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MODBUS RTU / TCP protocol
On COM1 Rs232 the MODBUS RTU protocol is always available.
The addresses shown in the tables follow the standard addressing specified in the Modicon
PI-MBUS-300 Rev.J reference guide (www.modbus.org).
The values of registers with an address greater than 41000 are stored permanently in
memory only after the save data command. If this function is not performed, turning off the
machine will restore the value previous to the modification.
Unless otherwise specified, numeric values (such as addresses, codes and data) are ex-
pressed as decimal values.
Communication error management
In the case of MODBUS RTU the communication strings are controlled by CRC (Cyclical Redundancy
Check), while in the case of MODBUS TCP the communication is automatically checked by the TCP / IP
protocol. In the event of a communication error, the slave does not respond with any string. The master
must consider a timeout for receiving the reply. If it does not get a response, it must deduce that a commu-
nication error has occurred.
Handling of errors of received data
In the case of a string received correctly but not executable, the slave responds with an EXCEPTION RE-
SPONSE as per the following table.
Code
Description
1
ILLEGAL FUNCTION (Function is invalid or not supported)
2
ILLEGAL DATA ADDRESS (The specified data address is not available)
3
ILLEGAL DATA VALUE (The data received have an invalid value)
Supported functions
READ HOLDING REGISTER
WRITE SINGLE REGISTER
Содержание RQ
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