Almost any instrument can be used with the IEEE-488 specification, because it says nothing about the function of the instrument itself, or about the form of the instrument’s data. Instead the specification defines a separate component, the interface, that can be added to the instrument. The signals passing into the interface from the IEEE-488 bus and from the instrument are defined in the standard. The instrument does not have complete control over the interface. Often the bus controller tells the interface what to do. The active controller performs the bus control functions for all the bus instruments.
At power-up time, the IEEE-488 card that is programmed to be the system controller becomes the active controller in charge. The system controller has several unique capabilities including the ability to send Interface Clear (IFC) and Remote Enable (REN) commands. IFC clears all device interfaces and returns control to the system controller. REN allows devices to respond to bus data once they are addressed to listen. The system controller may optionally Pass Control to another controller, which then becomes active controller.
There are 3 types of devices that can be connected to the IEEE-488 (Listeners, Talkers, and Controllers). Some devices include more than one of these functions. The standard allows a maximum of 15 devices to be connected on the same bus. A minimum system consists of one controller and one talker or listener device (i.e., a PC with a TransEra GPIB-900 board and a voltmeter).
It is possible to have several controllers on the bus but only one may be active at any given time. The active controller may pass control to another controller which in turn can pass it back or on to another controller. A listener is a device that can receive data from the bus when instructed by the controller and a talker transmits data on to the bus when instructed. The controller can set up a talker and a group of listeners so that it is possible to send data between groups of devices as well.
The IEEE-488 interface system consists of 16 signal lines and 8 ground lines. The 16 signal lines are divided into 3 groups (8 data lines, 3 handshake lines, and 5 interface management lines).
