First time subsurace usage surprises

[Linus Torvalds]

On Tue, Jul 31, 2012 at 3:39 PM, Lutz Vieweg <lvml at 5t9.de> wrote:
>
> Ouch, that's really weird. I cannot belive Suunto's engineers would
> bet on some current to be drawn out of a DTR signal pin... could
> be driven by CMOS tri-state gate, after all.

Do you see any other way to do a two-pin signal+ground thing out of a
standard serial line?

And I think this is why it has the 22k resistors. To make sure actual
current drain is minimal, and the resistors are supposed to be bigger
than any pull-up/down on the rs232 interface.

That said, I don't know what the *official* suunto cable did. I've
never seen the actual traditional serial version, only the USB version
(and with USB, you have actual power, and there's a real chip on
there, although it's just a normal serial chip, but at least you'd
have Vcc and GND line independently of the serial outputs from that
chip). See for example

    http://homepage.hispeed.ch/scubadiver/download/usb.pdf

for a USB version of the thing (notice how it doesn't use DTR, it just
goes to VCCIO)

Anyway, I'm not actually much of a hw person, but it all looks pretty
hacky to me. But if you want minimal electrical pins sticking out of
the dive computer, two is the lowest number you can do..

             Linus