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Modbus Serial Protocol

The Modbus serial protocol was one of the first industrial communication protocols ever developed. Developed by MODICON, the company that created the PLC, the protocol is a model of simplicity. As a result, it is used all over the world for industrial communications. Because it can be used over modems, it is the de facto communications standard for SCADA systems and remote terminal units (RTU).

The original Modbus ASCII protocol has largely been supplanted by the more versatile RTU protocol. The Modbus/RTU messaging structure is consistent with the message structure for Modbus/TCP, allowing for seamless operation between the two protocols. The Modbus Organization has even published specifications on how a gateway between the two protocols should operate.

The Modbus serial protocol can be implemented using point-to-point physical layers, such as RS-232 and RS-422, or it can be implemented in multi-drop bus configurations using RS-485.

The Modbus Organization, which maintains the protocol, is supported by dozens of members from all over the globe. There is no standardized conformance or interoperability test suite for Modbus serial devices, but informal conformance and interoperability testing is available.

What Can AB Tech Solution Do for Your Modbus Product Development?
  • Develop a fully-compliant product for you
  • Add Modbus capability to your existing product
  • Enhance the features and functionality of your product
  • Perform interoperability testing

Data Access

Faction Name

Faction Code

Data Access

Bit Access

Physical Discrete Inputs

Read Discrete Input

2

Internal Bit or Physical Coils

Read Coil

1

Write Single Coil

5

Write Multiple Coil

15

16-bit access

Physical Input Registers

Read Input Register

4

Internal Register or
Physical Output Register

Read Holding Registers

3

Write Single Register

6

Write Multiple Register

16

Read/Write Multiple Register

23

Mask Write Register

22

Read FIFO Queue

24

File Record Access

Read File Record

20

Write File Record

21

Diagnostics

Read Exception Status

7

Diagnostic

8

Get Com Event Counter

11

Get Com Event Log

12

Report Slave ID

17

Read Device Identification

43

Other

Encapsulated Interface Transport

43