PWM IN MC43, Signal Conversion

Controller: IQAN-MC43, Connector C1, Pin 12 (IN_34, pull-up type frequency input)

Issue: I'm feeding a 200 Hz, ~60% duty cycle open-collector signal into pin 12, using the module's internal pull-up (per manual: 1.5 kΩ to internal 5V, 10 nF) with no external resistor. Instead of a clean square wave, the signal reads as a sawtooth that never reaches 0V - high sits around 4.6V, low only drops to ~4.3V.

I've confirmed pin 12 floats at 4.6V unloaded, and leakage from my circuit is negligible. Tried several pull-up configurations externally; sink current into the pin stays very low (~0.2-0.5 mA) regardless.

Questions:

1. Is 1.5 kΩ/10 nF correct for this input type?

2. What sink current is needed to reliably trigger a logic-low on this pin?

3. Any recommended interfacing for low-current-sinking sources on these inputs?

Thanks for any guidance.