Re: BeagleBone Black doesn't boot without serial console (10.0 release)

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Fri, Oct 11, 2024 at 11:12:58AM -0400, Mouse wrote:
> >> I also infer that it is a logic-level signal, not an RS-232-level
> >> signal.
> > Yes, these are TTL-level signals.  I've never seen RS-232-level
> > signals in embeeded or developement boards (and if you connect theses
> > to a RS232 line you'll probably destroy your board)
> 
> Depends on the current-carrying capacity of the protection diodes.

Reading the schematic and data sheets, it seems that the BBB could accept
+-50mA on the console pins


> RS-232 specifies some kind of limit; I forget whether it's a minimum
> output impedance or a maximum current-drive capability or what, but
> there is a limit.  Provided the protection diodes on the input pin can
> withstand that current without ill effect, you should be fine.  (Fine
> from the point of view of frying the hardware, that is.  It still won't
> produce enough voltage swing on lines driven by the TTL end to satisfy
> RS-232's voltage constraints and thus won't work reliably unless your
> RS-232 receivers happen to have a threshold which falls in the
> logic-level voltage range.)

not only that, RS232 and TTL levels have reverse polarity.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--