User manual

The bus handshake lines operate in an interlocked sequence. This method ensures reli-
able data transmission regardless of the transfer rate. Generally, data transfer will occur at
a rate determined by the slowest active device on the bus.
One of the three handshake lines is controlled by the source (the talker sending informa-
tion), while the remaining two lines are controlled by accepting devices (the listener or lis-
teners receiving the information). The three handshake lines are:
DAV (DATA VALID) — The source controls the state of the DAV line to indicate to any lis-
tening devices whether or not data bus information is valid.
NRFD (Not Ready For Data) — The acceptor controls the state of NRFD. It is used to sig-
nal 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 Figure E-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 controllers
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 the following paragraph.
DATA
DAV
SOURCE
SOURCE
VALID
NRFD
NDAC
ACCEPTOR
ACCEPTOR
ALL READY
ALL ACCEPTED
Figure E-2
IEEE-488 hand-
shake sequence
IEEE-488 Bus Overview E-7
2000-900 (J - Aug 2010) BOOK.fm Page 7 Wednesday, October 12, 2011 12:29 PM