So i recently got an Arduino mega, then I moved into a house with an old security system. I can't find the plug for the control panel, but I want to attach the control keypad to my Arduino that way I can use the original keypad to kinda rebuild the system.
Welcome to the forum.
So, what are your questions?
I recommend you have a look at the "How to get the best out of this forum". They are available in each sub forum. Good posts with required information tend to get a lot of attention in this forum. Include some information about yourself and your skill level.
If the keypad has a matrix of buttons with one connection per row and one connection per column, I would use the Keypad library.
it has 2 power lines and 2 data lines, so there are only 4 connections
That could be almost anything. Have you got a two-channel oscilloscope?
No, i don't have an oscilloscope or any fancy equipment. something that might help is that i'm using a mega 2560, and the alarm keypad is this. Ademco 6127 Keypad [Honeywell 6127] - $0.00 : AlarmSuperStore.com, Your One Stop Alarm Shop
That does help. The inside of one such alarm keypad shows that the two data lines for a remote keypad are IN and OUT. That means it's not a Wiegand interface that uses 0 and 1 lines for send-only data. My guess would be 'async serial' and all you would have to do is figure out the baud rate and message protocol.