Signals and PLC · Glossary

What is Modbus RTU?

serial protocolSignals and PLC

Modbus RTU is a simple master slave serial protocol, usually on RS485, that reads and writes registers and coils in field devices.

Modbus RTU Explained

A master sends a request with slave address, function code, data and a CRC; the addressed slave replies. Frames are separated by at least 3.5 character times of silence.

Common function codes are 03 (read holding registers), 04 (read input registers), 06 (write single register) and 16 (write multiple registers).

Modbus RTU: Typical Values

Slave addresses	1 to 247
Error check	CRC 16
Typical baud rates	9600 or 19,200 bit/s

Calculators for Modbus RTU

CalculatorBCD ConverterFree online calculator with formula and worked example.

Free Tools

ToolModbus Address CalculatorFree interactive tool on instrumentationblog.in.ToolModbus CRC CalculatorFree interactive tool on instrumentationblog.in.ToolModbus Frame DecoderFree interactive tool on instrumentationblog.in.ToolModbus RTU Timing CalculatorFree interactive tool on instrumentationblog.in.

Frequently Asked Questions

What is Modbus RTU?

Modbus RTU is a simple master slave serial protocol, usually on RS485, that reads and writes registers and coils in field devices.

Why do register addresses differ by one?

Documentation often uses 1 based numbers like 40001, while the protocol sends 0 based addresses.

What causes CRC errors?

Noise, wrong baud or parity settings, missing termination or bad grounding.

Learn more on the blog

Modbus RTU troubleshooting: CRC, no response and fault table

A practical guide on instrumentationblog.in with more detail on modbus RTU and how it is applied on real plants.

Read Full Article

More Signals and PLC Terms

Signal termGalvanic IsolationGalvanic isolation separates two circuits so no current can flow directly between them, while the signal…Signal termGround LoopA ground loop occurs when a circuit is earthed at two points with different potentials, letting current…Signal termCable Shield GroundingCable shield grounding connects the screen of an instrument cable to earth, normally at one end only, to…Signal termElectromagnetic InterferenceElectromagnetic interference is unwanted electrical noise coupled into signal circuits from motors,…Signal termAnalog ScalingAnalog scaling converts the raw value from a PLC analog input into engineering units such as bar or °C.Signal termADC ResolutionADC resolution is the number of bits an analog to digital converter uses, which sets the smallest signal…

All signals and plc terms · Full glossary A to Z