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Appendix C
Operation of the GPIB
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
International Standards Organization (ISO) code set, in which case the
eighth bit, DIO8, is unused or is used for parity.
Handshake Lines
Three lines asynchronously control the transfer of message bytes among
devices. This 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 whether a device is ready to receive a message byte.
All devices drive NRFD when they receive commands, and Listeners
drive it when they receive data messages.
NDAC (not data accepted)
NDAC indicates whether a device has accepted a message byte. All
devices drive NDAC when they receive commands, and Listeners drive
it when they receive data messages.
DAV (data valid)
DAV tells whether the signals on the data lines are stable (valid) and
whether devices can accept them safely. The Controller drives DAV
when sending commands, and the Talker drives it when sending data
messages.
Interface Management Lines
Five lines manage the flow of information across the interface.
ATN (attention)
The Controller drives ATN true when it uses the data lines to send
commands and drives ATN false when a Talker can send data messages.
© National Instruments Corporation
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LabVIEW Function and VI Reference Manual