National Instruments GPIB-COM Appendix C Operation of the GPIB

The GPIB is a link, bus, or interface system through which interconnected electronic devices

communicate. Hewlett-Packard invented the GPIB, which they call the HP-IB, to connect and

control programmable instruments manufactured by them. Because of its high system data rate

ceilings of from 250 kbytes/sec to 1 Mbyte/sec, the GPIB quickly became popular in other

applications such as intercomputer communication and peripheral control. It was later accepted as

the industry standard IEEE-488. The versatility of the system prompted the name General

Purpose Interface Bus.

Devices on the GPIB communicate by passing messages through the interface system. There are

two types of messages:

• Device-dependent messages, often called data or data messages, contain device-specific

information such as programming instructions, measurement results, machine status, and data

files.

• Interface messages manage the bus itself. They are usually called commands or command

messages. Interface messages perform such functions as initializing the bus, addressing and

unaddressing devices, and setting devices for remote or local programming.

Note: The term command as used here should not be confused with some device instructions

which are also called commands. Such device-specific instructions are actually data

messages.

There are three types of GPIB communicators. A Talker sends data messages to one or more

Listeners. The Controller manages the flow of information on the GPIB by sending commands to

all devices.

Devices can be Talkers, Listeners, and/or Controllers. A digital multimeter, for example, is a

Talker and may also be a Listener. A printer or plotter is usually only a Listener. A computer on

the GPIB often combines all three roles to manage the bus and communicate with other devices.

The GPIB is a bus like a typical computer bus except that the computer has its circuit cards

interconnected via a backplane bus whereas the GPIB has standalone devices interconnected via a

cable bus.

The role of the GPIB Controller can also be compared to the role of the computer's CPU, 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).