Noise Rs232

KoyJa

Im having a problem with Noise coming into my circuit when I hook my BS2P40
to an external Serial Port. Im using a Maxim A/d for my measurments and it works
very good when the Serial connector from my PC is not connected. Im using an analog
switch to form the voltage divider for my measurements. The problem is Im getting
a 60HZ signal when I coonect the Rs232. The serial function work just fine, I can program
and receive data. I have put a scope on my analog switch and disconnected the RS232
lines 1 by 1, it appears the noise is coming from the common point·(Ground pin) from
the PC. I have thought about Opto Isolation but I already have Debug boards built from
expresspcb.com

I have attached parts of my schematic for reference. As you can see from the schematic
I do not have caps on the DTR line like it shows in the BS2P40-IC Mod Rev B schematic.

Any Help·would be appreciated.

Thanks

mikey

Bruce Bates

Mikey -

Does the computer have a 2-wire or 3-wire line cord plug on it? If it has a 2-wire plug, and it can be flipped around and reinserted, do so, and the problem will probably disappear. Using a 3 to 2 wire adapter on a 3-wire plug is can be the same as having a 2-wire plug especially if the ground tab is not connected. Same suggestion applies.

Regards,

Bruce Bates

KoyJa

Hello Bruce,

I did as you suggested, I reversed the plug (2-wire) and it did not work. So

then I unplugged my LapTop from the wall and ran off my battery and the noise

was removed. Thanks for your suggestion you hit the nail on the head. Now my

problem is trying to remove the noise when the unit is plugged into the wall, hopefully

I can remove the noise with minor mods and not respin the baord.

Thanks again

mikey

steve_b

You can work this a couple ways now that you know where the noise source is....

You can work some filtering to get rid of the 60HZ....RC filters are the easiest to play with.

You could also set up some opto isolation....there would be no connection from one end to the other.

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Steve
http://ca.geocities.com/steve.brady@rogers.com/index.html
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

KoyJa

Steve,

Im looking into the Opto solution .... But I have 25 Boards I need work through. Im going to try
an in-line RC Filter to see if it does the job for these 25.
Next time I will not develop with my LapTop unplugged.

Thanks

mikey

KoyJa