D-6 IEEE-488 Bus Overview Model 6487 Reference Manual
the correct status. If the source is a controller, NRFD and NDAC must be stable for at least
100ns 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 previous sequence 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 described in the following paragraphs.
Figure D-2
IEEE-488 handshake sequence
Bus commands
The instrument may be given a number of special bus commands through the IEEE-488
interface. The following paragraphs briefly describe the purpose of the bus commands
which are grouped into the following three categories.
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 ATN line true (low).
3. Common commands — Commands that are common to all devices on the bus;
sent with ATN high (false).
4. SCPI commands — Commands that are particular to each device on the bus; sent
with ATN (false).
DATA
DAV
SOURCE
SOURCE
VALID
NRFD
NDAC
ACCEPTOR
ACCEPTOR
ALL READY
ALL ACCEPTED
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