Appendix A Operation of the GPIB
© National Instruments Corporation A-3 GPIB-120A User Manual
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, 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.
NRFD (Not Ready For Data)
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.
NDAC (Not Data Accepted)
NDAC indicates when a device has or has not accepted a message byte. The line is driven by all
devices when receiving commands and by Listeners when receiving data messages.
DAV (Data Valid)
DAV indicates when the signals on the data lines are stable (valid) and can be accepted safely by
devices. The Controller drives DAV when sending commands and the Talker drives it when
sending data messages.