Operation of the GPIB Appendix E
GPIB-1014 User Manual E-2 © National Instruments Corporation
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.
• Controller - to manage the GPIB
• Talker - to send data
• Listener - to receive data
The Controller-In-Charge and System Controller
Although there can be multiple Controllers on the GPIB, only one Controller at a time is active or
Controller-In-Charge (CIC). Active control can be passed from the current CIC to an idle
Controller. Only one device on the bus, the System Controller, can make itself the CIC. The
GPIB interface board is usually the System Controller.
GPIB Signals and Lines
The interface system consists of 16 signal lines and eight ground return or shield drain lines.
The 16 signal lines are divided into the following three groups.
• Eight data lines
• Three handshake lines
• Five interface management lines

Data Lines

The eight data lines, DI01 through DI08, carry both data and command messages. All commands
and most data use the 7-bit ASCII or ISO code set, in which case the eighth bit, DI08, is unused or
used for parity.

Handshake Lines

Three lines asynchronously control the transfer of message bytes among devices. The process is
called a three-wire interlocked handshake, and it guarantees that message bytes on the data lines
are sent and received without transmission error.