Port Connection & Relay Interface Circuits BACK
Interfacing the printer port to external devices requires a driver circuit and a relay for each printer output pin to be used The driver circuit must be externally powered and interface should not load the printer port overloading printer port could damage your computer. The diagram below shows the pins on the printer port socket (D25) socket, only pins 2 to 9 are changed by this software. Centronics printer connector has different pin outs to this diagram
Printer D25 connector pin information
Pins 18 through 25 are grounded
Transistor Driver circuit
Below are two transistor circuit.
IC Relay driver circuits
Integrated Circuit Instead of the transistor circuit above for each data output from printer port and data driver IC can can used The UNL2803 $5:00 at www.dse.com.au this IC simplifies building your relay interface