Modbus General Description

smiffy

A lot of Modbus issues result from a general lack of understanding of the Modbus protocol and how it functions.
So, I have produced this General Description document for the Modbus Protocol.
This is a generic guide to the modbus protocol and is not intended as a replacement for the specific Modbus driver manuals supplied from IIA.

 thanks smiffy,

its really helpful

Thank you very much for the documents.

Sharing my experience.

For S7 200 I have to read input registers from clients. And the register number given are in range of 300000 instead of 30000.

I was confused how to do this and after the research I found that 30001 is same as 300001 as long as it does not exceed 39999. Extended address is used if you have to exceed 9999. In this case you can use upto 65535 registers.

So I configured 30700 instead of 300700 (for example) given in document, which worked perfectly.

This is the extended addressing of modbus. Important note: First digit is the type of data and rest digits indicate the address.

This is for S7 200 but I wonder how it is used in S7300 and S7400 because i saw 105535 in the Modbus general description document attached with this forum.

Thanks smiffy.

it's really help 

Please find the following documents which are helpful for Modbus

Art Science RS485 by BOB PERRIN