Anyway to find voltage on Serial Cable wire?

[Stojakapimp]

I'm trying to rig a speaker that will sound off when the Data Carrier Detect line on a serial cable is trasmitting data. I'm making this circuit by using an nmos, but I need to know what the voltage is on the DCD line that will be feeding into the base of my nmos.

Now I don't know much about serial cables, but is there a typical voltage? Or does it depend on what source is trasmitting? Or am I just totally in the wrong train of though?

 

[CTho9305]

A serial cable is some pieces of metal, and can have any voltage on it from 0 to probably a few thousand volts. Assuming one end is hooked up to something like a computer or modem though.... http://www.google.com/search?q=serial+voltage+rs232&btnG=Search&hl=en&lr=&ie=UTF-8

 

[Stojakapimp]

ok, so then it's dependent on what is trasmitting the signal. Am I just going to have to go in there with a multimeter and measure it? That's what I was going to do but I wanted to try to avoid all that

 

[CTho9305]

Originally posted by: Stojakapimp
ok, so then it's dependent on what is trasmitting the signal. Am I just going to have to go in there with a multimeter and measure it? That's what I was going to do but I wanted to try to avoid all that

Yes... but if you know either what's transmitting or what's receiving, you can find the voltages that are used.

 

[FrankSchwab]

If you read the RS-232 spec, the voltage on the line can be +5 to +15V for a high level, and -5 to -15V for a low level. In the PC world, as a practical matter, most RS-232 ports put out between 10 and 12 volts for a high level, and -10 to -12V for a low level.

Note that +12 V is the non-asserted state, and -12V is the asserted (logic 1) state.

Note also that you are limited on the amount of current you can pull from one of these pins. IIRC, it's around a few milliamps - enough to light an LED, but not much more.

/frank