Hi,
I think I would measure the input resistance of the input and try to work out why it differs
so wildly from the representitive circuit diagram in the manual.

With the Bob powered, with multimeter put the red lead to the input having previously dissconnected your
probe, and the black lead to earth. Is there any voltage at the input? Note if there is voltage on the pin that would indicate
that the buffer IC has failed.

With the BoB unpowered what is the resistance?

Craig

Hi,
I downloaded the manual and had another look, the reason that you required a 100Ohm pull up resistor is because of the input LED and current limit resistor attached to the input.
See the attached pic from the manual.

The good news it that the 100Ohm resistor is not required because of a faulty device. I have given you a bum steer there...sorry. Given that you are only talking one pin having 50mA flowing
continuously probably wont hurt but would be more of an issue if you had several pins sucking away at that level. If you felt inclined and could trace out the tracks and components on the
board you could open circuit either the resistor of the LED. Thereafter a 1kOhm pull up resistor would be adequate.

Craig