Chapter 8 NI-488.2 Programming Techniques© National Instruments Corporation 8-13 NI-488.2 User ManualService Requests from IEEE 488 DevicesIEEE 488 devices request service from the GPIB Controller by assertingthe GPIB SRQ line. When the Controller acknowledges the SRQ, it serialpolls each open device on the bus to determine which device requestedservice. Any device requesting service returns an 8-bit status byte with bit6 set and then unasserts the SRQ line. Devices not requesting service returna status byte with bit 6 cleared. Manufacturers of IEEE 488 devices use theremaining seven bits to communicate the reason for the service request orto summarize the state of the device.Service Requests from IEEE 488.2 DevicesThe IEEE 488.2 standard refined the bit assignments in the status byte.In addition to setting bit 6 when requesting service, IEEE 488.2 devicesalso use two other bits to specify their status. Bit 4, the Message Availablebit (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 enabledIEEE 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 whenESB or MAV are set, or when a manufacturer-defined condition occurs.Automatic Serial PollingIf you want your application to conduct a serial poll automatically whenthe 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 Configurationutility. (Autopolling is enabled by default.)2. When the SRQ line is asserted, the driver automatically serial polls theopen devices.3. Each positive serial poll response (bit 6 or hex 40 is set) is stored in aqueue associated with the device that requested service. The RQS bitof the device status word, ibsta, is set.
