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Modbus - Technical Background

Message Frames

emModbus supports all three Modbus standard protocols covered by the official Modbus documentation:

ProtocolDescription
RTUOriginal Modbus standard. Binary data is sent via serial connections such as RS-232 or similar.
ASCIISimilar to RTU. Instead of raw binary, data is encoded in ASCII.
Modbus/TCPBinary data is encapsulated in a TCP frame and sent via network connections such as Ethernet. This variant can also be used with UDP instead of TCP and is then called Modbus/UDP.

Message frame fields

Although the different message frames are each handled differently by the protocol, both RTU and ASCII frames consist of the same four specified fields. Field 2 and 3 constitute the Protocol Data Unit (PDU), which is included in the Modbus/TCP frame as well. 

ModbusFrame.png

RTU message frame

When using RTU frames, each byte contained in a message is sent as binary data. The main advantage of this mode is its greater density, allowing better data throughput for the same baud rate compared to ASCII frames. 

ModbusRTUFrame.png

ASCII message frame

When using ASCII frames, each byte contained in a message is encoded and sent as two ASCII characters. This allows time intervals of up to one second to occur between characters without causing an error. 

ModbusASCIIFrame.png

Modbus/TCP message frame

When using Modbus/TCP frames, an additional header called 'Modbus Application Header' precedes the Protocol Data Unit.

ModbusTCPFrame.png

Instruction Set

emModbus currently supports the following instructions:

Function codeDescription
1Read Coil.
2Read Discrete Input.
3Read Holding Register.
4Read Input Register.
5Write Coil.
6Write Register.
15Write Coils.
16Write Registers.

Data Types

Modbus uses four primary data types: 

Data typeDescription
CoilSingle bit, alterable by an application program, read-write.
Discrete InputSingle bit, provided by an I/O system, read-only.
Holding Register 16-bit, alterable by an application program, read-write.
Input Register16-bit, provided by an I/O system, read-only.