I don't think  "It should be a quite simple thing to do " for you if you are the 1st time using Siemens CNC/PLC.

You need looking for the manual 808D (advanced or simple 808D?) --you need to understand 

the HW configuration to find out spare PLC Input (2 input bits) and you need to understand where should be the signals those to buttons going to . for this I can give you a quick tips:

DB3000.DBX0.7---Reset  and DB3200.DBX71.---NC Start (or Cycle start)

You need have PLC programming tool package to down/upload PLC project /program, and modify PLC program. 

Attention, when you have good back up. and starting the modify job.

according to my experience --using "cross refence " function , locating the 2 signals in the PLC program (I mentioned above) --as it is possible some condition s already exist. you need understand "OR" function in order Not affect your original button functions on standard MCP.

If you think it is not easy for you, I suggest to call your machine builder to do it for you  

However you can download manual in this web. key words "SINUMERIK 808D Manual", download all you can find 

Himpini

Hi guys, I need to add 2 Buttons on my Lathe working with Sinumerik 808D

One for Cycle Start and other for Reset, in order not to touch the ones from the MCP

I was checking that is possible to do it from the programming tool but I have some doubts

Is there any document to help me to understand a bit more how it works the PLC programming tools

Thanks

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Split from Output signals Sinumerik 808D.