Despatch Protocol Plus Modbus Communications User Manual

Page 34

Chromalox Instruments and Controls

A-51643 Rev. 6 10/06/03

5.3 Diagnostics and Support Description for Protocol

Plus

All of the diagnostic and support functions are unclassified commands that tend to be application

dependent. The Modbus diagnostic function provides a series of tests and information exchange for

checking the integrity of the controller, the communication link, and the status of the controller.

Broadcast is not supported.

5.3.1

[08] Diagnostics

The function uses a two-byte sub-function code field in the query to define the type of test to be

performed. Most of the diagnostic queries use a two-byte data field to send diagnostic data or control

information to the slave. In sub-functions where the data is not required, $0000 is transmitted as data and

is ignored by the slave.

The slave echoes both the function code and sub-function code in a normal response as well as returning

any requested data. The diagnostic query form is shown below. The response is sub-function dependent

and may be application dependent. In cases where no return data is required and the data field was

included in the query, the query data field is echoed in the slave response.

Diagnostic Query Form

Field Name

Data

Slave address

$XX

Function

$08

Sub-function hi

$XX

Sub-function lo

$XX

Data hi

$XX

Data lo

$XX

CRC

$----

5.3.1.1 [08 00] Return Query Data

The data passed in the query data field is to be returned (looped back) in the response. The entire

response message should be identical to the query.

Example: Return Query Data $AA55 from slave device 2.