Reference MVI56E-MCM ā¦ ControlLogix Platform
User Manual Modbus Communication Module
Page 174 of 209 ProSoft Technology, Inc.
6.8.7 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 will
load the specified register with the contents specified.
Note Functions 5, 6, 15, and 16 are the only messages that will be 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 ā3ā to register 40002 in slave
11.
Response
The response to a preset single register request is to re-transmit the query
message after the register has been altered.
6.8.8 Diagnostics (Function Code 08)
Modbus function code 08 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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