Thread Starter

A Hull

I have a project in which there are some protection relays, they have an RS485 port which
talks J-BUS, I am led to believe that this is a sub set of MODBUS.
Will a MODBUS Master talk to a J-BUS slave ?
I have asked the manufactures but they are less than helpful !!


Yosef Feigenbaum

The only difference, to the best of my knowledge,
is that J-bus allows addresses of 0 (ie. Register
address 40000) while Modbus starts at addresses of 1 (ie. Register address 40001).

(8{)} ( .)
J-bus is a subset of modbus and a modbus master will communicate with a j-bus slave, but only using the subset instructions

Kenton Foster

J-bus uses has a superset of address over Modbus. Keep your j-bus device addresses in the Modbus device address range and your Modbus master should be able to talk to it. Some Modbus master devices can use the j-bus address range.
I have made this communication with a S7-400 and the CP441-2 with the Modbusmaster and that is working very good. The only you have to take account with is that adress 0 is used.

Best regards

Carlos Araya Mc.

Yes, I agree with Lowiek. I made a system with J-BUS, RS485 for Merlin-G relays (SEPAM 2000), using a normal MODBUS master driver in a PC.
May be you need to add 1 to any address in the memory map (address 0), but you need to check this conecting the device to your master.
Good luck!!

Carlos Araya Mackenney
Automation Manager - PLC INTERNATIONAL S.A.
(562) 269.8727 - Fax (562) 269.8728
Santiago de Chile
Title: Differences between JBUS and MODBUS protocols<br>
Number: 370

<p>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.

<p>The data exchange services offered by both protocols are the same, and so the terms MODBUS and JBUS are used interchangeably.


<p>- Addressing offset

<p>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.

<p>- Processing capacity restrictions

Jbus Modbus

Number of stations 255 247
Read n bits 2000 1920
Read n words 125 120
Write bits 1968 1920
Write words 123 120

<p>To communicate with all devices, the lowest common denominator for each parameter must be used.

<p>- Each processor has a corresponding list of communication functions supported. However, the following basic functions are always present:


Write 1 bit 05
Write n bits 15
Write 1 word 06
Write n words 16
Read n bits 01 & 02
Read n words 03 & 04