And that's before you deal with the rarity of an interface card that's literally impossible to purchase at any price today, so you better hope someone has saved spares.
Typically folks will go looking for these services after the device has failed :(
That said, as long as it works...
See, the situation was that IT management, who officially had the responsibility for the PC saw it as a risk because we in IT were unable to do anything if the system died. But the production line management, who had to pay for the upgrade, saw no reason to do it because it was "working fine!".
Sure it was working fine, and had been for almost 20 years. But WHEN it would inevitably give up the ghost they would be completely hosed.
I would also assume that the PLC just gets some commands from the master.
Granted, these might be too many assumptions.
Sniffing a protocol to reverse engineer stuff, is like trying to learn a new language by listening to conversations of a ground-level monitor and a crane operator.
Yep.
Modern USB/TCP connected PLCs just encapsulate RS-232 data into USB or TCP packets, nothing else (for backward compatibility). Also there are encapsulation and decapsulation devices which enables archaic PLCs and other industrial devices to be retrofitted to more modern systems.
Moreover, the OP noted that emulation speed was at critical importance so, the code is CPU speed sensitive. Missing specs, missing company, missing source plus time sensitivity makes this endeavor very risky if not hard.
Back in 2012 I worked at a large manufacturing company. They had an old IBM PC running IBM DOS which controlled a vital system at the beginning of their production line. This machine had been chucking along since 1992 and now someone high up had been made aware that it could be a potential risk if this machine stopped working. It was therefore decided that this PC should be replaced/upgrade if possible.
After investigating I learned it communicated with a PLC through the RS-232 interface and ran some special sauce software from a company that stopped existing in 1995. Previous upgrade attempts involved virtualization, but that did not work since the program ran too quickly on modern hardware.
My solution was to copy everything from the old HDD to a new computer, install DOSBox and configure the serial port. The first few attempts caused some sirens and alarms to go off in the building, which was "exiting", but after fiddling with the emulation speed I managed to get it to work.
To this day, unless something drastic has changed, a billion $ company is running DOSBox in production (and I literally mean production).
Also, a side note. The old PC was still connected to a modem with a dedicated phone line. At some point it had been remotely operated through some kind of Norton remote control software. I made sure to turn off the modem and did not bring it over to the new DOSBox setup. Imagine if someone had war dialed into the machine... They could have caused a major disturbance and potentially started a fire.