Serial Port Bug?

Mikkel L. Ellertson mikkel at
Tue Nov 27 17:01:03 UTC 2007

Tim wrote:
> On Tue, 2007-11-27 at 08:48 -0700, Karl Larsen wrote:
>>     With a volt meter I find some pins have a constant -10.7 volts on 
>> them which is the standard RS-232 for a no-data signal. In this static
>> measure the voltmeter sees no positive volts near +12 volts. 
> NB:  It's quite common for serial ports on computers, these days, to be
> nothing like "standard RS-232".  That can be a problem for some devices
> that you want to connect to it.
> I'd be trying some tests by connecting two computers together, and
> seeing what you get.  Or plugging in an old modem, and doing the old ATI
> command tests, etc.
I thought the standard for RS-232 was +/- 3-15 volts, with a
requirement that the device be able to handle +/- 25 volts. Even
though you may have 15 volts on one end, you have to allow for
voltage drop on long runs.

I am used to seeing +/-12v and sometimes +/-5v depending on the
supply available. Depending on the state of a port, I would not be
at all surprised to see all negative voltages to ground. Remember, a
negative voltage is a logic 1, so the status and data line will all
be negative when the port is ready. (TD, DTR, RTS)


  Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!

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