Pavone Systems DAT 200 Technical Manual page 27

MODBUS RTU PROTOCOL
WARNINGS: The addresses listed in the tables below follow the standard routing specified in the
reference guidelines of the Modicon PI-MBUS-300 Rev.J (www.modbus.org), referred to below is an
excerpt that helps the user to 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."
The values of the registers with address greater than 40100 are permanently stored in memory only
after the data save command (see Command Register table). If this function is not performed by turning
off the instrument, it will return to the value before the change.
If not otherwise specified, the numerical values (such as addresses, codes and data) are expressed as
decimal values.
COMMUNICATION ERRORS HANDLING
In case of MODBUS RTU, the communication strings are controlled by the CRC (Cyclic Redundancy
Check). In the case of a communication error, the slave does not respond with a string. The master must
consider a timeout for the receipt of the response. In case of no answer there is a communication error.
RECEIVED DATA ERROR HANDLING
In the case of string received correctly, but that cannot be executed, the slave responds with an EXCEP-
TION RESPONSE according to the following table.
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 received data have invalid value)
SUPPORTED FUNCTIONS
FUN 03 READ HOLDING REGISTER
FUN 06 WRITE SINGLE REGISTER
FUN 16 WRITE MULTIPLE REGISTERS
Page 25