Introduction 4
1. INTRODUCTION
Modbus is a request‑response software protocol implemented on
HI520
and intended for efficient and immediate
remote industrial process control.
Main Features
• Allows immediate response to a problem with equipment (even from different plants)
• Minimizes production downtime as mechanical issues are quickly identified
• Traceability of stored data
• Monitors and controls processes remotely
2. MODBUS PROTOCOL BASICS
Modbus protocol defines a communication structure that occurs in pairs. One device initiates a request and then
waits for a response. The initiating device is responsible for initiating every interaction (query).
The initiating device can address individual requests (client responds) or initiate a broadcast message to several
clients (client does not respond).
Default parameters
Modbus communication protocol is implemented on
HI520
through a serial communication channel that makes
controller data available to a remote user. Physical layer is implemented on a RS485 line with the following
default parameters:
half duplex
8 bit data
1 start bit
1 stop bit
no parity
RTU (Remote Terminal Unit) mode
Bytes are transferred as they are and checksum is calculated over CRC algorithm.
START
ADDRESS
FUNCTION
DATA
CRC CHECK
END
T1‑T2‑T3‑T4
8 bits
8 bits
N x 8 bits
16 bits
T1‑T2‑T3‑T4
Note:
T1-T2-T3-T4 = 3.5 characters times at no communication
•
Address Field
– client address range is 1 – 247, with 0 kept for broadcasting.
•
Function Field
– values range is 1 – 255.
Field is used by client device to echo server query function field for normal case or sets most significant bit
of query function to 1 if an error occurs.
•
Data Field
– information regarding required operation.
Data field can be of various lengths or 0. Data uses “big‑endian” convention, whereby numerical data with
a size greater than 1 byte is sent with the most significant byte first e.g. 0x1234 is sent as 0x12,0x34.
