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

[Bob Shanteau]

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?

Bob Shanteau