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MODBUS RTU PROTOCOL
The addresses set out in the tables follow the standard routing specified in the reference guide of Modicom
PI-MBUS-300 an extract of which is provided below to help the user communicate with the instrument.
“All data addresses in Modbus messages are referenced to zero. The first occurrence of a data item is
addressed as item number zero. For example:
The coil known as ‘coil 1’ in a programmable controller is addressed as coil 0000 in the data address
field of a Modbus message.
Coil 127 decimal is addressed as coil 007E hex (126 decimal).
Holding register 40001 is addressed as register 0000 in the data address field of the message. The
function code field already specifies a ‘holding register’ operation. Therefore the ‘4XXXX’ reference is
implicit.”
To confirm a new value entered in E2prom, run the MAKE – BACKUP function. If this function is not
performed, by switching off, the DAT 11, the value before the change will be restored.
Unless otherwise specified, the numerical values (such as addresses, codes and data) are expressed
as decimal values.
The MODBUS RTU protocol is available only on COM2 RS485.
HANDLING OF COMMUNICATION ERRORS
The CRC (Cyclic Redundancy Check) is carried out to check the communication strings. In the case of
a communication error, the slave does not respond with a string. The master must consider a timeout
for receipt of the response. If it does not receive a response, a communication error has occurred
HANDLING OF RECEIVED DATA ERRORS
In the case of a string that has been received correctly but cannot be executed, the slave responds with
an EXCEPTIONRESPONSE as indicated in the table below.
CODE
DESCRIPTION
1
ILLEGAL FUNCTION (The function is not valid or not supported)
2
ILLEGAL DATA ADDRESS (The address of the specified data is not available)
3
ILLEGAL DATA VALUE (The values of the received data are invalid)
FUNCTIONS SUPPORTED:
FUNCTION
DESCRIPTION
01
READ COIL STATUS (Reading the status of the logic outputs)
02
READ INPUT STATUS (Reading the status of the logic inputs)
03
READ HOLDING REGISTERS (Reading the programmable registers)
04
READ INPUT REGISTERS (Reading the “read only” registers”)
05
FORCE SINGLE COIL (Writing the status of each output)
06
PRESET SINGLE REGISTER (Writing a programmable register)
15
FORCE MULTIPLE COILS (Multiple writing of outputs)
16
PRESET MULTIPLE REGISTERS (Multiple writing of registers)
Funct + 80h EXCEPTION RESPONSE
Summary of Contents for DAT11
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