Many (we won't get into HOW many) years ago I worked with original Data
General Nova computers.  They really cheaped out and used standard TTL gates
(high around 4.7V, low around 0.3V) for the "rs232" interface.  It mostly
worked.  Palm uses a similar cheat to this day, with CMOS drivers and
receivers.

Ever since those days, I have tried very hard to make sure my rs232 outputs
were strictly kosher, and that my inputs were tolerant of anything remotely
resembling a legal signal level.  And NEVER count on getting any useful
power from the other end of the wire.

It's sooooo easy these days with cool chips like the MAX232 that I wouldn't
waste a minute considering any other way.  Maxim and other vendors have
whole families of teensy, low cost, easy-to-use, well-engineered chips that
really do the job right.

Figure out a good way to slip a little power past the isolation to a little
island containing the RS232 interface, then isolate it at the logic-level
interface with the optos or whatever you like.

> -----Original Message-----
> From: Robert Rolf [mailto:Robert.Rolf@UALBERTA.CA]
> Sent: Thursday, June 14, 2001 11:13 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Serial port drive specs. Noncomliant devices?
>
>
> Hi Gang,
>
> I'm using an opto isolator on my RS232 comm lines to a PIC system.
> I have discovered that many portable computing devices DO NOT MEET the
> RS232 spec in that their maximum output is 5V, rather than the 12V
> of the spec. Since I have to design for 'worst case' has anyone
> encountered devices that put out less that 5V and 10mA when
> signalling, or more than 12V and 30mA of the spec limit?
>
> TIA
>
> Robert
>
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