APPENDIX F
Handshake Lines
The bus uses handshake lines that operate in an interlock-
ed sequence. This method ensures reliable data transmis-
sion 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 information), while the remain-
ing two lines are controlled by accepting devices (the
listener or listeners receiving the information). The three
handshake lines are:
DAV (Data Valid&The source controls the state of the DAV
line to indicate to any listening devices whether or not data
bus information is valid.
NRFD (Not Ready For D&)-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
Figure F-2. IEEE-488 Handshake Sequence
device is ready.
NDCA (Not Data Accepted)-NDCA is also controlled by
the accepting device. The state of NDCA tells the source
whether or not the device has accepted the data byte.
The complete handshake sequence for one data byte is
shown in Fi re F-2. Once data is placed on the data lines,
the source A? ecks 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 con-
ditions are not met, the source must wait until NDCA and
NRFD have the correct status. If the source is a controller,
NRFD and NDAC must by stable for at least lOOnsec 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.
The instrument may be given a number of special bus com-
mands through the IEEE-488 intefiace. This section brief-
ly describes the purpose of the bus commands which are
BUS COMMANDS
grouped into the following three categories.
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 released to go high until all devices have ac-
cepted the data byte.
The sequence just desaibed 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
contains data, addresses or commands as desaibed in the
following paragraph.
1. Uniline Commands-Sent by setting the associated bus
lines true. For example, to assert REN (Remote Enable),
the REN line would be set low (true).
2. Multiline Commands General bus commands which
are sent over the data lines with the MN line true (low),
3. Device-Dependent Commands Special commands
whose meanings depend on device configurations; sent
with ATN high (false).
These bus commands and their general purposes are sum-
marized in Table F-l.
Uniline Commands
MN, IFC and REN are asserted only by the controller. SRQ
is asserted by an external device. EOI may be asserted
either by the controller or other devices depending on the
direction of data transfer. The following is a description
of each command. Each command is sent by setting the
corresponding bus line true.
F-3
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