Protocol Description

Overview

MODBUS Protocol is a messaging structure, widely used to establish master-slave communication between intelligent devices. A MODBUS message sent from a master to a slave contains the address of the slave, the 'command' (e.g. 'read register' or 'write register'), the data, and a check sum (LRC or CRC).
Since Modbus protocol is just a messaging structure, it is independent of the underlying physical layer. In AMB-12I-4O is implemented using RS485 interface.


MODBUS RTU Framing

The AMB-12I-4O module supports standard RTU Modbus protocol for getting the input status and setting/getting the relays status. RTU Modbus Protocol frame is illustrated below:

Start Address Function Data CRC Check End
≥ 3.5 Char 8 Bits 8 Bits  N*8 Bits 16 Bits ≥ 3.5 Char
  • Start is at least 3.5 char time of silent interval between master and slave to start communication 
  • Address is a slave ID Address
  • Function is the function code of MODBUS protocol
  • Data is the appropriate data for function code
  • CRC is Cyclic Redundancy Check for Error Checking (here, CRC-16,Big Endian(ABCD))
  • End is at least 3.5 char time of silent interval between master and slave to start communication

MODBUS Serial Port Parameter

Parameter Option/range
Baudrate 9600 – 128000 configurable
Data bit 8
Parity None
Stop bit 1

ID Configuration of MODBUS Card

User can set the slave ID address using 8 Bit DIP switch found on board. This allow 0 to 255 different IDs to be set. The number below the switches are added together and the result will be identifier of the device slave ID.
To set the slave ID 125, you need to turn ON switch 1,3,4,5,6,7 on DIP switch and OFF the others (In this, convert slave ID number 125 into 8bit binary form – 01111101, turn ON DIP switch 1,3,4,5,6,7 and turn OFF DIP switch 2,8) and so on….  



Baudrate Configuration of MODBUS

User can set the baudrate using 4 Bit DIP switch found on board. To set baudrate, first turn off power supply of card. This allows to set baudrate between 2400bps to 128000bps as shown below table.

Baudrate SW1 SW2 SW3 SW4
9600 OFF OFF OFF OFF
2400 ON OFF OFF OFF
4800 OFF ON OFF OFF
14400 OFF OFF ON OFF
19200 OFF OFF OFF ON
28800 ON ON OFF OFF
38400 OFF ON ON OFF

57600

OFF OFF ON ON
115200 ON OFF OFF ON

128000

ON ON ON ON