G-6 IEEE-488 Bus Overview Model 2700 Multimeter/Switch System User’s Manual
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
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 G-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 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 G-1.
Data
DAV
Source
Source
Valid
All Ready
Acceptor
NRFD
NDAC
All Ready
Acceptor
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