I have come across these kind of systems. One such system was running on a “top secret” soft PLC written in c language. My guess is mostly it will be a Modbus PLC. Find the communication cable, connect it to your laptop, and use some data capturing tools or protocol sniffer which is freely available on internet. Copy paste the data packets and do some google research. You will get some clue. I did this technique to develop a communication driver for a top secret control panel and found it was Modbus. You need to do some reverse engineering…
If it’s Modbus, you can use some Modbus PLC programming software like Twidosoft from Schneider to go online. Mostly it will work. You can also contact softplc manufacturers like codesys. They can supply you the programming software. Good luck
http://softplc.com/products/comm/gateways/
You can contact Gateway manufacturers like chipkin. They have very good tools to help you.
http://store.chipkin.com/
https://www.airicom.ae/communication-gateways-and-protocol-converters/