Interfacing

The hardware connection between your computer and printer may be the easiest to manage among all computer peripherals. Most printers, still, use the vintage printer port. A few (and soon, more than a few) printers use a USB connection. Both of these connection systems are covered in Chapter 11, "Ports." In either case, you need do little more than plug in the printer. All the details of the linkup are automatically taken care of ”or passed off to software.

And with the software, the interface gets interesting. The interface doesn't just have to get signals to the printer (the hardware does take care of that), but it must ensure that the right signals, those that the printer understands, get there. Modern software considers every dot on the page individually and has to tell the printer what to do with it. Your computer, through its operating system and printer driver, describes what the printer should put on paper using a command language or by sending a bit-image of the entire page to print.

At one time the control language used by a printer determined what applications could take advantage of all the features and the quality the printer had to offer. You had to match your software to the command set the printer understood .

Modern operating systems have eliminated this concern. Instead, the chief issue is the printer driver . The driver matches the printer's native commands to the operating system and all the applications that run under it. Once you install a driver, the printer's command set doesn't matter.

That said, printers usually fit three classes: Windows GDI, PCL, or PostScript.

Windows GDI printers have drivers that directly mate with the Windows Graphic Device Interface. They use proprietary commands for eliciting each printer's features. In most cases, graphics and even the entire page may be sent through the GDI to the printer as a bit-image. In most cases, the GDI is the fastest way of communicating with a printer. The computer, with its powerful processor, does all the work of rasterizing the image. Most Windows printer drivers now use the GDI.

PCL is the abbreviation for Printer Control Language , a command system originally developed by Hewlett-Packard for its inkjet printers and later adapted to laser machines as well. The commands in PCL tell a specific printer what to do to make an image on a page of paper. PCL thus focuses on how to draw the image on the page.

Although PCL is now a common language used by a wide variety of printers, it is a device-dependent language. That is, the driver for a printer may send out somewhat different codes for another output device, even if it is another printer.

Through the years , PCL has gone through six distinct versions (four that apply to laser printers), each building on the features of the earlier versions. PCL ships the information to be printed as a combination of characters and graphics commands, relying on a raster image processor in the printer.

PostScript is a page-description language developed by Adobe Systems, now in its third major revision (PostScript 3). It describes every dot that appears on a printed page. The language is intended to be device independent. That is, the PostScript code that yields a printed page will be the same for all devices that understand PostScript, and not necessarily only printers. PostScript focuses on the final output, what the image on paper looks like. How that image gets drawn doesn't matter to PostScript.

This device independence is a blessing when you need to preview something on a draft-quality machine before sending it out to a production house ”both the draft printer and the production house's typesetting equipment work from the same code. At the same time, PostScript attaches a penalty. It requires substantial processing to convert the language commands into a raster image of a page. Using PostScript usually results in slower throughput from the printer.