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Protocol Overview
The instrument and controller communicate using program messages and
response messages. These messages serve as the containers into which sets
of program commands or instrument responses are placed. Program
messages are sent by the controller to the instrument, and response
messages are sent from the instrument to the controller in response to a
query message. A query message is defined as being a program message
which contains one or more queries. The instrument will only talk when it
has received a valid query message, and therefore has something to say. The
controller should only attempt to read a response after sending a complete
query message, but before sending another program message. An important
rule to remember is that the instrument will only talk when prompted to, and
it then expects to talk before being told to do something else.
Protocol Operation
When the instrument is turned on, the input buffer and output queue are
cleared, and the parser is reset to the root level of the command tree.
The instrument and the controller communicate by exchanging complete
program messages and response messages. This means that the controller
should always terminate a program message before attempting to read a
response. The instrument will terminate response messages except during a
hardcopy output.
If a query message is sent, the next message passing over the bus should be
the response message. The controller should always read the complete
response message associated with a query message before sending another
program message to the same instrument.
The instrument allows the controller to send multiple queries in one query
message. This is referred to as sending a "compound query." As noted in
chapter 1, "Multiple Queries," multiple queries in a query message are
separated by semicolons. The responses to each of the queries in a
compound query will also be separated by semicolons.
Commands are executed in the order they are received.
Message Communication and System Functions
Protocols
6–4