Bart, I think you'll find the spec for the 89C2051 says it can SINK 20mA. This means that it can pull a load to 0V at 20mA. Historically, TTL logic could always sink more current than it could source so one would connect a led from 5V (via a led) and have the logic device pull it to 0V.

On reset, the port pins on most 8051's are pulled to 5V via an internal resistor - this is the same as writing a '1' to the port. When you write a '0' to the port the N channel mosfet in the 8051 is activated and pulls the output pin to 0V (or close enough to it). To use the port pin as an input make sure you don't write a '0' to it otherwise all you will read is '0's.

8051's differ from most micros in this respect. Micros like the PIC,HC05,08,11... have a DDR (data direction register) that sets the port pin to an input or output. You will not find this on a 8051 (except a MSC12xx series from TI and there could be more!!).

Back to my original post - I would suggest you use a buffer and I would expect things will work as you require. However, if you know the circuitry of the stepper controller, you may be able to omit said buffer. I would suggest you try it and measure the '0' and '1' voltages to ensure you are driving the circuit correctly.