10.1. Preparing the Parallel Port

Although the parallel interface is simple, it can trigger interrupts. This capability is used by the printer to notify the lp driver that it is ready to accept the next character in the buffer.

Like most devices, the parallel port doesn't actually generate interrupts before it's instructed to do so; the parallel standard states that setting bit 4 of port 2 (0x37a, 0x27a, or whatever) enables interrupt reporting. A simple outb call to set the bit is performed by short at module initialization.

Once interrupts are enabled, the parallel interface generates an interrupt whenever the electrical signal at pin 10 (the so-called ACK bit) changes from low to high. The simplest way to force the interface to generate interrupts (short of hooking up a printer to the port) is to connect pins 9 and 10 of the parallel connector. A short length of wire inserted into the appropriate holes in the parallel port connector on the back of your system creates this connection. The pinout of the parallel port is shown in Figure 9-1.

Pin 9 is the most significant bit of the parallel data byte. If you write binary data to /dev/short0, you generate several interrupts. Writing ASCII text to the port won't generate any interrupts, though, because the ASCII character set has no entries with the top bit set.

If you'd rather avoid wiring pins together, but you do have a printer at hand, you can run the sample interrupt handler using a real printer, as shown later. However, note that the probing functions we introduce depend on the jumper between pin 9 and 10 being in place, and you need it to experiment with probing using our code.