Modbus Communication Protocol; Characteristics Of Links; Rtu Transfer Mode - INVT IVC2H Programming Manual

Programming manual of IVC series small PLC

10.4 Modbus communication protocol

10.4.1 Introduction
For the serial port communication of IVC series small PLC, Modbus communication protocol is available. Two communication
modes: ASCII and RTU (IVC1 only supports RTU mode) are supported. The PLC can be set as the master or slave station.

10.4.2 Characteristics of links

1. Physical layer: RS232, RS485
2. Link layer: asynchronous transfer mode
1) Data bit: 7 bits (ASCII) or 8 bits (RTU)
2) Transfer rate: 1200, 2400, 4800, 9600, 19200, 38400
3) Check method: even check, odd check or no check
4) Stop bit: 1 or 2 stop bits
3. Networking configuration: up to 31 sets of equipment. Address range: 1~31. Broadcast is supported.

10.4.3 RTU transfer mode

1. Hexadecimal data.
2. The interval between two characters shall not be less than the time of 1.5 characters.
3. There is no frame head or tail, and the interval between two frames is at least the time of 3.5 characters.
4. Use CRC16 check.
5. The maximum length of RTU frame is 256 bytes and the frame structure is as follows:
6. Calculation of interval among characters:
If the communication baud rate is 19200, the interval of 1.5 characters is 1/19200×11×1.5×1000=0.86ms.
The interval of 3.5 characters is 1/19200×11×3.5×1000=2ms.
Structure of frame Address
Number of bytes 1
Chapter 10 Using communication function
Different from "Example 1", when sending the
high&low bytes of a word element, the element
must be divided into high&low byte parts.
For instance, if you want to send the content of
D2, you can store its high byte and low byte
separately in D3 and D4, and then send D3 and
D4. You can also store the data in a K4MX (such
as K4M0 of in this example) element. Take K2M0
as high byte and K2M8 as low byte.
Function code Data
1 0~252
CRC
2
277