D-6 IEEE-488 Bus Overview
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 contains data,
addresses or commands as described in the following paragraph.
Bus commands
The instrument may be given a number of special bus commands through the IEEE-488
interface. This section briefly describes the purpose of the bus commands which are grouped
into the following four 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).
These bus commands and their general purpose are summarized in Table D-1.
DATA
DAV
SOURCE
SOURCE
VALID
ALL READY
ACCEPTOR
NRFD
NDAC
ALL ACCEPTED
ACCEPTOR
gure
-
EEE-488 handshake
sequence
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