ATC Mini Chiller - ModBUS Communication Protocol
ATC MINI Chiller Modbus Protocol v1 - 03 May 2019
3
2 Communication Protocol
The protocol adopted by the ATC MINI Chiller is a subset of the widely used MODBUS RTU (JBUS, AEG
Schneider Automation, Inc. registered trademark) protocol, so that connections are easy for many commercial
PLCs and supervisory programs.
The MODBUS RTU communication functions implemented are:
From master to slave
From slave to master
Function 3: read n registers request
Function 3: return data content of n registers
Function 6: set one register request
Function 6: set one register and acknowledge
Function 16: set multiple registers request
Function 16: set multiple registers and acknowledge
Every message contains four fields: Slave Id, Function, Data, CRC
Slave address (from 1 to 247): MODBUS RTU (JBUS) reserves address 0 not used in MINI Chiller
Function code: contains 3, 6 or 16
Data field: contains data for the function used like register address, number of registers and actual data;
Check word: a cyclic redundancy check (CRC) performed with particular rules for CRC16.
Cyclic Redundancy Check
The CRC is a check word that permits verification of message integrity.
Every message, sent or received, has the CRC check word in the two last characters .
After receiving a request, the slave checks the received message by comparing the received CRC with the
calculated one.
When a reply is ready to be sent by the slave, it calculates the CRC word and adds two characters to the
prepared message.
CRC calculation is performed on every character of the message, excluding the last two bytes being the CRC.
Error Condition Response
Data
Byte
Slave address
1
Function code + 128
1
Error code
1
CRC-16 (LSB)
1
CRC-16 (MSB)
1
Errors generate a code indicating the cause as follows: 00H = No Error
01H = Illegal Function
– code received by the slave device is not recognized
02H = Illegal Data Address
– the address does not exist in the slave
03H = Illegal Data Value
– the value is not accepted by the slave
04H = Slave Device Failure
– an unrecoverable error occurred during slave processing
05H = Acknowledge
– the slave accepts the data but needs timeout extension
06H = Slave Busy
– the master should try again later
07H = NACK
– the slave cannot perform the command, master should request diagnostic
Annex L-11M 190503 SA00011M RS485 ModBUS RTU for Mini
30
