Data Acquisition

• TalkersA talker is a one-way communicating device that can only send data to alistening device. It does not receive data. The talker waits for a signal from thecontroller and then places its data on the bus. Only one device can talk at atime. Common examples are simple DVMs (digital voltmeters) and some A/Dconverters.• ListenersA listener is a one-way communicating device that can only receive data fromanother device. It does not send data. It receives data when the controllersignals it to read the bus. Common examples are printers, plotters, andrecorders.• Talkers/ListenersA talker/listener has the combined characteristics of both talkers and listeners.However, it is never a talker and a listener at the same time. A commonexample is a programmable DVM, which is a listener while its range is beingset by the controller and a talker while it sends the results back to thecontroller. Most modem digital instruments are talker/listeners as this is themost flexible configuration.• ControllersA controller manages and controls everything that happens on the GPIB. It isusually an intelligent or programmable device, such as a PC or amicroprocessor-controlled device. It determines which devices will send data(talkers) and which will receive data (listeners), and when. To avoidconfusion in any GPIB application, there can only be one active controller,called the controller in charge (CIC). The key word is active. There can beseveral controllers, but to avoid confusion, only one can be active at any time.A controller also has the features of a talker/listener. In some cases, whenseveral PCs are simultaneously connected on a GPIB, one is usuallyconfigured as the controller and the others configured as talkers/listeners. Thecontroller needs to be involved in every transfer of data. It needs to address atalker and a listener before the talker can send its message to a Listener. Afterthe message is sent, the controller un-addresses both units. Some GPIBconfigurations do not require a controller, e.g. when only one talker isconnected to one or more listeners. A controller is necessary when the activeor addressed talker or listener must be changed. In this case, a device sees itstalk address on the bus, and knows that it has to act as a talker and hencerequired to send data. Conversely, when it sees its listen address on the bus, itknows it is required to act as a Listener and hence receive data.8.5 Bus structureThe GPIB interface system illustrated in Figure 8.5 consists of 16 signal lines and 8ground return or shield drain lines.