Data Acquisition

they read the byte on the data lines and immediately assert the NRFD line to indicate thatno further data should be sent. As each listener accepts the data, it releases NDAC. Afterthe last listener has accepted the data, the NDAC line voltage goes high (‘false’),signaling the Talker that the data has been accepted. Only when the data byte has beenaccepted by all the listeners, can the talker allow DAV voltage to go high (‘false’) andremove its data from the bus. The listeners then assert NDAC to signify that the datatransfer has ended, in preparation for the next cycle. Figure 8.6 illustrates this handshakingsequence for one message byte.Figure 8.6GPIB handshaking timing diagramIt should be noted that since a talker waits until all listeners are ready (NRFD is ‘false’)before sending a message byte and waits for all listeners to accept the message byte(NDAC line is ‘false’) before transferring any more data, the maximum data transfer rateof the GPIB is determined by the slowest listener on the bus.8.7 Device communicationGPIB devices communicate with other interconnected GPIB devices by passing devicedependentmessages and interface messages one byte at a time through the parallel datacommunications interface.• Device-dependent messages contain specific information related to aparticular device, including programming instructions, measurement results,device status, and data files. These messages are often called data messages.• Interface messages manage the operation of the communications bus,performing such tasks as initializing the bus, addressing, and disablingdevices, setting modes for remote or local programming. Such messages areusually referred to as command messages. The term ‘command message’,used here, can sometimes be confused with device-specific commands that arecontained in data messages. For example, the identification query command*IDN? that is used to identify a particular device, is a command that a deviceunderstands but is sent in what is known as a GPIB data-message.