Preset Single Register (Function Code 06); Diagnostics (Function Code 08) - ProSoft Technology CompactLogix MVI69E-MBS User Manual

MVI69E-MBS ♦ CompactLogix™ Platform
Modbus Serial Enhanced Communication Module

8.2.9 Preset Single Register (Function Code 06)

Query
This Function Code allows you to modify the contents of a Modbus 4x range in the slave.
This writes to a single register only. Any holding register that exists within the controller
can have its contents changed by this message. However, because the controller is
actively scanning, it also can alter the content of any holding register at any time. The
values are provided in binary up to the maximum capacity of the controller. Unused high
order bits must be set to zero. When used with slave address zero (Broadcast mode), all
slave controllers load the specified register with the contents specified.
Note Functions 5, 6, 15, and 16 are the only messages that are recognized as valid for broadcast.
Note: This is the structure of the message being sent out to the Modbus network. The byte values below are
in hexadecimal display.
The example below is a request to write the value '3' to register 40002 in slave 11.
Function
Node
Code
Address
06
0B
Response
The response to a preset single register request is to re-transmit the query message
after the register has been altered.
Function
Node
Code
Address
06
0B

8.2.10 Diagnostics (Function Code 08)

This function provides a series of tests for checking the communication system between
a master device and a slave, or for checking various internal error conditions within a
slave.
The function uses a two-byte sub-function code field in the query to define the type of
test to be performed. The slave echoes both the function code and sub-function code in
a normal response. Some of the diagnostics commands cause data to be returned from
the remote device in the data field of a normal response.
In general, issuing a diagnostic function to a remote device does not affect the running of
the user program in the remote device. Device memory bit and register data addresses
are not accessed by the diagnostics. However, certain functions can optionally reset
error counters in some remote devices.
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Data Start Bit Data Start
High Bit Low
00 01
Data Register Data Register
High Low
00 01
Preset Data Preset Data
Register High Register Low
00 03
Preset Data Preset Data
Register High Register Low
00 03
Reference
User Manual
Error Check
Field (2 bytes)
CRC
Error Check
Field (2 bytes)
CRC
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