I have a DSC 1864 House Alarm and I want to connect up my Raspberry Pi's GPIO to the Keybus (a serial data communication protocol used by the alarm).
I would say my knowledge of electronics is novice to intermediate.
The Keybus is a 4 wire serial data bus - +12V, Ground, DATA and CLK.
The clock runs at 1khz from what I've heard.
The Logic is 0-12V I believe (I tested with a multimeter since I dont have a scope and got 12.84v on the +12v and a continually changing 6-8V on both Data and Clk lines as to be expected.)
The Raspberry Pi has a 0 - 3.3V logic level, With anything above 2.8V being HIGH.
My question is how best to connect these two, with the Pi purely just eavesdropping on the keybus - I dont need to write to it, just read from it.
I'm concerned that if I dont have a high impedance circuit, that the Raspberry Pi will act as a drain on the circuit and disrupt communications between the alarm panels (bad!).
Use an Optocoupler - Not a good solution as I've heard they are slow and draw a lot of current on the transmitting side.
Use a logic shifter chip - Possibly, but I dont know how to find the right one for my needs, and it has to have high impedance so as to not disrupt the keybus circuit.
Use resistors as a voltage divider - This is the route I was planning on going. I calculated that the following:
Will give 3v at the Pi when DATA goes to 12v, but will fry the GPIO pin if the voltage goes above 13.5v.
It should provide 1.05mA to the Pi - I'm not sure if this is enough for input purposes.
Resources I've found while researching this:
Any help / suggestions here would be greatly appreciated.