Appendix A
Operation of the GPIB
© National Instruments Corp.
A-3
GPIB-140 User Manual
Handshake Lines
Three hardware handshake lines asynchronously control the transfer of message bytes
between devices. This process is a three-wire interlocked handshake, and it guarantees
that devices send and receive message bytes on the data lines without transmission error.
Table A-1 summarizes the GPIB handshake lines.
Table A-1. GPIB Handshake Lines
Line
Description
NRFD (not ready for data)
Listening device is ready/not ready to receive a message
byte. Also used by the Talker to signal high-speed
transfers (HS488).
NDAC (not data accepted)
Listening device has/has not accepted a message byte.
DAV (data valid)
Talking device indicates signals on data lines are stable
(valid) data.
Interface Management Lines
Five GPIB hardware lines manage the flow of information across the bus. Table A-2
summarizes the GPIB interface management lines.
Table A-2. GPIB Interface Management Lines
Line
Description
ATN (attention)
Controller asserts ATN when it sends commands and
unasserts ATN when it sends data messages.
IFC (interface clear)
System Controller drives the IFC line to initialize the bus
and make itself CIC.
REN (remote enable)
System Controller drives the REN line to place devices
in remote or local program mode.
SRQ (service request)
Any device can drive the SRQ line to request service
from the Controller asynchronously.
EOI (end or identify)
Talker uses the EOI line to mark the end of a data
message. Controller uses the EOI line when it conducts
a parallel poll.
