Questions about Modbus registers

HamzaAhmed said:

Hi guys,

i am trying to program a Scada/HMI.
The values i will take them from PLC using Modbus.

In the manual they said:
This address is not an absolute address of a Modbus register. It indicates register offset from the beginning of the address space for registers of selected Type.

For example, Address (offset) 1 for a register of a Holding Register type will match to:

Protocol address 40000
Data model address 40001.

So i put the offset to 0 so the programe take the vaule from Register D0 and then the offset to 150 so it take the value from D150.

But i don't know how get the value from coils or outputs, i don't know how much the offset supposed to be to take the status of M0 for example.

can you help?
Thanks,

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tragically1969 said:

Which PLC and HMI/SCADA ?

The offset means that if you want register 0 you request register 1 on your SCADA/HMI, register 150 would be 151 etc.

You would need to change your ModBus function code to get access to other data areas assuming your ModBus Server (slave) will accept them

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lfe said:

Address 40001 or 400001, with 6-digit addressing, in the Holding Registers field, corresponds to offset 0, address 40000 does not exist.

The relationship between internal registers of your PLC, such as D0 or M0, and Modbus addresses must be explained in your PLC manual, there is no fixed rule applicable to all PLC

To access other Modbus address fields, like Coils, you should use the appropriate Modbus function

https://en.wikipedia.org/wiki/Modbus

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HamzaAhmed said:

Yes i understand the process now,, thanks a lot.

But what i should do now? search for what? converting the plc register to modbus or what?

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tragically1969 said:

Check what Modbus Function codes your PLC and HMI/SCADA support, if it doesn't support FC1 and FC2 then you will need to map the Mx registers to PLC data registers which will be available on FC3 (4xxxxx)

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HamzaAhmed said:

i didn't understand

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tragically1969 said:

Which bit don't you understand ?

Firstly you need to understand how ModBus works in its simplest terms, then you need to check the ModBus support in both your PLC and SCADA system as the PLC may not accept the requests for certain types of data from the SCADA system

ModBus masters use function codes to request information from slaves, have a read here:

https://www.csimn.com/CSI_pages/Modbus101.html

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HamzaAhmed said:

Thank you, i understand now the modbus
Just i want to understand how to communicate the Delta PLC registers with the modbus, i couldn't find any resources or ways for that.

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tragically1969 said:

I had a quick search and here is numerous YouTube videos shown Delta PLC Modbus setup, not sure if is the same model as yours though

I don't use Delta PLC's so i have no idea what their ModBus capabilities are so I cant help any more than i have, sorry

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