RS232 Protection Circuit

DailyDose

I have attached two different schematics of circuits that I thought would be capable of protecting RS232 signals. The second one (RS232_Protect_2) I feel is all I would need to protect the RS232 chip from any mayhem that may occur on the actual device side. But, someone referred me to the first one (RS232_Protect_1) and I honestly don't understand it all. Would anyone be willing to give me an explanation as to why the first one may be better than the second and also perhaps an explanation as to how it works. Thank you.

Attached Files

	RS232_Protect_1.pdf (12.2 KB, 19 views)
	RS232_Protect_2.pdf (6.7 KB, 14 views)

berkeman

Both circuits appear incorrect. The 2nd one with the series diodes is just plain wrong, IMO. And the first one is ignoring the fact that RS-232 signals are bipolar (symmetric about ground), so you can't return your clamp diodes to ground....

DailyDose

Thank you for quick response. The second is meant to protect from any shorts...which I believe is really the only main concern. But the first, I have no idea what it is doing. Why is second wrong?

berkeman

You can't put diodes in series with signalling lines. The drivers need to be able to drive the lines both high and low (or negative in the case of RS-232).

In the second circuit, you tell us what is wrong. Check the drive voltage levels of RS-232, and compare those to the voltages being clamped to by the diodes...

DragonPetter

DailyDose, are you able to purchase an RS-232 Transceiver that has these features built in? If so, that is your best solution.

There are some tricky pitfalls in trying to protect an RS-232 pin in the case of short circuits and shorts to high voltage because the voltage swing is so high and has to be able to go to + and - directions. To sustain a short to high voltage for a long period of time can blow up a normal diode protection circuit, and current limiting resistors need to be very high watt rated. The only transceiver I've seen come close to having this kind of protection was made by Linear Tech.

Do you have specific requirements? I built and tested an active protection circuit that would protect from short circuit and short to 34V DC before. I can provide you with a schematic of that

DailyDose

@Berkeman, you're right the second is ridiculous. But the first, I got a decent explanation as to its purpose and it is protecting transients after dsp. I believe you were referring to right before comm w/ PC.
@DragonPetter, I would greatly appreciate that schematic if you are willing.

DragonPetter

I will be able to share it in a few hours. But you did not tell me, what are the exact specifications/requirements that you are protecting for? My circuit is to protect against short circuit to 30+ volts for over a minute (it actually can protect indefinitely).

DailyDose

Your protection may actually be more than necessary for our applications. But 30V for a second I would say is bare minimum...least that is what first design is capable of.