Difference between Modbus & Jbus

S

Thread Starter

Sandeep Shroff

Dear List Members

What is the difference between Modbus & JBUS protocols? Are this protocol the same.

Where will I find more information on the JBUS protocol.

Thanks & Regards,

Sandeep
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Sandeep Shroff
Sr. Consultant
Remote & Embedded Technologies

rapidEffect (P) Ltd. 25, Napier Road, Pune 411001, India
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J

Jim Gosselin

You will find more info on Modbus, you can get info at Modbus.org on the web. Jbus is for the most part Modbus with a few added commands
 
B
MODBUS is a master / slave type protocol developed by MODICON.
It was further developed for the APRIL PLC 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

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

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

Rob Hulsebos

>- 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.
Athough true because it is specified as such in the Modbus spec, in practice many Modbus interfaces do not follow this rule, a minor source of irritation. But it makes it more compatible with Jbus ;-)

>- Processing capacity restrictions
>
> Jbus Modbus
>
>Number of stations 255 247
There are also Modbus masters that allow 255 slaves. Where
that limit comes from? It's not message-related. Probably Modicon PLC related, a long time ago...


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

I've seen many a Modbus slave that supported only 03 and 16.

In practice, I'd always check the manual of the device to find out which functioncodes are really supported and how many bits / registers can be accessed in a single message, irregardless of what the Modbus specification says.

Rob Hulsebos
 
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