So, reversing it was sort of all over the place. I first had to reverse the front desk system and all that; that was primarily done by sitting between the equipment with a serial proxy and working from there. Once I had a good bit of data captured, I'd write software to emulate being one side or the other. Everything is RS232 and RS485, pretty straightforward.

In terms of reversing the actual lock protocol, that was a bit more tricky. First step was tapping the line between the portable programmer and lock with an o-scope (a 70s-era HP scope; only thing I could afford at the time, haha) to figure out the voltage levels involved and the basic properties of the communication. From there, I hit it with the Saleae Logic to see what the communication actually looked like.

From there, I wrote some Python scripts to walk over the data from the logic analyzer and attempt to decode the data. With some tweaking, I managed to finally see some data that I knew, specifically the site code (which I knew from other parts of the system).

After I knew all that, it was a matter of figuring out the actual hardware level. Given that I have effectively no experience with this level of things, this was a lot of asking questions, googling, and experimenting. I knew that it was a one-wire protocol, so by reading up on other one-wire protocols I managed to figure out a lot. Once that was done, everything just fell into place; making the opening device work initially took maybe a day given all the info I had.