[E3-hacking] Another hello

Ian Stirling ian.stirling at mauve.plus.com
Thu Jan 19 17:03:10 GMT 2006

Cliff Lawson wrote:
> I'd have thought the protection was vital. It's not so much whether the PC
> can detect the "weedy" 0V..5V swing coming out of an E2/E3 but whether the
> UART input buffers on the OMAP/Sharp can stand getting -12V shoved up them
> coming from the PC (I'm assuming that PCs still transmits true RS232 even if
> they're a bit more lenient about what they can detect?)
> (Or maybe my h/w designing colleague did splash for sufficient inbound
> protection on the board - I can't remember off hand)
Sorry - I was just addressing the fact that it might work.
It's a very bad idea - as you say, 20ma is not a nice current to shove 
into a nominally 5V port.

But, along with the trend to lower threshold voltages, has come more and 
more things with rather weedy outputs, rather than 'proper' 12V/20ma.
And many 'TTL' level ports won't immediately, or even soon die from a 
12V signal limited to 20mA.
A simple resistor will limit the current, and would be good enough for 
some prototyping, but be a bad idea for a number of reasons in production.

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