Ppio Description & Connection

The PPIO makes use of the eight output pins (pins 2 through 9) at address 378H to drive its eight outputs. It uses the upper four bits of address 379H (pins 13, 12, 10, and 11) for the upper four input bits of the PPIO. It uses the lower four input bits of address 37AH (pins 1, 14, 16, and 17) for the lower four input bits of the PPIO. This assumes you are using the same port addresses as in Table 1. In this way the PPIO can have eight inputs or eight outputs. Refer to Figure 1 for the PPIO schematic.

Each pin of the PPIO is bi-directional. It can be used as either an input or an output. Referring to Figure 1 you will see that PPIO bit 0 can be driven by pin 2 of the parallel port or it can be read by pin 1 of the parallel port. The drivers are open collector Darlington transistors that can sink up to 500 mA and are protected by “kick back” diodes that are connected to the positive power supply. These outputs can handle voltages as high as 50 Volts DC. If bit 0 of port 0378H is HIGH, pin 2 of the parallel port is HIGH and the PPIO output pin will be LOW and can sink 500 ma. If bit 0 goes LOW, pin 2 will go LOW and the PPIO transistor will go OFF.

To use PPIO I/O bit 0 as an input you first set bit 0 of port 378H LOW to turn OFF the driver transistor. From then on, if you force the PPIO bit 0 to ground, a LOW will show up on pin 1 of the DB-25 connector. If you look at Table 1 you will note that pin 1 is inverting in the computer parallel port. This means that a LOW on pin 1 will show up as a HIGH in the computer. This is called negative true logic. The PPIO receivers are set up as inverters or non-inverters to compensate for the inverting and non-inverting inputs of the parallel port.

All you have to remember is that if you force one of the PPIO I/O bits to ground, it will be a ONE when you read it in the computer. If you leave the PPIO I/O pin open or force it HIGH (above 2.5 volts), it will be a ZERO when you read it in the computer.

PPIO I/O bits 0 through 3 are connected to bits 0 through 3 of port 037AH and PPIO I/O bits 4 through 7 are connected to bits 4 through 7 of port 0379H. (This assumes that you are using a parallel port at 0378H in your computer.) A LOW on any PPIO I/O pin will show up as a HIGH on the corresponding bit in the computer.

PPIO2899 Manual

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