Operation of the GPIB
Appendix A
GPIB-130 User Manual
A-2
© National Instruments Corporation
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
•
Three 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.
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.
Not Ready For Data (NRFD)
NRFD indicates when a device is ready or not ready to receive a message byte. The line is
driven by all devices when receiving commands and by Listeners when receiving data messages.
