus to determine which device requested
service. Any device requesting service returns an 8-bit 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 the
remaining seven bits to communicate the reason for the service request or
to summarize the state of the device.
Service Requests from IEEE 488.2 Devices
The IEEE 488.2 standard refined the bit assignments in the status byte.
In addition to setting bit 6 when requesting service, IEEE 488.2 devices
also use two other bits to specify their status. Bit 4, the Message Available
bit (MAV), is set when the device is ready to send previously queried data.
Bit 5, the Event Status bit (ESB), is set if one or more of the enabled
IEEE 488.2 events occurs. These events include power-on, user request,
command error, execution error, device dependent error, query error,
request control, and operation complete. The device can assert SRQ when
ESB or MAV are set, or when a manufacturer-defined condition occurs.
Automatic Serial Polling
If you want your application to conduct a serial poll automatically when
the SRQ line is asserted, you can enable automatic serial polling.
The autopolling procedure occurs as follows:
1. To enable autopolling, use the board-level configuration function,
ibconfig , with option IbcAUTOPOLL , or the GPIB Configuration
utility. (Autopolling is enabled by default.)
2. When the SRQ line is asserted, the driver automatically serial polls the
open devices.
3. Each positive serial poll response (bit 6 or hex 40 is set) is stored in a
queue associated with the device that requested service. The RQS bit
of the device status word,
Ibsta , is set.