Data Acquisition

Figure 8.5The GPIB bus structureThe 16 signal lines consist of data lines (D101–D108) and 8 control lines. Three of theeight control lines are the handshaking lines that coordinate the transfer of data (DAV,NRFD and NDAC), while the remaining five lines are used for bus control andmanagement (ATN, REN, IFC, SRQ and EOI). The 8 ‘ground’ lines provide electronicshielding and prevent bus control signals from interfering with one another or from beinginfluenced by external signals. A summary of the signal lines on the GPIB is as follows:• Data bus lines D101 – D108• Handshaking lines DAV – Data availableNRFD – Not ready for dataNDAC – No data accepted• General interface ATN – Attention• Management lines FC – Interface clearSRQ – Service requestREN – Remote enableEOI – End or identifyThe eight data lines D101 to D108 carry both data and command messages. Allcommands and most data use the 7-bit ASCII code, in which case the eighth bit, D108, iseither unused or used for parity. The state of the attention (ATN) line determines whetherthe information is data or commands. Command messages are sent with the ATN lineasserted, while data messages are sent with the ATN line unasserted.Five signal lines manage the flow of information across the GPIB. These are describedbelow.• ATN (attention) – The controller asserts the ATN line ‘true’ when it uses thedata lines to send commands. All devices become listeners and participate inthe communication. When ATN is unasserted, information on the bus isinterpreted as data.• IFC (interface clear) – This line can only be controlled by the systemcontroller, which drives the IFC line to initialize the bus and become