Unipolar stepper motor interfacing with 8051 variants

[Revestroke]

Hello friends, I am interfacing a unipolar stepper motor with P89V51RD2 controller using ULN2803.
-I have connected Pin0.0 to Pin0.3 of Port0 to ULN2803 and connected phase 1 to 4 of stepper motor to the o/p of the driver IC(ULN2803).
-Motor drive voltage is 12V. I'm getting appropriate readings using multimeter regarding h/w connections.
-Program code I am using is a basic C program given in Muhommad Ali Mazidi book for 8051 for stepper motor clockwise /anticlockwise rotation, but motor is not rotating.
-However whenever I connect center-tap and a phase of stepper motor it takes a step and turns with a single step clockwise and similarly when latch up center-tap with other winding phase it rotates anti-clockwise.

I want to rotate the motor continuously in both directions giving a little delay. Kindly suggest.
Thank You.

 

[FvM]

From a 8051 datasheet:

Port 0 is an 8-bit open drain bi-directional I/O port

It's not able to drive an ULN2803 without additional means, e.g. sufficient strong pull-up resistors.

I don't believe that you ever checked the Port 0 output voltage with a multimeter, otherwise you'll have noticed that it's zero all the time.