8-9
IM 704420-01E
Communication Function
8
Precautions to Be Taken when
Transferring Messages
• If a program message that does not contain a query
is sent, the next program message can be sent at
any time.
• If a program message that contains a query is sent,
a response message must be received before the
next program message can be sent. If the next
program message is sent before the response
message is received in its entirety, an error occurs.
The response message that was not received is
discarded.
• If the controller tries to receive a response message
when there is none, an error occurs. If the
controller tries to receive a response message
before the transmission of the program message is
complete, an error occurs.
• If a program message containing multiple message
units is sent, and the message contains incomplete
units, the instrument will attempt to execute the
ones that are believed to be complete. However,
these attempts may not always be successful. In
addition, if the message contains queries, the
responses may not be returned.
Deadlock
The instrument can store response messages of length
1024 bytes or more in its buffer (The number of
available bytes varies depending on the operating
conditions). When both the transmit and receive
buffers become full at the same time, the instrument
can no longer continue to operate. This state is called
a deadlock. In this case, operation can be resumed by
discarding the program message.
Deadlock will not occur if the program message
(including the <PMT>) is kept below 1024 bytes.
Furthermore, deadlock never occurs if a program
message does not contain a query.
8.6.3 Commands
Commands
There are three types of commands (program headers)
that are sent from the controller to the instrument.
They differ in their program header formats.
Common Command Header
Commands that are defined in the IEEE 488.2-1987
are called common commands. The header format of
a common command is shown below. An asterisk (
*
)
is always placed in the beginning of a command.
*
<Mnemonic>
?
An example of a common command;
*CLS
Compound Header
Dedicated commands used by the instrument are
classified and arranged in a hierarchy according to
their functions. The format of a compound header is
shown below. A colon (:) must be used to specify a
lower hierarchy.
:
<Mnemonic>
?
:
An example of a compound header;
MEASURE:FUNCTION
Simple Header
These commands are functionally independent and do
not have a hierarchy. The format of a simple header is
shown below.
<Mnemonic>
?
:
An example of a simple header;
START
Note
<mnemonic> is a character string made up of alphanumeric
characters.
8.6 Program Format
