The circuit illustrated in Figure allows external loads (8) to be driven from parallel port signals and also to acquire data through the same port, simply by switching the appropriate I/O function enablements. Coupling (both input and output signals) is done through optical couplers, which means a high level of safety for the integrity of the PC circuits. The source for the TTL circuits must be external with a common ground, and the inverter ports can be of any TTL type such as the 7805.
