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

[Bob Shanteau]

Bob Shanteau wrote:
> ... 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.
> ...
> 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?

To partially answer my own question, I just tried hooking up various 
size resistors across the RTS and GND of the serial port of my desktop 
computer (an HP DX5150 with a reported 16550A UART). The drop in voltage 
does indeed depend on the resistance and the resultant current draw:

 10K   6.7V  670uA
 25K   8.2V  328uA
 50K   9.1 V 182uA
100K  9.6V  100uA
200K  9.9V   50uA
400K 10.2V  25uA

So the voltage regulator must have a low quiescent current and the 78L05 
voltage regulator specified for the homebrew IR receiver will not work 
on this serial port.

It's interesting that Thomas Breuer specified an LP2950CZ5 in his 
circuit design <http://www.lirc.org/PCB.pdf>, the datasheet for which is 
here <http://www.national.com/ds/LP/LP2950.pdf>. It has a typical 
quiescent current of 75uA. I have ordered some from Digi-Key and will 
let you know how they work in the homebrew IR receiver circuit. (Anybody 
want some slightly used 78L05 voltage regulators?)

That still leaves the question of why there is almost no voltage on DTR 
and TXD. Anybody hazard a guess?

Bob Shanteau