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MODBUS RTU General Information
Overview
The hardware version number of this document is V2.0; the software version number is V5.9 and
above. This document describes the MODBUS RTU interface in details and the target object is a
software programmer.
MODBUS command structure
Data format description in this document;
Binary display, suffix B, for example: 10001B
- decimal display, without any prefix or suffix, for example: 256
Hexadecimal display, prefix 0x, for example: 0x2A
ASCII character or ASCII string display, for example: "YL0114010022"
Command Structure
The MODBUS application protocol defines the Simple Protocol Data Unit (PDU), which is
independent of the underlying communication layer.
Figure 1: MODBUS Protocol Data Unit
MODBUS protocol mapping on a specific bus or network introduces additional fields of protocol
data units. The client that initiates the MODBUS exchange creates the MODBUS PDU, and then
adds the domain to establish the correct communication PDU.
MODBUS PDU
Figure 2: MODBUS architecture for serial communication
On the MODBUS serial line, the address domain contains only the slave instrument address. Tips:
The device address range is 1...247
Set the device address of the slave in the address field of the request frame sent by the host.
When the slave instrument responds, it places its instrument address in the address area of the
response frame so that the master station knows which slave is responding.
Function codes indicate the type of operation performed by the server.
CRC domain is the result of the “ redundancy check” calculation, which is executed according to
the information content.
Function Code
Data
Address Domain
Function Code
Data
CRC
MODBUS SERIAL LINE PDU
