Re: EE question: convert RS232 voltages to RC servo
 

Benton Electronics Supply. Nothing like it in a hundred-mile radius.

Re: EE question: convert RS232 voltages to RC servo
 

Thanks Al.

After some research and some experimentation on three old PCs I had languishing here, I've determined that the RS232 control pins typically¹ output only about 8 milliamps when shorted. A simple 1N914 diode² is all that is required to turn that into a PWM signal suitable for driving a Jag or Vic motor controller³.


¹ warning: YMMV

² available at Radio Shack! 3 bucks for a pack of 50

³ I hope. I haven't looked at the waveform on a scope yet to see how crisp it is, nor tried running a motor controller with it.

Re: EE question: convert RS232 voltages to RC servo
 

Yeah, I'm pretty sure the west side of Chicago and the northwest side of Indy are over a hundred miles from them (at least if you're driving around the lake). That's why so many retailers allow online ordering--lets 'em get to customers who move too far away from the store.;)

Re: EE question: convert RS232 voltages to RC servo
 

With the very gracious assistance of Richard Wallace and various engineers and technicians in his motor lab this afternoon, I can now report that this works quite well indeed. Please note two things though:

1) Before doing this, check the short-circuit current output of the RTS pin of the RS232 port you plan to use. All the ones I tested were limited to approx 7 milliamps, but yours may not be. If not, add a series resistor to limit the current.

2) If using this simple diode circuit, note that the polarity of the PWM input to the motor controller becomes important. Don't reverse the polarity. I'm not sure the LED in the photocoupler would like that. To paraphrase a famous quote, measure twice (with your meter), and connect once.

More details later.

Re: EE question: convert RS232 voltages to RC servo
 

Just to be clear, which circuit were you using? I've been lurking in this thread and have been curious to the solution myself.

Re: EE question: convert RS232 voltages to RC servo
 

FigureH is what we used.

• Measure the open-circuit voltage and short-circuit current provided by your particular RS232 hardware.
  
  
• Size the resistor to limit the current if necessary.
  
  
• Be careful not to reverse the polarity on the signal to the Victor. RS232 voltage can be as high as 25 volts. Reverse polarity may damage the photocoupler LED in the Vic.

Re: EE question: convert RS232 voltages to RC servo
 

The zener would be useful only if there was a maximum voltage tolerable; that's not the case here. So Figure H would be fine IMHO (and seems like it is). R can be adjusted to limit current if necessary

Re: EE question: convert RS232 voltages to RC servo
 

If you'd like more information, I've just posted a more detailed explanation in a collection of PDFs here:

http://www.chiefdelphi.com/media/papers/2702

It includes circuit diagrams and complete source code for a demo app to generate the PWM signal with an old unwanted laptop or desktop PC.

Thanks to Eric, Don, Hugh, Al, and others who helped with the circuit stuff.