Hello guys!

I have searched an information for my question, but haven't yet found a clear answer.

Technical sheets of SM 1221 (8 in) says, there are 2 isolation groups in the module. So If I need an input connection from an external automation system, am I able to connect external + 24 VDC to the inputs of the SM1221 module and the negative side of the external power supply to the 1M or 2M of the SM1221? 

I have 1214 DC/DC/Relay as a CPU.

eltcam

Hello guys!

I have searched an information for my question, but haven't yet found a clear answer.

Technical sheets of SM 1221 (8 in) says, there are 2 isolation groups in the module. So If I need an input connection from an external automation system, am I able to connect external + 24 VDC to the inputs of the SM1221 module and the negative side of the external power supply to the 1M or 2M of the SM1221? 

I have 1214 DC/DC/Relay as a CPU.

 Hello,

put both M (negative) of the two systems toghether

Hello,

in my opinion is this not a good idea to wire the external signals direct to the PLC.

I would connect the 2 machines with optocoupler .
e.g. like this from Phoenix Contact

Solid-state relay module - PLC-OPT- 24DC/ 48DC/100/SEN - 2900358

If you use this coupler you have no problems with different potentials, current over the Ground connection, overvoltage  and more.

This solution is not the cheapest but safe.

regards
CaOd

Hi eltcam,

in priciple it is possible to connect the M of your external system to 1M ( for Input 0-3) or 2M (for Input 4-7). But you have to be aware that then you can use all of the 4 Inputs only for the connection to the external system. It's no possible to use e.g. one or two inputs for the connection and the other inputs for different purposes.

But I agree with CaOd  It's always the best solution to you use opto-couplers or relays to have an galvanic isolation between the two systems.

Regards

Thank you very much for your prompt answers.

Yes I thought to use the whole module separated for that certain external signal. But that sounds a safer way to use the galvanic isolation.

Maybe also for the SM 1222 relay output module the galvanic isolation is also better, even though there are relays already in the module. At least it saves the module, if there happens something bad with the external signal...     What do you think?

Hi eltcam,

originally you asked for the SM1221. This is an input module. And as I unterstood you want to receive input signals. For this the proposed solution with opto couplers would be right.

Now you are asking for SM1222, but this an output modul. Is this a new question?

Or is your external device also a S7-1200 and you want to use the SM1222 to "send" Signals to your S7? In this case the SM1222 would be a solution too.

Regards

I am sorry of my obscure question.

Yes I have planned to use both input and output modules for external PLC system, other than Siemens, but with the voltage level of 24 VDC.

I was not sure is it possible at all to use direct connections to input module SM1221. Direct connection to output module SM1222 (relay) I had thought it is not a problem but when I got these good advices about galvanic isolation, I also started to doubt, maybe it is not good to connect the SM1222 outputs direct to the external system either, even though the outputs are relays. So I wrote that also here thinking if I'll get some thoughts, is there a certain practice for that kind of cases...