> With a voltage drop of 3 x 0.7V, maybe they were concerned about getting > too close to the high level threshold for the input pin ? I just checked the datasheet for the upd650. Minimum input voltage for a high level is 0.7 * Vcc At 6V, that gives you 4.2V minimum, so those three diodes can't drop any more than 1.8V. Cheers, Colin f