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November 1985
Data Lines
The eight data lines, DIOl through D108, carry both data and
command messages. All commands and most data use the 7-bit
ASCII or IS0 code set, in which case the 8th bit, D108, is unused
or used for parity.
Appendix A lists the GPIB command messages.
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 tells 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.
The way in which NRFD and NDAC are used by the receiving
device is called the Acceptor Handshake. Likewise, the sending
device uses DAV in the Source Handshake.
APPENDIX D - OPERATION OF THE GPIB
110
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