Specifications

Document 12245 Rev. I Sheet 9 of 39
If further data is included, if follows the instruction code and is referred to as the message body.
Some instruction codes require a sub-instruction. If required, the sub-instruction is sent as the first byte of the
message body. If there is a reply, the reply will contain the same sub-instruction.
Data values, if required, are sent in the message body following the instruction code, or the sub-instruction (if
it is present).
2.1 Message Types
The message type is determined by the instruction code.
The message type determines:
 Whether or not a reply will be sent.
 Whether or not the reply (if any) contains data.
There are three general types of message: Status, Control/Configuration, and Command.
2.1.1 Status Messages
Status messages request data without affecting any controls or settings. The unit will generate a response
message containing the requested data in the message body.
Normally status messages may be used regardless of the operational mode of the unit.
2.1.2 Control/Configuration Messages
Control messages set the value of an operational control on the unit, whose value can be either set or
queried. Controls usually correspond to some important feature, such as the gain of an amplifier.
To set the value of a control, a message is sent containing the desired value. The control will be set, but no
reply will be generated. To determine if the control was actually set to the desired value, another message
should be sent to query the value of the control.
To query the value of the control, a message is sent without any setting value (although some messages do
require other data values, which should be sent). A reply will be generated containing the current value of the
control.
Configuration messages are identical to control messages in their behavior, but the data in them corresponds
to some setting, that is usually only used once when the system is set up.
There may be conditions under which certain messages will not be obeyed (see the specific equipment
manual). Normally the value of any control or configuration setting may be queried regardless of the
operational mode of the unit.
2.1.3 Command Messages
Command messages cause the device to perform some action or function that can't necessarily be
evaluated. Commands may (but don’t have to) contain data in the message body. No message is returned
from the unit in response to a command.
Command messages are also used for controls that may take a significant amount of time to change. There
will be another status message that can be used to determine the value of such controls.
There may be conditions under which certain messages will not be obeyed (see the specific equipment
manual).
2.2 Data Values
Data values are used in the message body to transfer measurements, settings, and other information. Data
values are normally represented as ASCII characters, to conform to the limitations imposed by the framing
protocol.