I've successfully connected an outdated Graupner MC-15 to the parport of my outdated notebook with the 680Ohm resistor and 4.7V z-diode circuit. But no joy with my Athlon board. After some tests with ppdev running linux, I've found out that the interrupt generation fails. The PWM signal level at the "nautic modul" port swaps between 0V and 1.5V, that might be to low for the low active ACK line to return to logic low (ttl high).
First bugfix was a npn-transistor (BC547A) between ACK (collector) and GND (emitter), wired to the rc signal via an 10k resistor (on basis). The signal got inverted, althought it worked.
Final involves the existence of a charger/externel supply connected to the remote while simulating at home. My charger's output and of course my computer is connected with GND, this could lead to some trouble, if the rc will "close" the circle between the computer and the charger ... this circumstance legitimates an optocoupler to draw a line between the rc and the pc.
Notes on the circuit: I use the active low ERROR line as power source for the transistor. The current from ACK to GND will be about 2mA, leading to a basis current of about 2mA : 200 = 10µA from the ERROR to GND, this will not disturb the ttl high state of ERROR (which is given using ppjoy and ppdev@epp). The 100kOhm resistor R2 assures this. The value of the input resistor R1 depends mainly on the high level voltage and strength of the PWM signal. The emitter of the 4N28 needs about 1V and 100µA to light up the receiving photo transistor. The current through the emitter can rise up to 60mA, but that might be to much for the comfort of rc's PWM signal driver. R = (U_PWM_HIGH - U_EMITTER) : I_EMITTER = (1.5V - 1V) : 100µA = 5kOhm => 4k7.
There was no special intention choosing the transistor and the optocoupler, both are fully exchangeable, as long as the transistor's amplification factor suites the power source and all components fit into the hood of a sub-d 25 plug
Greetings, Michael