Title: Differences between JBUS and MODBUS protocols
Number : 370
Aim of the
resolution
To define the differences between JBUS and MODBUS protocols.
Customer
activity affected
Research
Products
affected
TSX SCY21600, TSX SCY21601, TSX SCP11x
TSX P57xx, TSX P57xxx, TSX 37xx
Associated
documents
TSXDGMDB, TLXDSCOMPL7
Details/
Resolution
Presentation
MODBUS is a master / slave type protocol developed by MODICON.
It was further developed for APRIL and is now called JBUS.
The two protocols are compatible and there are many applications of the MODBUS protocol
produced by different manufacturers on the market.
The data exchange services offered by both protocols are the same, and so the terms
MODBUS and JBUS are used interchangeably.
Differences
- Addressing offset
To access object n, in JBUS use address n and in MODBUS use address n+1. There is an
addressing offset of 1 which must be managed.
- Processing capacity restrictions
Number of stations
Read n bits
Read n words
Write bits
Write words
Jbus
Modbus
255
2000
125
1968
123
247
1920
120
1920
120
To communicate with all devices, the lowest common denominator for each parameter must
be used.
- Each processor has a corresponding list of communication functions supported
However, the following basic functions are always present :
Functions
Write 1 bit
Write n bits
Write 1 word
Write n words
Read n bits
Read n words
05
15
06
16
01 & 02
03 & 04