ADOS S.R.L. Buccinasco (MI)
417-MTA200I1
Rev. 9
Page 36 of 77
2.14.6 MODBUS protocol
The RTU version of MODBUS is used.
Received and transmitted frame structure
General frame structure is as follows:
ADDRESS FUNCTION
DATA
CHECK
8 bits
8 bits
N x 8 bits
2 x 8 bits
ADDRESS
8 bit defining the slave address and ranging from 1 to 32
FUNCTION 8 bit defining the required function.
The functions supported by A200E are the following
Code 03
Read Holding Registers
Code 05
Force Single Coil
Code 06
Preset Single Register
Code 16
Preset Multiple Registers
DATA
All data relevant to the specific function
CHECK
CRC-16 (Cyclic Redundancy Check) frame validation
Exception responses
When A200E receives a request involving illegal functions or illegal data an exception response is
generated containing address, function code, error code and checksum.
To indicate that the response is a notification of an error, the high order bit of the function code is
set to “1”.
Supported error code are:
01
Illegal function. The message function received is not an allowable action for the
addressed slave.
02
Illegal data address. The address referenced in the data field is not an allowable
address in the address slave location
03
Illegal data value. The value referenced in the data field is not allowable in the
addressed slave location
A200E supported functions
For a complete description of MODBUS available functions, please refer to detailed MODBUS
documentation.
Function 03 - Read Holding Registers
Allows the host to obtain the binary value of the content of A200E registers
All registers can be transferred in a single read request.