[mythtv-users] LIRC initialization of serial port in MythDora 12.23

[Ozzy Lash]

On Wed, Apr 21, 2010 at 4:56 PM, Bob Shanteau <rmshant at gmail.com> wrote:

> Many years ago on another list, Thomas Breuer wrote:
> > The lirc_serial driver obviously sets RTS positive and DTR and TXD
> negative. So we can get a positive (as used in the standard circuit) and a
> negative supply.
>
> That's not what I am seeing on the serial port of my desktop computer
> running MythDora 12.23. With LIRC set to use the the lirc_serial driver
> (labeled COM1/Iguanaworks) and nothing connected to the serial port, what I
> see is that as MythDora boots, the voltage on the RTS line changes from
> -10.5V to +10.5V (probably when LIRC initializes the serial port). And when
> I connect a homebrew IR receiver <http://www.lirc.org/receivers.html>, the
> voltage on RTS changes from -10.5V to +4.5V, which is too low for the
> circuit to work. Also, both DTR and TXD are near 0V in both cases.
>
> I was impressed by Mr. Breuer's design that uses TXD as a negative voltage
> source <http://www.tb-electronic.de/vdr/lirc/lirc_rx.html> and would like
> to use it, but I cannot as long as the voltages on DTR and TXD are
> negligible.
>
> How do I get lirc_serial to set RTS positive and DTR and TXD negative? And
> how do I keep the voltage on RTS from dropping?
>
> Another data point is that an IR receiver from <irblaster.info> works on
> the same serial port. It uses only 4 components: the IR receiver module and
> 3 surface mounted parts, a voltage regulator labeled M2954-03WS <
> http://www.micrel.com/_PDF/mic2954.pdf>, a component labeled 472, most
> likely a resistor, between RTS and the output of the IR receiver and ground
> and an unlabeled component, most likely a capacitor, between GND and the
> output of the voltage regulator. Any idea why this circuit does not cause
> the RTS line to change to +4.5V whereas the circuit I built using a 78L05
> voltage regulator does, even without the IR receiver in the circuit? Might
> it be that the M2954 is labeled as a low-dropout voltage regulator with a
> low quiescent current (120uA typical)  vs the 78L05's draw of 5.5 mA
> typical?
>
>
Are you sure you wired the voltage regulator correctly?  the 4.5V sounds
like you may have the RTS line connected to the output of the regulator
instead of the input.  I can't think of any reason why TXD would be near 0,
since it is an output of the serial port, it should always be either low or
swinging between low and high when sending data.   The IR receiver from
irblaster.info that you mention sounds exactly like the home brew one that
you built with the exception of using a different regulator and not having a
protection diode on the RTS line.  the 472 part is a 4700 ohm resistor (4.7
kohm).
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