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Appendix A
GPIB Basics
The GPIB is like a typical computer bus, except that the typical computer has circuit cards
interconnected via a backplane bus, whereas the GPIB has standalone devices interconnected via
a cable bus.
The role of the GPIB Controller is similar to the role of the CPU of a computer, but a better
analogy is to the switching center of a city telephone system. The switching center (Controller)
monitors the communications network (GPIB). When the center (Controller) notices that a party
(device) wants to make a call (send a data message), it connects the caller (Talker) to the receiver
(Listener).
The Controller addresses a Talker and a Listener before the Talker can send its message to the
Listener. After the message is transmitted, the Controller may unaddress both devices.
Some bus configurations do not require a Controller. For example, one device may always be a
Talker (called a Talk-only device) and there may be one or more Listen-only devices.
A Controller is necessary when the active or addressed Talker or Listener must be changed. The
Controller function is usually handled by a computer.
With the GPIB interface board and its software your personal computer plays all three roles.
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Controller—to manage the GPIB
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Talker—to send data
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Listener—to receive data
Controller-In-Charge and System Controller
You can have multiple Controllers on the GPIB, but only one Controller at a time can be the
active Controller, or Controller-In-Charge (CIC). The CIC can be either active or inactive
(standby). Control can pass from the current CIC to an idle Controller, but only the System
Controller, usually a GPIB interface, can make itself the CIC.
GPIB Signals and Lines
Devices on the bus communicate by sending messages. Signals and lines transfer these messages
across the GPIB interface, which consists of 16 signal lines and 8 ground return (shield drain)
lines. The 16 signal lines are discussed in the following sections.
Data Lines
Eight data lines, DIO1 through DIO8, carry both data and command messages.