Data Acquisition

Table 8.2IEEE 488.2 controller protocolsThe functions performed by each of the defined protocols are as follows:RESETInitializes the GPIB bus, clears and sets all devices to aknown state.FINDRQS The controller senses the ‘FALSE’ to ‘TRUE’ transitionof the SRQ line and then services the most criticaldevices first.ALLSPOLL Serial polls each device and returns the status byte ofeach device.PASSCTL Passes bus control to other devices.REQUESTCTL Requests bus control.FINDLSTN The controller issues this command with a particularlistener’s address and monitors the NDAC handshakeline to determine if a device exists at that address.SETADD Used in conjunction with FINDRQS sets a deviceaddress.TESTSYS The controller instructs each device to perform a selftestand report back whether it has any problems or isOK.As an example of the usefulness of protocols, consider the serial polling of one or moredevices on a GPIB system. Multiple devices can asynchronously request service from thecontroller by asserting the service request (SRQ) line. It is the responsibility of thecontroller to determine which device(s) made the request by performing a poll of theknown active devices. Serial polling of a particular device can be implemented with theIEEE 488.1 protocol.The following sequence of events would occur:• Controller sends the un-listen (UNL) command message, which disables allListeners from listening.• Set the listen address of the controller.• The controller sends the command message containing the serial poll enable(SPE) command to the device. This command directs the device to return itsserial poll status byte, by setting the IEEE 488.1 serial poll mode in thedevice. When the device is addressed to talk, it will return the serial poll statusbyte.