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???
11/27/07 06:08
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 		 #147439 - So problem is in "modern" PCs?!
Responding to: ???'s previous message
Another non-clear situation:
1)Suppose user device is powered , serial output is idle at about minus 8V. PC is not powered. Device is connected to PC.
2)Then PC is turned on and , suppose, +5V and +12V are rising faster than -12V. At some moment +5V and +12V are stale, and negative supply is -4V.

Now we have overvoltage on RX pin at PC side. Datasheet for popular buffer 75188 says that protective diodes are needed between +-12V supply and supply pins of buffer.
On "modern" serial PC cards these diodes often are
not mounted, not even routed. I dont know about motheboards, but maybe situation is same.
So we have computers with serial ports to attach serial mouse,
but these ports cannot work with standard RS232 devices - serial
port MUST handle as normal input voltages +-25V.
In particular case of OT maybe current limiting resistor of 1000 ohm will help.

regards

List of 60 messages in thread
TopicAuthorDate
PC COM Ports Damaging !!!            01/01/70 00:00      
   Earth loops?            01/01/70 00:00      
      Why Now and this circuit ?            01/01/70 00:00      
   Damage?            01/01/70 00:00      
      I am Sure ...            01/01/70 00:00      
   Common mode            01/01/70 00:00      
      You haven't mentioned            01/01/70 00:00      
   do you have the shell of DB9 grounded?            01/01/70 00:00      
   Serial port damage            01/01/70 00:00      
      for serial port damage            01/01/70 00:00      
         No            01/01/70 00:00      
      No, that can't be right!            01/01/70 00:00      
         re:3.3V            01/01/70 00:00      
            are you sure about that, Andy?            01/01/70 00:00      
   Are You SURE?            01/01/70 00:00      
      I'm sure Dear ...            01/01/70 00:00      
         not to be argumentative ... but ...            01/01/70 00:00      
   check it            01/01/70 00:00      
      Checks            01/01/70 00:00      
         We haven't seen how the DB-9 is wired            01/01/70 00:00      
            It\'s probably not a DB-9!            01/01/70 00:00      
               sure, but            01/01/70 00:00      
               Sloppy terminology            01/01/70 00:00      
   My Burned out ports            01/01/70 00:00      
      What?            01/01/70 00:00      
         What I mean is            01/01/70 00:00      
   a common misunderstanding is            01/01/70 00:00      
      So problem is in "modern" PCs?!            01/01/70 00:00      
      So problem is in "modern" PCs?!            01/01/70 00:00      
         maybe i'm wrong            01/01/70 00:00      
   try with opto coupler and +9 volt supply            01/01/70 00:00      
      Back when folks cared whether things work ...            01/01/70 00:00      
      Assuming He does not have one            01/01/70 00:00      
         There is a way to be sure.            01/01/70 00:00      
            Standard            01/01/70 00:00      
               Power Supply ...            01/01/70 00:00      
                  Dwhatever9 shell connection to GND...            01/01/70 00:00      
                     about that shell-to-shell connection            01/01/70 00:00      
                        generally...            01/01/70 00:00      
                  Truth emerges            01/01/70 00:00      
                     developed?            01/01/70 00:00      
                  that\'s the problem!            01/01/70 00:00      
         HUH?            01/01/70 00:00      
      why??            01/01/70 00:00      
         Broken COM port suggestions            01/01/70 00:00      
            he is not wrong - if            01/01/70 00:00      
               ...he is not wrong... only said watch it            01/01/70 00:00      
                  BULL!!!            01/01/70 00:00      
                     Bull?            01/01/70 00:00      
                        yes, genuine male cow manure            01/01/70 00:00      
                           Eric, you are a sad man            01/01/70 00:00      
                           Amateurs and pros            01/01/70 00:00      
                              of course I do            01/01/70 00:00      
                     Well, it has to do...            01/01/70 00:00      
                        Kai, I now you know this stuff ... but            01/01/70 00:00      
                           Different CMOS design?            01/01/70 00:00      
   that's right, when you can not argue tha facts ...            01/01/70 00:00      
      My Dear Friends ...            01/01/70 00:00      
         look in the datasheet            01/01/70 00:00      
            Y E S . . .            01/01/70 00:00      

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