30
11.0 MODBUS & JBUS COMMUNICATIONS
12.1
Introduction……………pg 31
12.2
Transmission Format...............pg 31
12.3
Communication Procedure...............pg 32
12.4
Error Check (CRC-16 Cyclical Redundancy Check……………pg 33
12.5
Function Code 3 & 4: Words Reading……………pg 33
12.6
Notes……………pg 35
12.7
Error Codes……………pg 37
12.8
Operative Mode Addresses……………pg 38
12.1 Introduction
This half duplex protocol accepts one master and one or more slaves. The physical interface
should be of the RS-485 type.
A single multidrop link can take up to 128 devices having the same “High Input Impedance” as
the transceiver used.
The computer should be programmed to serve as a master controlling which slave has access to
the link. All other slaves are in a waiting state. Each slave has a unique address ranging from 1
to 255. Address “0” is a broadcast one. When the master sends a message with address “0”, all
slaves receive it and no one replies.
NOTE:
the numerical value present in this text are expressed as:
binary value if they are followed by b
decimal value if they are not followed by any letter
hexadecimal value if they are followed by h
12.2 Transmission Format
The protocol uses the RTU (Remote terminal unit) mode of transmission. RTU is a binary
method with byte format composed as follows:
1 start bit, 8 data bit, 1 parity bit (optional), 1 stop bit. The communication speed is selectable
among 150, 300, 600, 1200, 2400, 4800, 9600 and 19200 baud.