S7-300 comms with another device over Ethernet

That looks like a modbus TCP to modbus RTU gateway?

The S7300 requires a license for its modbus TCP blocks. I think the block exists under Communications -> Other -> Modbus TCP, but you need to contact your Siemens rep for a license for each CPU.

In the newer 1200 and 1500, modbus TCP is free.
 
mk42 said:
That looks like a modbus TCP to modbus RTU gateway?

The S7300 requires a license for its modbus TCP blocks. I think the block exists under Communications -> Other -> Modbus TCP, but you need to contact your Siemens rep for a license for each CPU.

In the newer 1200 and 1500, modbus TCP is free.
Click to expand...

Under Comms -> Other on my TIA Portal I have the option for MODBUSPN.

In the description it says "Communicate via Profinet as Modbus TCP server or client".

Is this the block you mean, or is it the MB_SERVER block which doesn't appear?

Thanks
 
Last edited:
MODBUSPN - Modbus TCP comms for S7-300, S7-400 over integrated interface
MODBUSCP - same but for comms over ethernet CP.

MB_SERVER and MB_CLIENT is Modbus TCP communication blocks for S7-1200, S7-1500 PLC
 
OK so I have started to try and get this up an running but not having much luck.

For testing I have actually setup a red lion DSPSX with the ethernet protocol MODBUS TCP/IP Slave.

What I cannot figure out is what to do with the MODBUSPN instruction. Siemens have sent me an example (https://support.industry.siemens.com/cs/gb/en/view/75330636)

but what is confusing me is the values in the DBs. They all have a default start value so where are the values I need actually coming from. Am I just supposed to change the start value to the value I want?

For example, the start value under the SIMATIC 300(Client) then DB1 for the tag START_ADDRESS I assume is the address of the holding register I am pointing too? So would I set the start value to 400001?

Thanks!
 
Start address should be memory address. For 400001 it should be 0.
Modbus address is memory address + offset. In case of holding register offset is 400001.
If you want to read address 402001.
Data type should be 3 (function 3 - read holding register)
Start address 2000
Length 1 (request for 1 register)
 
marcius said:
Start address should be memory address. For 400001 it should be 0.
Modbus address is memory address + offset. In case of holding register offset is 400001.
If you want to read address 402001.
Data type should be 3 (function 3 - read holding register)
Start address 2000
Length 1 (request for 1 register)
Click to expand...

Thanks - do you have an example of one of these blocks you would be willing to share?
 

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