
Originally Posted by
Nayyi It is true that it could be fatal if one would directly connect a circuit, with an external power source, to the parallel port of the PC.
Depending on your load, it can be fatal to you or another people that touches the PC
If the external power source and the PC have a common ground connection, the power supply should be isolated from the mains (ie it must have a transformer)
If you must control something that is not isolated from the mains you should use optoisolators between the PC and your load.
Use safety precautions, remember that line voltages (110V or 220V) are dangerous

Originally Posted by
Nayyi There will be no inputs that will go back to the PC's parallel port yet there aren't aven diodes to protect the PC from feedback. The parallel port will be driving a DC Motor, a Stepper Motor, an Electromagnet and a flashing LED
As dknguyen posted, only one of your loads (the led) is "safe" for your PC.
Both motors and the electromagnet should have, at least, diodes in parallel to protect the port from the EMF spikes they generate.
Without the diodes they "can be fatal" to the PC.