Data Acquisition

controller in charge (CIC). The IFC line is the master reset of the GPIB andwhen asserted all devices return to a known quiescent state.• REN (remote enable) – The system controller drives the REN line to putdevices into a remote state. When the REN line is asserted and a device isaddressed to listen, the device is placed into a remote programming state.• SRQ (service request) – Any device can drive the SRQ line to asynchronouslynotify the CIC that it needs service. It is the responsibility of the CIC tomonitor the SRQ line, poll the device, and determine the type of service thedevice needs. SRQ will remain asserted until the CIC polls the devicerequesting service.• EOI (end or identify) – The EOI line has two purposes. Its first use is when atalker asserts the EOI line to indicate the last byte of data in the messagestring. A listener stops reading data when EOI is asserted ‘true’. A second usefor the EOI line is to tell devices to identify their response in a parallel poll.Three handshake lines asynchronously control the transfer of message bytes betweendevices. The GPIB uses a three-wire interlocking handshake scheme that guarantees thatmessage bytes on the data lines are sent and received without error. The handshake linesand their use are discussed below.• NRFD (not ready for data) – The NRFD handshake line indicates whether adevice is ready to receive a message byte or not. When receiving commands,the line is driven by both talkers and listeners, and only by listeners whenreceiving data messages.• NDAC (no data accepted) – This line indicates whether a device has or hasnot accepted a message byte. NDAC is driven by all devices (i.e. talkers andlisteners) when receiving commands, and only by listeners when receivingdata messages.• DAV (data valid) – The DAV handshake line indicates whether signals on thedata lines are stable and therefore valid and can be accepted by devices. Thecontroller controls DAV when sending commands and the talker controlsDAV when sending data.8.6 GPIB handshakingData is transmitted asynchronously on the GPIB parallel interface one byte at a time. Thetransfer of data is coordinated by the voltage signals on the three bus-control ‘handshake’lines (DAV, NDAC and NRFD). The process is called a three-wire interlockedhandshake. Handshaking ensures that a talker will put a data byte on the bus, only whenall listeners are ready and will keep the data on the bus until it has been read by alllisteners. It also ensures that listeners will accept data only when a valid byte is availableon the bus.The talker first un-asserts DAV then monitors the NDAC and NRFD lines. The talkermust wait for the NRFD line to go high (‘false’) before any data can be put onto the bus.The NRFD line is controlled by the listeners. Only when NRFD voltage is high (‘false’)are all listeners ready to receive data. A short delay after NRFD goes high, the talkerasserts DAV ‘true’ (voltage low) to indicate valid data is available on the bus. The delayis determined by the type of drivers the talker uses on the data lines (trio-state requiresless delay than open collector). When the listeners detect the low voltage level on DAV,