144 Rockwell Automation Publication 825-UM004D-EN-P - November 2012
Chapter 10 Modbus RTU Communications
7Eh NOP Command
This function code the allows a Modbus master to perform a control operation
and is used inside of the 7Dh when no regular Modbus query is required.
Table 83 - 17Eh NOP Command
Modbus Password Control
and Parameter Modification
The 825-P parameters MID, TID, Password, and the User Map Registers are
settable via Modbus. Any settable parameter or reset that requires a valid
password write will timeout 15 minutes after the last valid write to any of these
restricted registers.
Writing the password for access level change requires the 10h (preset multiple
register) command. Changing the password can be done one register at a time. A
device error is returned during settings save if the relay is disabled or settings are
being changed on another port. Device error is also returned for attempts to write
to settable values if the access level has not been changed.
To enable modification of the settable parameters, a valid Access Level E (EAC)
password must be written to the password registers using function code 10h.
Note changing the password will change the password for all ports.
Once a valid password has been written, then the values may be changed using
standard single or multiple register writes (06h or 10h). Until a command is
issued to save or discard the settings, the value returned when reading the settable
parameter registers is a temporary copy.
To save the modified parameters, write a 0x0001 to the Save Settings register of
the Control I/O region. This is the only method that saves the changes.
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (7Eh)
2 bytes CRC-16
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (7Eh)
2 bytes CRC-16
An example of a 7D message using 7E will have the following format:
1 byte Slave Address
1 byte Function Code (7Dh)
2 bytes Subfunction (2100h)
2 bytes Subfunction (2101h)
1 byte Function Code (7Eh)
2 bytes CRC-16
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