Keithley 2500 User Manual page 384

D-6 IEEE-488 Bus Overview
NRFD (Not Ready For Data) — The acceptor controls the state of NRFD. It is used to
signal to the transmitting device to hold off the byte transfer sequence until the accepting
device is ready.
NDAC (Not Data Accepted) — NDAC is also controlled by the accepting device. The
state of NDAC tells the source whether or not the device has accepted the data byte.
The complete handshake sequence for one data byte is shown in
placed on the data lines, the source checks to see that NRFD is high, indicating that all
active devices are ready. At the same time, NDAC should be low from the previous byte
transfer. If these conditions are not met, the source must wait until NDAC and NRFD have
the correct status. If the source is a controller, NRFD and NDAC must be stable for at least
100nsec after ATN is set true. Because of the possibility of a bus hang up, many control-
lers have time-out routines that display messages in case the transfer sequence stops for
any reason.
Once all NDAC and NRFD are properly set, the source sets DAV low, indicating to accept-
ing devices that the byte on the data lines is now valid. NRFD will then go low, and NDAC
will go high once all devices have accepted the data. Each device will release NDAC at its
own rate, but NDAC will not be released to go high until all devices have accepted the data
byte.
The sequence just described is used to transfer both data, talk, and listen addresses, as well
as multiline commands. The state of the ATN line determines whether the data bus con-
tains data, addresses, or commands as described in
Figure D-2
IEEE-488 handshake sequence
Data
DAV
NRFD
NDAC
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Valid
Acceptor
All Ready
All Accepted
Acceptor
Models 2500 and 2502 User's Manual
Figure D-2. Once data is
"Bus commands," page
Source
Source
D-7.