Keithley 6512 Instruction Manual page 137

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.
NDAC (Not Data Accepted)—NDAC is also controlled by
the accepting device.
The complete handshake sequence for one data byte in
shown in Figure A-2. Once data is 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 con-
trollers have timeout routines that display messages in case
the transfer sequence stops for any reason.
DATA
DAV
VALID
NRFD
NDAC
DATA
TRANSFER
BEGINS
Figure A-2
IEEE-488 handshake sequence
SOURCE
SOURCE
ACCEPTOR
ACCEPTOR
DATA
TRANSFER
ENDS
Once all NDAC and NRFD are properly set, the source sets
DAV low, indicating to accepting 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 re-
leased to go high until all devices have accepted the data byte.
Once NDAC goes high, the source then sets DAV high to in-
dicate that the data byte is no longer valid. NDAC is returned
to its low state, and NRFD is released by each device at its
own rate, until NRFD goes high when the slowest device is
ready, and the bus is set to repeat the process with the next
data byte.
The sequence just described is used to transfer data, talk and
listen addresses, as well as multiline commands. The state of
the ATN line determines whether the data bus contains data, ad-
dresses, or commands as described in the following paragraph.
A.4 Bus commands
While hardware aspects of the bus are essential, the interface
would have minimal capabilities without appropriate com-
mands to control communications among the various devices
on the bus. This paragraph briefly describes the purposes of
the various device commands, which are grouped into the
following three general categories:
Uniline Commands—Sent by setting the corresponding bus
line true.
Multiline Commands—General bus commands, which are
sent over the data lines with ATN true (low).
Device-dependent Commands—Special commands whose
meanings depend on device configuration; sent over the data
lines with ATN high (false).
These bus commands and their general purposes are summa-
rized in Table A-1.
IEEE-488 Bus Overview
A-3