What is the Industrial Communication - What is Modbus

Understanding Industrial Communication Series: Modbus 

Modbus is the widely used common communication protocol in SCADA. In this protocol, multiple devices communicate with each other in a central network. This scheme primarily consists of a master and slave(s) having their own identity i.e.,. device address. 

Modbus Serial & Modbus TCP/IP

Based on the hardware signal cable, Modbus Serial comprised of the following types
1.RS 232 (mainly used for short communication, less than 15 meters)

2. RS 485 (it is used for longer wire lengths, beyond 15 meters)

3. RS 422 (longer wire length, )

Modbus TCP/IP:

Modbus TCP/IP works as Client and server over ethernet. 

In the current world, nearly all RTU/RTU, RTU/PLCs use this protocol as the base communication tool. 

What are the Two modes of Modbus:

1. Modbus ASCII
2. Modbus RTU 

Modbus RTU is basically a binary system and the most commonly implemented mode. 

On the other hand, Modbus ASCII uses characters for transfer of data. This uses more number of bytes as compared to RTU for communication. 

What are the Data Types

There are 4 data types: 

1. Coil (single-bit output)
2. Discrete Input (single-bit input)
3. Input Register (16-bit input)
4. Holding Register (16-bit input)

How Does Modbus Communication Protocol Work

Modbus message comprises of following four components: 

SLAVE ADDRESS

FUNCTION CODE

DATA

CRC ERROR CHECK

Communication is initiated by the Master. The master sends a message to the network and all slaves connected over the network interpret this message. A slave having the same address as in the message responds back to the master while all other nodes remain silent.