PC Output Interface

  
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The PC parallel port can be easily damaged by wiring mistakes, short circuits or the circuits you connect to it. If the parallel port is integrated into the motherboard, as is the case with most computers repairs can be expensive. You can buy inexpensive I/O card which connect to a spare slot on most motherboards, these are ideal for experimentors
PC Output Interface - schematic

and easy to replace. Every reasonable care has been taken in producing this information. However, the author can accept no responsibility for any damage arising from mis-use of this information. Please also see this sites general Disclaimer. A versaile output interface that can control external devices direct from your computer. Uses freely available software and works with both windows and Linux. Any PC that has a free parallel printer port available should work with this circuit. The schematic shown above requires little explanation. Any data sent to the printer port is already "latched" and just requires buffering to be able to drive an external ciruit. In this case I have chosen to use the ULN2083 octal transistor array to buffer the output current from the parallel port. The IC pinout for the ULN2083 is shown below. The pararell printer port on your PC is a 25 way D type connector. Its pinout is shown below. Pins 2 to 10 are the data output lines D0 to D7. Pins 18 through 25 are ground pins, the other pins are not used and left unconnected. The output current is amplified by the ULN2083 and each output pin can supply a load of up to 50mA. The outputs are active low, meaning that a high logic input gives a low logic output. To get around this the output load is routed to a positive voltage, so that any high input will now produce a high output. The positive supply can be any value from +5 to 50Vdc, I used 12V relays...



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