Parallel Port

The parallel port was originally strictly for printers. This is a 25-pin female D-Sub connector, often called a DB-25F connector. A printer cable had a 25-pin male D-Sub connector to match the female connector on the computer. The other end of the printer cable is a 36-pin Centronics connector. The printer port is synchronous. In this case, a wire signals the printer when a byte of data is ready. This takes nine wires to send eight bits. Five pins were used to communicate back to the computer. These wires had the following functions:

ACK (acknowledge)
busy (logical low when busy, high when ready for data)
paper out
select
error

It wasn't long before someone made a hack to use these five wires to send data to the computer four bits at a time. Before the parallel port was made bi-directional, a few devices used this hack to send data to the computer a nybble at a time.

When IBM introduced the PS/2 in the late 1980s, they also introduced the bi-directional parallel port. The original parallel port then became known as the Standard Printer Port (SPP). The first bi-directional parallel port was called the Enhanced Parallel Port (EPP). The final generation of parallel port was called the Extended Capabilities Port (ECP). The Institute for Electrical and Electronic Engineers (IEEE) published a standard for this port designated IEEE 1284. Now, all parallel ports, cables, and devices are IEEE 1284 compliant, but in the mid to late 1990s, it was essential to ensure that cables and devices were IEEE 1284 compliant.

The parallel port is a rarity today, but here is some legacy information in case you encounter some:

• The first parallel port (LPT1:) typically uses IRQ 7 and I/O address 0378h (h for hexadecimal)
• A second parallel port (LPT2:) typically uses IRQ 5 and I/O address 0278h (sound cards typically use IRQ 5, so watch for conflicts)
• Set the compatibility mode (in the system setup program) for EPP/ECP for best compatibility. Try other modes if you have problems.
• If your parallel port seems slow (probably very slow), be sure it is in ECP mode.
• In ECP mode, the parallel port uses a DMA channel.