The port pins on a 8051 are what's called 'open drain' which is similar to 'open collector'. This means they only actively pull the signal to 0v. A '1' is pulled up to 5V via internal pullup resistors (except on port0). So in answer to your problem I would suggest you use a buffer of some sort (74hc244) to give the port pins some current drive (especially if you have a length of cable between the stepper drive and the 8051).

So if you write a '1' to the 8051 port in software, it will be output as a '1' and vice-versa.

Hopefully this points you in the right direction.