sending RS-232 or RS-485 data via 2-wire

[wakanga]

guys just want to ask question, i know that if you want to communicate with a micro-controller on the field using RS-232 or RS-485 data you need to use serial cable right? how about if i want to use just a 2-wire common cable (lamp cord) to communicate to the micro-controller using RS-232 or RS-485 data? how do you do this? thanks

 

[Papabravo]

With RS-232 you have two single ended signals and a ground for three wires. If you want to use two then you have to have a mechanism for turning transmitters on and off on a single wire. This is difficult but not impossible.

RS-485 is differential and you could do this, but you need to estimate the characteristic impedance of "lamp cord" to determine the value of the termination resistor. At low speeds it won't make much difference, but at higher speeds reflections will become a problem.

 

[wakanga]

thanks for the reply,

with regards to the RS-232 signal, what kind of mechanism your are referring to? another MCU to control the transmitter on/off on a single wire?

with RS-485 how low is the baudrate speed your are referring? and how high is the baudrate? if the low speed is negligible for what i want to do,i might go that way..

 

[Papabravo]

One mechanism would be to have one end of the connection talk periodically, then guarantee to be off for some period of time. The other end would always be listening. Then after some small period after the last character it would have a chance to talk. If it had nothing to say then it could send a null meassage.

I think 250kbaud might be a reasonable stab at having to worry about reflections or not. Yes for higher speeds, and no for lower speeds. With reflections, you are worried about 1's changing to 0's and vice versa. It also depends on the legth of tha cable. Shorter is better.