Computer with electrically isolated printer port using shunt transistors controlled by a common charge-robbing node

摘要

A parallel port adapter, which provides a special buffering circuit on the four control lines from the system to the printer. This circuit provides a low-impedance bidirectional path for communication between the CPU and the printer, whenever any of the control lines is active. The circuit uses blocking diodes on all four lines, and uses pass transistors in parallel with the blocking diodes, and uses power-robbing to turn on the pass transistors whenever the port is in use.