Register to reply

RS232 Protection Circuit

by DailyDose
Tags: circuit, protection, rs232
Share this thread:
DailyDose
#1
Mar9-12, 12:19 PM
P: 25
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
File Type: pdf RS232_Protect_1.pdf (12.2 KB, 22 views)
File Type: pdf RS232_Protect_2.pdf (6.7 KB, 17 views)
Phys.Org News Partner Engineering news on Phys.org
Tricorder XPRIZE: 10 teams advance in global competition to develop consumer-focused diagnostic device
Study shows local seismic isolation and damping methods provide optimal protection for essential computing equipment
New filter technology uses inert gas to bore holes in high-quality steel
berkeman
#2
Mar9-12, 12:33 PM
Mentor
berkeman's Avatar
P: 41,085
Quote Quote by DailyDose View Post
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.
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
#3
Mar9-12, 01:03 PM
P: 25
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
#4
Mar9-12, 01:05 PM
Mentor
berkeman's Avatar
P: 41,085
RS232 Protection Circuit

Quote Quote by DailyDose View Post
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?
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
#5
Mar9-12, 01:08 PM
P: 834
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
#6
Mar9-12, 01:58 PM
P: 25
@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
#7
Mar9-12, 02:01 PM
P: 834
Quote Quote by DailyDose View Post
@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.
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
#8
Mar9-12, 02:14 PM
P: 25
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.


Register to reply

Related Discussions
Circuit Breaker Ratings and Electrical Protection Electrical Engineering 1
Help with prototype-circuit protection Electrical Engineering 0
I need help designing an overload protection circuit Electrical Engineering 6
Short Circuit Protection Diagrams Electrical Engineering 2
Diode protection circuit help Electrical Engineering 3