User manual

MODBUS RTU, ASCII, TCP 5

English

1. Description

MODBUS ASCII/RTU is a master-slave communication protocol, able to support up to 247 slaves connected in

a bus or a star network.

The protocol uses a simplex connection on a single line. In this way, the communication messages move on a

single line in two opposite directions.

MODBUS TCP is a variant of the MODBUS family. Speciﬁcally, it covers the use of MODBUS messaging in an

“Intranet” or “Internet” environment using the TCP/IP protocol on a ﬁxed port 502.

Master-slave messages can be:

Reading (Function code $01 / $03 / $04)• : the communication is between the master and a single slave.

It allows to read information about the queried counter

Writing (Function code $10)• : the communication is between the master and a single slave. It allows to

change the counter settings

Broadcast (not available for MODBUS TCP)• : the communication is between the master and all the

connected slaves. It is always a write command (Function code $10) and required logical number $00

In a multi-point type connection (MODBUS ASCII/RTU), slave address (called also logical number) allows to

identify each counter during the communication. Each counter is preset with a default slave address (01) and

the user can change it.

In case of MODBUS TCP, slave address is replaced by a single byte, the Unit identiﬁer.

COMMUNICATION FRAME STRUCTURE

ASCII mode

Bit per byte: 1 Start, 7 Bit, Even, 1 Stop (7E1)

Name Length Function

START FRAME 1 char Message start marker. Starts with colon “:” ($3A)

ADDRESS FIELD 2 chars Counter logical number

FUNCTION CODE 2 chars Function code ($01 / $03 / $04 / $10)

DATA FIELD n chars Data + length will be ﬁlled depending on the message type

ERROR CHECK 2 chars Error check (LRC)

END FRAME 2 chars Carriage return - line feed (CRLF) pair ($0D & $0A)

RTU mode

Bit per byte: 1 Start, 8 Bit, None, 1 Stop (8N1)

Name Length Function

START FRAME 4 chars idle At least 4 character time of silence (MARK condition)

ADDRESS FIELD 8 bits Counter logical number

FUNCTION CODE 8 bits Function code ($01 / $03 / $04 / $10)

DATA FIELD n x 8 bits Data + length will be ﬁlled depending on the message type

ERROR CHECK 16 bits Error check (CRC)

END FRAME 4 chars idle At least 4 character time of silence between frames