Will my MODBUS RTU work with SIEMENS on this way

I' have done following inside my C application and works fine. I'm able to read data from MODBUS RTU Slave.

I would like to do the same inside siemens PLC in order to communicate with some MODBUS RTU slave.

This is a procedure:

1. Ensure port is open
2. Build outgoing modbus message according MODBUS rules
3. Send modbus message to Serial port
4. Listen and read incoming message on port
5. Evaluate reveived message

So I would like that my siemens S7-300 is master without any additional software or hardware to buy except PLC.

Will this be enough and why not if so?

S7300 doesn't have an open port so you need cp for sure. The driver you can maybe avoid by doing all the scripting..

In s7 200 you can do it nodriver no cp, in s7 1200 no driver but cb for rs485.

S7 300 is the worst for modbus...

m_turk said:

S7300 doesn't have an open port so you need cp for sure. The driver you can maybe avoid by doing all the scripting..

In s7 200 you can do it nodriver no cp, in s7 1200 no driver but cb for rs485.

S7 300 is the worst for modbus...

Click to expand...

Pretty much agree with this. The different Siemens platforms over the years have very different requirements for modbus, which Siemens never really embraced.

The reason you want to avoid the 300/400 is because you need a license (maybe a dongle, too?) for the modbus driver, even for modbus TCP. For the newer platforms, 1200/1500, there is no license required. Never tried in the older 200s, but I'll take m_turk's word that it isn't needed there.

mk42 said:

Pretty much agree with this. The different Siemens platforms over the years have very different requirements for modbus, which Siemens never really embraced.

The reason you want to avoid the 300/400 is because you need a license (maybe a dongle, too?) for the modbus driver, even for modbus TCP. For the newer platforms, 1200/1500, there is no license required. Never tried in the older 200s, but I'll take m_turk's word that it isn't needed there.

Click to expand...

ET200S comes with 3 profinet interfaces. What if I built modbus message according modbus TCP/IP rules and I just send/throw message on the 1 of 3 interfaces. Will slave respond?

1.So function No.1 send bytes to an profinet interface .
2. function No.2 Listen/ read incoming bytes(message from slave) on interface.
3. function No. 3 -> handle received message(from Slave).

naturephoenix said:

ET200S comes with 3 profinet interfaces. What if I built modbus message according modbus TCP/IP rules and I just send/throw message on the 1 of 3 interfaces. Will slave respond?

1.So function No.1 send bytes to an profinet interface .
2. function No.2 Listen/ read incoming bytes(message from slave) on interface.
3. function No. 3 -> handle received message(from Slave).

Click to expand...

The IM 151-8 has three ports, only one interface ( only 1 IP address). It is a built in switch, for lines/rings.

If you want to write your own modbus TCP blocks, based on the TSEND block family, that should work. Not a trivial task, though.

I've done some tests programming modbuc tcp from scratch with a S7-315 with profinet interface.

You use siemens' blocks to open and close ports (sockets) and to send and receive bytes. You need timeouts and stuff like that to make it robust because it needs to be able to handle transmission errors and interrupted links.

Modbus TCP and modbus rtu is an almost identical protocol. I haven't tried communicated over serial port on S7-300 though but I assume it's similar, well probably easier.