Creating a powered serial bus using Rx and Tx

Hey,
I'm looking for some help with serial connection. I have a design constraint where I need to have 2 way serial communication with another Arduino, but it needs to be done over only two wires. I've been contemplating this concept for a while now and I am still drawing blanks. Can anyone explain how this concept works or how I can build something that will perform this function?
Thanks,
Will

Are you asking how to bet both power and comm over only two wires? Or comm on only two wires (without a separate ground) to an otherwise powered device?

If I ignore the rather misleading subject of the entry I'd say that an RS-485 line would fulfill the requirements mentioned. Only two wires needed, the possibility of two way communication (although not concurrently but that wasn't explicitly requested) and the communication is serial.

If that's not what you're looking for describe in more detail what functionality you need and what constraints you have (and why you have them).

I am currently in the process of constructing a fire alarm control unit for experimentation and I am looking to use a 2-wire serial connection because of NFPA (National Fire Protection Association), ANSI, and UL requirements for life safety systems. I currently am using a 4-wire system which gives the initiating device power and data on two separate sets of wires. I am trying to reduce this to 2 wires to meet the code specs. The way real fire alarm systems work is that each device is polled by the control panel every so often. The panel will ask it what condition it is in by sending the character 'P' and the device will reply with a character that corresponds to the condition of the device. 'D' for dusty, 'A' for alarm, and 'T' for trouble (also if the panel gets no reply, it goes into trouble). The way the circuit works is that the 2 wire SLC (signaling line circuit) alternates between power and data at a very rapid rate. What I am looking to understand is how I can do that with my arduino. I have already done the programming and what not; however, I need help constructing a circuit capable of both powering and communicating with the other arduinos on the same pair of wires.

Also apologies about the misleading title. It sounded good at the time but I was not very specific.

Well, if there's a standard protocol, then there is surely standard driver circuitry, and you probably have to adhere pretty closely to the standard to meet the requirements. I got nothing, though. Do these standards have a name/designation?