DC Jan:
In general it is normally required to place a resistor in series with the base of a transistor. If this is not done the current flow from the driver chip into the base of the transistor will be rather larger than what you may want. In addition the driving IC may be damaged. The actual size or the resistor is quite depenent on your application.

In your application with 82C55 chip output A0 driving the base of transistor it would be a good idea to put a resistor. If you use 1K resistor then the base current into the TIP41C will be on the order of 4 milliamperes. The TIP441C has a current gain of between 15 to 75. Assuming a typical gain of 30, then this base current translates into a collector current of 30*4 = 120 milliamperes. This may or may not be a satisfactory current to drive the stepper motor that you have. You have not provided nearly enough information to analyze things further. In order to design a satisfactory connection to the stepper motor is going to require either much more information than you have provided or a bit more expertise on your part in applying the use of the transistor. This expertise can be gained by getting a book on transistor circuit design and I would suggest that you should pick one up. There are also places on the WEB where you can get tutorial information on transistors. You can find dozens of such links by doing a search on Google with a search key of words transistor tutorial.

On the 82C55 the Port A operates as a full byte wide port. All bits are either simultaneously output or input. Software from the bus side of the chip can set the port A as input or output. It is possible to use the port as input or output in the simple Mode 0 of operation by having software select which mode is needed at any given time. It is also possible to use another mode setting where the port can be set into a bi-directional mode as set by a handshake line (one of the port c pins.

I do suggest that if you are trying to drive a stepper motor with some of the pins of Port A set as outputs then you want to leave Port A as full time output. You can use Port B as inputs in this case, or Port C. The bits of port C can be set as input or output in two 4 bit groups. Here is a full data sheet for the 82C55 so you can read what all can and can not be done with this chip.

Good Luck
Michael Karas