Output 2 - Printer Port Schematic

The schematic below represents the standard Centronics printer port. (It is considerably more complex than this, but it will give you an idea of the basic principles.) Eight lines are available for output and nine for input. The remaining pins (pins 18 - 26) are all connected to GROUND (negative). The 74LS244 integrated circuit acts as a 'buffer' and ensures the signal is a reliable HIGH, or LOW. It also acts as a level of protection between the data bus and the outside world.

Notice that the input pins 17, 14, 1 and 11 all go through an "inverter" before appearing on the data bus. This means that if a HIGH (+ 5 volts) appears at one of these pins, the CPU will read it as a LOW and vice versa. The "ERROR" pin does go though a 'buffer', but its HIGH/LOW state is combined with other data signals. It is a little more confusing to work with.

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