Appendix A GPIB Basics© National Instruments Corporation A-3 NI-488.2 User Manual for WindowsHandshake LinesThree hardware handshake lines asynchronously control the transfer ofmessage bytes between devices. This process is a three-wire interlockedhandshake, and it guarantees that devices send and receive message byteson the data lines without transmission error. Table A-1 summarizes theGPIB handshake lines.Interface Management LinesFive hardware lines manage the flow of information across the bus.Table A-2 summarizes the GPIB interface management lines.Table A-1. GPIB Handshake LinesLine DescriptionNRFD(not ready for data)Listening device is ready/not ready to receive amessage byte. Also used by the Talker to signalhigh-speed GPIB transfers.NDAC(not data accepted)Listening device has/has not accepted a messagebyte.DAV(data valid)Talking device indicates signals on data lines arestable (valid) data.Table A-2. GPIB Interface Management LinesLine DescriptionATN(attention)Controller drives ATN true when it sendscommands and false when it sends data messages.IFC(interface clear)System Controller drives the IFC line to initializethe bus and make itself CIC.REN(remote enable)System Controller drives the REN line to placedevices in remote or local program mode.SRQ(service request)Any device can drive the SRQ line toasynchronously request service from the Controller.EOI(end or identify)Talker uses the EOI line to mark the end of a datamessage. Controller uses the EOI line when itconducts a parallel poll.
