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Printer Communication Protocols and Hardware

Printers that don't use proprietary vendor codes communicate with computers using one or more of three major printing protocols. The communication is done over a hardware cable that can be a parallel connection (printer port) or a serial connection (COM port).

ASCII Printing Protocol

The ASCII protocol is the simplest protocol used, as well as the oldest. ASCII is also used to represent text files internally in the DOS, UNIX, and Windows operating systems. Therefore, data taken from a text file or a directory listing generally requires little preparation before being sent to the printer other than a newline-to-carriage return/linefeed conversion for UNIX. Printers usually follow the DOS text file convention of requiring an explicit carriage return character followed by a linefeed character at the end of a line of text. Since UNIX uses only the linefeed character to terminate text, an additional carriage return character must be added to the end of each line in raw text print output; otherwise, text prints in a stairstep output. (Some printers have hardware or software switches to do the conversion.)

PostScript Printing Protocol

Adobe introduced the PostScript language in 1985; it is used to enable the printout of high-quality graphics and styled font text. PostScript is now the de facto print standard in the UNIX community and the only print standard in the Macintosh community. Numerous UNIX utilities exist to beautify and enhance text printing with PostScript. PostScript can be used to download font files into a printer as well as the data to be printed. PostScript commands can be sent to instruct the printer CPU to image, rotate, and scale complex graphics and images, thus freeing the host CPU. Scaling is particularly important with fonts since the document with the font has been produced on a computer screen with far lower resolution than the printer. For example, a 1,024 x 768 computer screen on a 17-inch monitor allows for a resolution of approximately 82dpi, but a modern desktop printer prints at a resolution of 600dpi. Therefore, a font must be scaled at least seven times larger for WYSIWYG output.

PostScript printers generally come with a number of resident fonts. For example, the NEC Silentwriter 95 contains Courier, Helvetica, ITC Avant Garde, Gothic Book, ITC Bookman Light, New Century Schoolbook Roman, Palatino Roman, Times Roman, and several symbol fonts. These are stored in ROM in the printer. When a page prints from a Windows client that contains a font not in the printer, a font substitution table is used. If no substitute can be made, Courier is usually used. The user should be conscious of this when creating documents—documents with fonts not listed in the substitution table may cause other users problems when printing. Avoid use of strange fonts for documents that will be widely distributed.

The user program can choose to download different fonts as outline fonts to the PostScript printer if desired. Fonts that are commonly used by a user are often downloaded to PostScript printers that are connected directly to the user's computer; the fonts are then available for successive print jobs until the printer is turned off. When PostScript printers are networked, clients must download any fonts desired with each print job. Since jobs come from different clients, clients cannot assume that downloaded fonts are still in the printer.

PostScript print jobs also contain a header that is sent describing the page layout, among other things. On a shared network printer, this header must also be downloaded with each print job. Although some PostScript drivers allow downloading of the header only once, this usually requires a bidirectional serial connection to the printer instead of a unidirectional parallel connection.

PostScript print jobs can be sent either as binary data or as ASCII. The main advantage of binary data transmission is that it is faster. However, not all PostScript printers support it, and fonts generally cannot be downloaded in binary. When FreeBSD is used as a printserver, ASCII PostScript printing should be selected on the clients; this is the default with most PostScript drivers.

The Adobe company licenses PostScript interpreters as well as resident fonts to printer manufacturers, and extracts a hefty license fee from any printer manufacturer who wants to use them in its printer. This presents both a benefit and a problem to the end user. Although a single company holding control over a standard can guarantee compliance, it does significantly raise the cost of the printer. As a result, PostScript has not met with much success in lower-end laser and inkjet Windows printing market, despite the fact that Adobe distributes PostScript software operating system drivers for free.

One issue that is a concern when networking PostScript printers is the selection of banner page (also known as header page or burst page) printing. UNIX shared printing began with ASCII line printers, and since UNIX is a multiuser system, often many different user print jobs would pile up in the printer output hopper. To separate these jobs the UNIX printing system programs support banner page printing if the client program that submits jobs asks for them. These pages print at the beginning or end of every print job and contain the username, submittal date, and so on. By default, most clients, whether remote (e.g., a Windows LPR client) or local (e.g., the /usr/"bin/"lpr program) trigger a banner page to be printed. One problem is that some PostScript printers abort the entire job if they get unformatted ASCII text instead of PostScript. (In general, PostScript printers compatible with Hewlett-Packard Printer Control Language [HPPCL] handle banners without problems.) Banner printing should be disabled for any printers with this problem unless PostScript banner page printing is set up on the server.

HPPCL Printing Protocol

The Hewlett-Packard (HP) company currently holds the largest market share of desktop inkjet and office laser printers. Back when Windows was released, HP decided to expand into the desktop laser jet market with the first LaserJet series of printers. At the time there was much pressure on Microsoft to use Adobe Type Manager for scalable fonts within Windows and to print PostScript to higher-end printers. Microsoft decided against doing this and used a technically inferior font standard, Truetype. They thought it was unlikely that the user would download fonts to the printer since desktop publishing was not being done on PCs at the time. Instead, users would rasterize the entire page to the printer using whatever proprietary graphics printer codes the selected printer needed. HP devised HPPCL for their LaserJets and made PostScript an add-on. The current revision of HPPCL allows for many of the same scaling and font download commands that PostScript does. HP laser jet printers that support PostScript can be distinguished by the letter "M" in the model number. (M is for Macintosh, because Macintosh requires PostScript to print.) For example, the HP 6MP has PostScript; the 6P doesn't.

HPPCL has almost no support in the UNIX applications market, and it is very unlikely that any will appear soon. One big reason is the development of the free Ghostscript PostScript interpreter. Ghostscript can take a PostScript input stream and print it on a PCL printer under UNIX. Another reason is the UNIX community's dislike of reinventing the wheel. HPPCL has no advantage over PostScript, and in many ways there are fewer problems with PostScript. Considering that PostScript can be added to a printer, either by hardware or use of Ghostscript, what is the point of exchanging an existing working solution for a slightly technically inferior one? Over the life of the printer, taking into account the costs of toner, paper, and maintenance, the initial higher cost of PostScript support is infinitesimal.


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