History
Most PC-compatible systems in the 1980s and 1990s had one to three ports, with communication interfaces defined like this:
- LPT1: I/O port 0x3BC, IRQ 7 (usually in monochrome graphics adapters)
- LPT2: I/O port 0x378, IRQ 7 (dedicated IO cards or using a controller built into the mainboard)
- LPT3: I/O port 0x278, IRQ 5 (dedicated IO cards or using a controller built into the mainboard)
If no printer port is present at 0x3BC, the second port in the row (0x378) becomes LPT1 and 0x278 becomes LPT2. Sometimes, printer ports are jumpered to share an interrupt despite having their own IO addresses (i.e. only one can be used interrupt-driven at a time). In some cases, the BIOS supports a fourth printer port as well, but the base address for it differs significantly between vendors. Since a fourth printer port is conflictive with other features on PS/2 machines, it typically requires special drivers in most environments.
The original implementation of the parallel port card for the IBM PC worked bidirectional, a feature soon dropped until its reappearance many years later in the IBM PS/2 series, where it could be enabled or disabled for compatibility with applications hardwired not to expect a printer port to be bidirectional. A wide variety of devices were eventually designed to operate on a parallel port. Most devices were uni-directional (one-way) devices, only meant to respond to information sent from the PC. However, some devices such as Zip drives were able to operate in bi-directional mode. Printers also eventually took up the bi-directional system, allowing various status report information to be sent.
In DOS, the parallel ports could be accessed directly on the command line. For example, the command "TYPE C:\AUTOEXEC.BAT > LPT1" would direct the contents of the AUTOEXEC.BAT file to the printer port. A PRN device was also available as an alias for LPT1. Some operating systems (like Multiuser DOS) allow to change this fixed assignment by different means. Some DOS versions use resident driver extensions provided by MODE, or the mapping can be changed internally via a CONFIG.SYS PRN=n directive (as under DR-DOS 7.02 and higher). DR-DOS 7.02 also provides optional built-in support for LPT4 if the underlying BIOS supports it.
A special "PRINT" command also existed to achieve the same effect. Microsoft Windows still refers to the ports in this manner in many cases, though this is often fairly hidden.
In the Linux operating system the first LPT port is available via the filesystem as /dev/lp0.
Read more about this topic: LPT
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