I'm attempting to rebuild a security system for a school department, and i'm stuck trying to find the best way for the main controller to communicate with the alarm panels (the panels with keypads and LCDs that are used to arm/disarm the system) that will be by each entrance. I will be replacing everything except the sensors (which are simple NC devices that open when tripped). I'm thinking the rs485 protocol may be the best, but i'm still looking for options. I'd like to reuse the existing wiring if possible. The existing wiring is 4 wires in a twisted pair format. On the old controller they are labeled +, -, RX, and TX, so I assume they are using some form of serial.
The primary concern is range. For the first installation there is only one remote alarm panel and its only 5 feet away from the main controller. For the other installations, the panels may be as far away as 600'. Does anyone know what method these alarm systems usually use to communicate over these distances? I imagine the manufacturer must use something that works in a wide variety of distances.
I also plan to use a Rasberry Pi running Apache as the main controller, so anything that can interface with the Pi directly would be great. If not, it would not be a problem if I have to put an arduino in the middle to facilitate communication.
NOTE THE RESISTORS AT THE BOTTOM OF THE SCHEMATIC.
The 120 ohm resistor is called a Termination resistor. There needs to be one of these at the two extreme ends of the bus.
The 470 ohm resistors are biasing resistors to keep the signal centered in the 0 to 5V range
RS485 will not work without the termination resistors to prevent reflected wave.
The 120 ohm resistor is called a Termination resistor. There needs to be one of these at the two extreme ends of the bus.
The 470 ohm resistors are biasing resistors to keep the signal centered in the 0 to 5V range
RS485 will not work without the termination resistors to prevent reflected wave.
The alarm panels are fairly dumb devices. They just need to transmit 7 digit codes when they are armed/disarmed, and receive 10 character names when a zone is faulted. I think anything over 1kbit/s would be sufficient.
EDIT: you added some more since I originally read the post.
The termination resistors for r485 were what were worrying me, since this use case wont match that model. There will be many (say, up to 5) devices that all connect back to a central point over varying distances. I was thinking that some type of rs485 hub would work.
They're called Termination resistors because they terminate the bus. In my experience they are only needed at each end of a "drop" and each end of the "trunk" . The trunk runs the length of the building and the drops branch out left and right.