Bar Code Basics

When a bar code scanner is passed over the bar code, the light source from the scanner is absorbed by the dark bars and not reflected, but it is reflected by the light spaces. A photocell detector in the scanner receives the reflected light and converts the light into an electrical signal.

As the wand is passed over the bar code, the scanner creates a low electrical signal for the spaces (reflected light) and a high electrical signal for the bars (nothing is reflected); the duration of the electrical signal determines wide vs. narrow elements. This signal can be "decoded" by the bar code reader's decoder into the characters that the bar code represents. The decoded data is then passed to the computer in a traditional data format.

Many of this booklet's readers have to comply with their customer's or industry's bar coding specifications; no choice is possible, just compliance. Look at the following samples of printed bar codes:

The classic bar code type is Code 39, (also called Code 3 of 9) which has 9 bars and spaces; three are wide, and the other 6 are narrow. In Code 39, 3 of 9 total bars and spaces are wide; hence the name, Code 3 of 9. For example, look at the following character representations with Code 39:

Notice there are two widths of bars and two widths of spaces. If you wished to print a bar code of ABCD, you would need to start and end it with a special Start/Stop code character - the * (asterisk) is used for Code 39. So to print a bar code of ABCD, it would need to be printed as *ABCD*. There should be at least 1/4" of white space to the left and right of the code; this helps the reader pick out where a bar code begins and ends.

Other bar code types are similarly constructed. UPC and EAN bar codes have four widths of bars and spaces; so does Code 128.

For new bar coding projects that don't have industry or customer standards, Code 39 is the typical non-food standard, because almost all bar code equipment reads/prints Code 39. However, Code 39 produces relatively long bar codes; it is not particularly efficient in bar code density, (the maximum density is 9.4 characters per inch including 2 start/stop characters). Where the label width is an issue and there is numeric data or lower case data, Code 128 is the best alternative; Code 128 also has an extra efficient numeric only packing scheme to produce very dense bar codes, and Code 128 has all 128 ASCII characters. Not all readers read Code 128, so before you settle on it as a standard, be sure that your reader is 128 capable. Code 93 has been promoted by only one vendor; it requires two characters to make Full ASCII; and it doesn't have a numeric packing option. For these reasons, Code 128 is preferable over Code 93.

The larger the width of the elements, the more space it takes to print the bar code; therefore, the lower the bar code density. The thinner the bar and spaces, the less space is required and the higher the bar code density. Look at the samples below of different densities:

Lower density bar codes are more reliably printed and more consistently read than higher density bar codes, because minor variations (due to printing or damage) are much more serious with high density bar codes - the percentage of distortion is larger.

There are three basic types of bar code readers: fixed, portable batch, and portable RF. Fixed readers remain attached to their host computer and terminal and transmit one data item at a time as the data is scanned. Portable batch readers are battery operated and store data into memory for later batch transfer to a host computer. Some advanced portable readers can operate in non-portable mode too, often eliminating the need for a separate fixed reader. Portable RF Readers are battery operated and transmit data real-time, on-line. More importantly, the real-time, two-way communication allows the host to instruct the operator what to do next based on what just happened.

A basic bar code reader consists of a decoder and a scanner, (a cable is also required to interface the decoder to the computer or terminal). The basic operation of a scanner is to scan a bar code symbol and provide an electrical output that corresponds to the bars and spaces of a bar code. A decoder is usually a separate box which takes the digitized bar space patterns, decodes them to the correct data, and transmits the data to the computer over wires or wireless, immediately or on a batch basis.

Personal Computer Keyboard Wedge Readers

If the bar code reader is attached through the keyboard interface, the bar code reader sends data in key codes, exactly as though the data had been keyed on the keyboard. Keyboard interface readers are nicknamed "wedge readers", because they physically wedge between the keyboard and the computer (or mainframe terminal) and attach as a 2nd keyboard. The great advantage of "wedge readers" is that bar code reading can be added with no software changes necessary; the software thinks that the data received was produced by a fast typist. (Of course the keyboard remains usable too!). With a wedge reader, any program that accepts keyed data will accept bar code data with no change. The following figure shows a keyboard wedge reader attachment.

A keyboard wedge reader which emulates all of the keys including function keys, Ctrl, Alt, Page Up, etc. is preferable. You cannot place a keyboard wedge reader more than 10 feet from the computer. You can get an extension cable for the scanner, allowing you to range up to 35-100 feet from the computer. For these applications a cordless radio frequency wand would be better; the wand has a transmitter and the decoder has a receiver so that the wand can transmit digitized data to the decoder over RF instead of a cord. RF wand readers transmit up to 150 feet; at these distances the trick is to hear the decoder's beep.

Serial Bar Code Readers
Another method of data transmission from the bar code reader to the computer is by RS-232 Serial ASCII format. If you have a multi-user computer, (for example a UNIX system), with serial ASCII terminals for each user, the bar code reader can attach between the terminal and host computer, transmitting ASCII data just like the terminal; in fact the bar code data looks just like keyed data. when attached like the following figure:

Single user computers without an external keyboard (most notebooks) must use the serial port for interface of a bar code reader; to get the bar code data to appear as keyed data, a TSR or device driver program is also necessary. Typically requiring only 2K of RAM memory, the program takes data from the COM port and places it into the keyboard data buffer, so bar code data appears to have been keyed. If your computer program can read a serial port directly, no additional program is necessary.

Serial readers can be placed several hundred feet from the computer, (keyboard wedge readers cannot be placed beyond 10 feet.). Also multiple serial readers can be attached to the same computer, (keyboard wedge readers cannot). The PC runs a program to poll the readers one at a time.

