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.