Questions Beginners Ask

Tom R. Halfhill, Staff Editor

Are you thinking about buying a computer for the first time, but you don't know much about computers? Or maybe you recently purchased a computer and are still a bit baffled. Each month in this column, COMPUTE! will answer questions often asked by beginners.

I am interested in purchasing a printer to go along with my computer. Several questions come to mind. First of all, what is meant by parallel and serial as related to printers? Do most new printers have a built-in interface of one type or another? I know the Atari printers do (I own an Atari 800XL), but what about third-party printers? What type of peripheral port does Atari use? The RS-232C or the Centronics standard? Numerous advertise­ments for printers in your publication claim compatibility with one brand or another. Does that mean the printer can be directly connected without an extra interface device? Most of the ads for printers do not specify the brand of computer that they are compatible with. Is it because the appropriate interface can be purchased separately? I'm sure there are plenty more folks out there who, like me, need answers to these questions.

A Welcome to the world of personal computing, where frequently used terms often have ambiguous meanings and few things are as simple as they should be.

There are indeed plenty of folks out there who would like answers to those questions. Similar questions are asked by practically everybody who starts shopping for a printer for the first time. Let's tackle them one by one.

The terms serial and parallel refer to the two general types of computer interfaces. They apply to all computer devices, not just printers. (An interface is simply a connection between two parts of a computer system.)

A serial interface transfers information one bit at a time, one after the other. Since it takes eight bits to represent one character, a computer hooked up with a serial interface to a printer must send a stream of eight signals each time a character is to be printed.

A parallel interface, on the other hand, transfers information eight bits at a time, all at once. A computer hooked up with a parallel interface to a printer can send the eight signals simultaneously for every character to be printed. Therefore, when all other things are equal, a parallel interface is much faster than a serial interface.

Here's an analogy: Think of a multiplex movie theater at a shopping mall, one with eight separate screens showing eight different movies. If the ticket booth is staffed by only one person (as they frequently are), everyone has to wait in one very long line, no matter which movie they want to see. The line moves relatively slowly. But if the ticket booth is staffed by eight people, one for each screen, the lines move about eight times faster. That's the difference between serial and parallel.

So, you might conclude that a printer with a parallel interface is preferable to one with a seria interface. But in practice, the printer interface's speed isn't too important for average home users Under-$1000 printers are generally limited by the speed of their own printing mechanisms, not by the speed at which the interface can transmit data. Instead, your decision should be based on which interface is more readily available for less money.

Nearly all printers come with one type of interface built-in, either parallel or serial. Some have both. Some have neither. So you can order them with the one you want. And some printers have one interface plus the option of adding a second. If this information is not in the advertise­ment, you'll have to contact the manufacturer, distributor, or dealer.

There are many different kinds of serial and parallel interfaces, but over the years two have become accepted as de facto standards for personal computer printers. The most common serial interface is called the RS-232C, and the most common parallel interface is called the Centronics standard (named after the manufacturer which made it popular). Probably 90 percent of the printers you see will have one interface or the other, especially printers made by third-party companies (independent firms which are not connected with a computer manufacturer).

Many personal computers—including the Atari, Commodore 64, VIC-20, Apple II/IIe, TI-99/4A, and IBM PC—do not include an RS-232C or Centronics interface as standard equipment. This means you either have to buy a printer made to plug directly into the computer, or buy an interface that will connect your computer to an RS-232C or Centronics-standard printer.

Atari computers do have a built-in serial interface, but it's not RS-232C standard. The Atari 600XL and 800XL also have a built-in parallel interface, but it's not Centronics standard. Both interfaces are unique to Atari, and they're made for plugging in Atari-compatible disk drives, cassette recorders, and other peripherals. The serial interface—that large socket on the right-hand side of the computer—works directly with the new line of Atari printers, including the 1025 dot-matrix printer, the 1020 color printer, and the 1027 letter-quality printer. No extra interface is required.

Although the computer manufacturer's own printers are usually the safest bet for full compatibility, you may want to buy a third-party printer for certain features or for a lower price. To hook up an RS-232C or Centronics-standard printer to your Atari, you'll need the Atari 850 Interface Module. It has one Centronics port and four RS-232C ports. Unfortunately, these modules cost about $175, and they're hard to find. Fortunately, equivalent interfaces are available from third-party companies for less money, and at least one third-party Atari-compatible disk drive has such an interface built-in.

Also, Atari planned to introduce something called the 1090XL Expansion System for the 600XL and 800XL. This is a box which plugs into the rear parallel expansion port found only on the 600XL/800XL, adding five expansion slots. The slots would accept more memory and various types of interface cards. However, Atari's recent sale and massive layoffs might affect future plans for such new products.

Anyway, once you add an RS-232C or Centronics interface to your computer, all you need is a compatible printer and a cable. When an advertisement states that a certain printer is "compatible" with your computer, it can mean two things: Either the printer is directly compatible (no extra interface required), or it's compatible with your computer only if you already have the RS-232C or Centronics interface. It's up to you to determine which. Always check before you buy, and make sure the proper cable is available, too. Strange as it may seem, not all RS-232C or Centronics ports take the same plugs. Sometimes the pins are wired differently. Specify the exact configuration of your system so the dealer can steer you to the printer, interface, and cable which will match together correctly.

Of course, everything we've discussed so far is limited to hardware compatibility. If you're planning to use the printer with a certain program—such as a word processor—you should also think about software compatibility. Certain programs can't take advantage of all the special features built into certain printers, and vice versa. But that's a topic for another column. For more information on matching printers and word processors, see "Questions Beginners Ask," COMPUTE!, March 1984.