MainFrame Bar Code Readers
Mainframe computers often have terminals with unique data connectors and data formats, (different from ASCII or PC key codes). The IBM System 36-38, AS/400, 4300, 9000, etc., have such terminals. To use bar codes with these computer systems, you must use a keyboard wedge reader specifically designed for the terminal to be attached to. Vendors such as Compsee, Intermec, and Welch-Allyn specialize in readers which attach to mainframe terminals.

The alternative is to have a PC with a terminal emulation card in it attached to the mainframe; then a less expensive PC bar code reader and laser printer can be used on the PC.

Portable Readers

Portable readers are handheld battery operated readers which store the data in memory for later uploading. In addition to a bar code scanner, a portable reader usually has an LCD display to prompt the user what to do; and they usually have a keyboard to enter variable data such as quantities. Ease of programmability is a key issue in selecting a portable, and that depends on your programming abilities; lots of vendors say it's easy, (as long as you can program in C++ or go to their two week school). Other variables to consider are: battery life (at least 20,000 scans), ease of reading the display, size/weight of the unit, who repairs it, and where it is to be repaired in the event of a malfunction.

Radio Frequency Readers

Radio frequency readers are the ultimate solution to many applications' needs - especially any computer remote application that can benefit from the computer checking and instructing the operator. Warehousing applications such as picking, put-aways, shipping, and receiving are typically better performed by RF readers because the computer can instruct the operator where to go and what to do, plus the computer files are current as to exact status and location of available inventory.

RF Readers are like on-line terminals, but wireless. The user can roam around his local facility scanning and keying data and getting a response from the computer with each entry. Therefore the computer can very carefully edit the data for errors as well as prompt the user for what to do next considering the data that has just been entered. The classic RF applications and associated advantages are:

RF Readers that Talk to a Serial Port
These readers require programming on the host computer to read and write to a serial port. Such programming is relatively trivial and can be written in almost any language or any platform. Existing application packages can be modified to include these simple RF readers, but programming is required. It could be argued that the amount of effort is considerably less than with terminal emulation, because all programming is on the host computer; the terminal emulation programs require host programming (for the upper left hand corner) and programming on the terminal too.

Readers that communicate to the host by serial port are usually less than 1/2 the price of the more complicated "Terminal Emulators"; they also often have faster response time due to less software overhead. No network is required - even a slow can drive them at maximum speed. They are far simpler - thus less costly, BUT they do require programming to get their full potential. Even though you can run them in "One-Way" mode without programming, that misses the greatest potential of computer-led activities, (often referred to Event Driven Applications).

Spread Spectrum Terminals vs. Narrow Band Terminals
Narrow band refers to radios that operate within a narrow band of radio frequencies. Spread spectrum refers to radios that jump around on a wide band of frequencies to avoid interference. Narrow band can be licensed at high power and unlicensed at low power. Spread spectrum is almost always unlicensed at high power. Spread spectrum is superior for very large networks of RF terminals - 150 plus terminals in the same building. For terminal networks below that number, unlicensed narrow band is usually less expensive, far less difficult to program and it usually transmits just as far with considerably less power and battery size/weight.

Narrow band radios with a user changeable frequency has the same advantage as spread spectrum in avoiding interference. In fact, it can be argued that 10 plus channels of user selectability are at least as safe as spread spectrum with a fixed pattern. What you really want to avoid is narrow band terminals with a fixed frequency that cannot be changed unless sent back to the manufacturer. More and more devices are going wireless; so, the channel interference is expected to increase substantially in the future.

Wand Scanners

Wand Scanners are the least expensive and the oldest type of bar code scanner. A wand is typically made from 1/2" stainless steel tubing or from plastic; optics are in the front with a cord out the back. The wand scanner must be moved by the user's hand across and in contact with the bar code. While the wand is moving across the bar code, the reflected light is converted to electrical signals through a photocell in the wand.

Laser scanners have a very precise beam of light which can be reflected accurately several inches to several feet. Almost all laser scanners today have a moving beam which sweeps back and forth, (some older laser scanners required the user to move the beam across the bar code). The advantages of moving beam laser scanners are:

	Reading bar codes from a distance (typically 3-18 inches, or up to 17 feet with reading low density bar codes).
	Reading moving objects on an assembly line.
	No-hands operation. Some lasers can be mounted to turn on automatically when an object passes under the scanner. Typically used in blood banks, library check out, etc.
	Reading through glass windows or thick laminates.
	Reading bar codes on curved surfaces, (bags of parts).
	Reading bar codes inside difficult to reach enclosures.

Laser scanners emit a laser light beam which sweeps back and forth across the bar code 36 times per second. At this rate, unsuccessful reading attempts go unnoticed; you will only be aware of the one successful decode. Once a read has occurred, the laser turns off, requiring you to release and pull the trigger again to reactivate the laser scanner.

The lower the density of the bar code, the further the laser scanner can read a bar code. The higher the density of the bar code, the closer to the bar code the laser scanner must be.

Triggered Laser Scanners
Triggered Laser Scanners are virtually "can't miss". Just "point and shoot". Face the bar code so that the bars point up (the laser light will then form a red line across the bar code when the trigger is pulled). Aim the gun scanner at a bar code and pull the trigger; reading is instantaneous. You may have to move the scanner closer to the bar code to get a read, but that's it. Triggered laser scanners are about 5-10 times more expensive than a wand scanner, but scanning is significantly easier. Pay attention to the length of the warranty on laser scanners; it could prove to be very important with heavy usage.

The basic laser scanners read up to 10-20" distance, depending on the brand of the laser scanner. There are hand held triggered long range laser scanners that can read up to 33 feet distance, (using retro-reflective low density bar codes) or 10 feet distance using paper low density bar codes. Long range laser scanners are naturally more expensive than the standard laser scanners.