Modbus Addressing 0 or 1-Base (How to get Communications Working)
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Modbus Addressing 0 or 1-Base (How to get Communications Working)

Author JonH ...
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#1 | Posted: 23 Dec 2014 08:25  
Often when using Modbus products and Modbus masters the first hurdle is to get the Master to read the Modbus Tables. With many Modbus devices the addresses for the parameters start from 0, whereas the Modbus master addressing starts from 1. Therefore in this case it is required to add 1 to the Device table addresses when the matching configuration is required. When table addresses start from 0, we call this 0-based addressing. When the table addresses start from 1, this is called 1 based addressing.
Author JonH ...
Registered
#2 | Posted: 23 Dec 2014 08:27  
And this can be reverse also.
Author Simo S...
Registered
#3 | Posted: 7 Oct 2016 09:20  
I have a question relating to this topic.

I'm working on trying to get Fidelix CPU to communicate with SRC200MOD conroller. I've gotten so far that Im able to read most of the register from the controller. Unfortunately Im not happy with this but I want to read/write all holding registers.

Now I have been trying my modbus master settings back and fort with out any luck, for example, I've got it to work so that my first (holding)register starts from 0 and is 48 long. Now is I try to put there more than 48, my CPU reports communikation error. Also, I cant start the next register from 48, but only from 64, on top of that, the next register can only be 14 long, any longer will give me communication error.

All this I got by testing the communication setting.
Is there some rules that how long register can be read, or in how many (what size) the holding register must be read?
Author Jukka H...
Moderator
#4 | Posted: 11 Oct 2016 17:34  
The standard Modbus specification states (in strict format) that if the Modbus register does not exist, then the Modbus slave will need to send back "Illegal Data Address" message if the register does not exists. When using the Modbus master to read multiple Holding Registers, if the SRC-200 does not have all the registers as requested by the Master, it will return this illegal data address message. This is because SRC-200 follows strictly the Modbus specification.

To find the registers available in the SRC-200 controller, please look at the data sheet. Please note that on the bottom of the first page, next to the data sheet number states the controller firmware version number (e.g. V2.24) that the data sheet reflects. Versions from this number onwards have the registers specified in the data sheet. (Over the time new versions introduce more functionality and additional Modbus registers)
Author Puneet K...
Registered
#5 | Posted: 15 Dec 2016 06:04  
Thanks Jhon for sharing Modbus base address information, It help me to understand the datasheet.
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