Foris Danny:

Please go to www.goggle.com and do a search for "8255 stepper motor". You will find LOTs of information.

One link is....

http://etidweb.tamu.edu/projects/...tm

...You will find there a report about a project that involved connecting a stepper motor to an 8255 chip. The project shown was somewhat different than the one you are trying to do because they were using a PC computer instead of an 80C51. Also the 8255 chip was a part of another plug in board that was in the PC. But there is plenty of information there to get you started.

In this example they showed using a National Semiconductor LM18293 chip as the driver to buffer between the 8255 chip and the stepper motor. This chip is now obsolete but here is a link to the old data sheet so that you can look at this chip and find another one that would give you equivalent functionality.

http://www.national.com/pf/LM/LM18293.html

A free wheeling diode is one used in a circuit where a high speed switching device, such as transistor or IC, is used to drive an inductive load. When the high speed device shuts off the current that was flowing in the inductive load wants to continue to flow becasue of the inductance of the load. Remember the rule of thumb -- you cannot instantaneously change the current flow through an inductor. Now since the high speed device has shut off there is no place for this current to go and so a voltage develops across the inductor terminals. This voltage goes higher and higher as the magnetic field around the inductor dissapates. The voltage is in opposite polarity as the voltage that was just prior being applied to the coil by the driving circuit. This voltage will almost always be of a magnitude that can destroy the driver IC or transistor. In the simplest case of a NPN transistor driving a relay coil where the connection for the current flow was: +V -> Coil 1 -> Thru Coil -> Coil 2 -> Collector of NPN -> Thru NPN -> Emitter of NPN -> GND. You would connect the free wheeling diode across the relay coil in the polarity so that it is reverse biased while the NPN transistor is ON. The diode will become forward biased when the above described voltage is developed. The diode will thus act as a shut across the relay coil to dissapate the energy thus keeping the voltage spike from going high and killing the transistor. The same thing applies to driving motors because their windings are also inductive.

You can use a 1N4007 as a free wheeling diode. That diode is rated for about 1 Amp and can work OK for small motor coils. There is a preference by designers however to use diodes that switch faster and have lower forward voltage drop than the 1N4007 when they are building higher power motor drivers. The faster the diode the sooner it can start shunting the voltage spike from the inductor and the lower forward voltage drop reduces the heating of the diode caused by it conducting the shunting current. For this reason, in higher power applications, the high speed high current Shottkey diodes are preferred over 1N4007 diodes. For small motors and relays the 1N4007 should work just fine. I have even used small diode like 1N4148 as the free wheeling diode across small relay coils on PC board becasue of small package size.

Good Luck
Mike Karas