Operation of the GPIB
Appendix A
GPIB-140 User Manual
A-2
© National Instruments Corp.
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.
•
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 8 ground return or shield drain lines.
The 16 signal lines are divided into the following three groups.
•
Eight data lines
•
Thre e handshake lines
•
Five interface management lines
Data Lines
The eight data lines, DIO1 through DIO8, 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, DIO8, is unused or used for parity.
