Appendix D
Operation of the GPIB
© National Instruments Corp.
D-5
GPIB-232CT User Manual
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.
Interface Management Lines
Five lines are used to 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 false when it allows a Talker to send data messages.