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Hewlett-Packard Printer Control Language (PCL)

Hewlett-Packard developed its printer control language (PCL) as a more advanced printer language to supersede simple ASCII codes. PCL features a set of printer commands greatly expanded from ASCII. Hewlett-Packard designed PCL with text-based output in mind; it does not support advanced graphical functions. The most recent version of PCL, PCL6 features scalable fonts and additional line drawing commands. Unlike PostScript, however, PCL is not a true page description language; it uses a series of commands to define the characters on the page. Those commands must be supported by each individual printer model, making PCL files less portable than PostScript files.

Windows GDI and XPS

Windows 2000/XP use the graphical device interface (GDI) component of the operating system to handle print functions. Although you can use an external printer language such as PostScript, most users simply install printer drivers and let Windows do all the work. The GDI uses the CPU rather than the printer to process a print job and then sends the completed job to the printer. When you print a letter with a TrueType font in Windows, for example, the GDI processes the print job and then sends bitmapped images of each page to the printer. The printer sees a page of TrueType text, therefore, as a picture, not as text. As long as the printer has a capable enough raster image processor (explained later in this chapter) and plenty of RAM, you don’t need to worry about the printer language in most situations. We’ll revisit printing in Windows in more detail later in this chapter.

Windows Vista supports GDI printing, but it also includes a new printing subsystem called the XML Paper Specification (XPS) print path. XPS provides several improvements over GDI, including enhanced color management (which works with Windows Color System) and better print layout fidelity. The XPS print path requires a driver that supports XPS. Additionally, some printers natively support XPS, eliminating the requirement that the output be converted to a device-specific printer control language before printing.

Printer Connectivity

Most printers connect to one of two ports on the PC: a DB-25 parallel port or a USB port. The parallel connection is the classic way to plug in a printer, but most printers today use USB. You’ll need to know how to support the more obscure parallel ports, cables, and connections as well as the plug-and-play USB connections.

Parallel Communication and Ports

The parallel port was included in the original IBM PC as a faster alternative to serial communication. The IBM engineers considered serial communication, limited to 1 bit at a time, to be too slow for the “high-speed” devices of the day (for example, dot-matrix printers). The standard parallel port has been kept around for backward compatibility despite several obvious weaknesses.

Parallel ports may be far faster than serial ports, but they are slow by modern standards. The maximum data transfer rate of a standard parallel port is still only approximately 150 kilobytes per second (KBps). Standard parallel communication on the PC also relies heavily on software, eating up a considerable amount of CPU time that could be used better.

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NOTE Although the phrase “Centronics standard” was common in the heyday of parallel ports, no such animal actually existed. Prior to the development of IEEE 1284, a very loose set of standards were adopted by manufacturers in an attempt to reduce incompatibility issues somewhat.

Parallel ports are hindered by their lack of true bidirectional capability. Although one-way communication was acceptable for simple line printers and dot-matrix printers, parallel communication also became popular for a wide range of external devices that required two-way communication. Although it is possible to get two-way communication out of a standard parallel port, the performance is not impressive.

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NOTE Many techs confuse the concept of duplex printing—a process that requires special