SPP - standard parallel port..
Pin 1 of the parallel port is the strobe , this is an active low signal (TTL)
that tells you when data is available ( to be sent to your relay)..
If you use a 74LS374 then you must use an Inverter between the clock (pin 11) and the strobe line..
then just send your data to the parallel port and the 374 will latch the data
Data is first applied on the Parallel Port pins 2 to 7. The host then checks to see if the printer is busy. i.e. the busy line should be low. The program then asserts the strobe, waits a minimum of 1uS, and then de-asserts the strobe. Data is normally read by the printer/peripheral on the rising edge of the strobe. The printer will indicate that it is busy processing data via the Busy line. Once the printer has accepted data, it will acknowledge the byte by a negative pulse about 5uS on the nAck line.
**broken link removed**