The objective of this document is to provide a practical and easy to use guide to those considering or seeking to use bar codes for the first time. As such it is hoped that this guide should provide a concise and easy to understand introduction to the "what" and "how" of using and benefiting from bar codes. We have deliberately kept the contents as non-technical as is possible to enable the widest possible range of readers to benefit; hence this guide should not be regarded as a technical reference document as such.

What is a Bar Code & how do they work?

A bar code is a way of representing characters (be they numbers and/or letters) in a printed form that can be read and decoded by suitable reading equipment. There are a variety of different "types" of bar code (the various types are called "symbologies") and a range of different technologies available to read bar codes. All a bar code itself actually contains though are encoded characters. The act of "reading" or decoding the bar code makes the letters or characters encoded available to the host computer equipment that the decoder is attached to.

The structure of a bar code varies from Symbology to Symbology, but the diagram below represents the components of a typical bar code:

Fig 1 – The analogy of a bar code
 Start/Stop Character is special codes that define each end of the bar code. Different bar code symbologies use different start and stop characters.

Check Character is a character appearing in the bar code data that is generated from the data in the bar code. The cheek character is used as a verification check that the data in the code has been correctly decoded. Different types of bar code use different methods for calculating the check character. In some symbologies a check character is optional while in others it is always present.

Quiet Zones are areas of blank space to the left and right sides of the bar code.

Bearer Bars are horizontal bars printed across the top and bottom of the bar code. Bearer Bars can help avoid partial reads should the reader move off the top or bottom of the code.

When a bar code is scanned, the optical elements of the reader convert the black and white bars of the code into an analogue (i.e. non-digital) electrical signal that varies according to the light/dark parts of the code. So the optics of a reader scanning a code would "see" the bar code as an electrical signal with a corresponding high/low pattern.

Scanner beam passing over bar code	Wand output analogue signal

Fig 2 - Analogue signal of bar code
Now the code has been represented in an electrical form, the next step is for the decoding element of the reader to convert the analogue signal into digital data that the attached computer can understand. The decoder analyses and decodes the signal according to a pre-defined set of rules. The decoding rules (correctly referred to as algorithms) are defined by the Symbology type being decoded. So, providing the bar code has been properly printed and accurately scanned, the decoder can apply the decode algorithm to output the encoded characters as data.

Why do bar codes often have numbers and letters below them?

The practice of printing the encoded characters below the bar code is really a case of common sense. If for some reason a bar code cannot be scanned (it may damaged, badly printed, or the scanner might fail) then a sensible back up measure is for the characters to appear in human readable form and for any associated application software to allow the user to manually enter the characters or digits.

Some bar code Symbology rules explicitly state that the numbers should always be printed below. There may though be applications where having the numbers printed below is not required, not desirable or not possible for other reasons, but generally speaking representing the bar code in 'human' readable form is usually good practice.

 I need to print bar code labels - what are the options?

There are a variety of ways to print bar coded labels, but each requires some consideration of what the nature of the application is. Generally there are two main approaches to producing your own bar code labels:

a. Using office printers.
Buying an 'off the shelf' bar code labeling and printing package that can run on your office PC along with your existing dot matrix or laser printer is a viable option for many. The principal benefit of this approach is the relatively low initial cost. However you may need to consider the following issues:

• You will need to load the printer with label stationary, so unless you dedicate a printer to the task other printing must be suspended.
• You are generally limited to paper labels, so if you need to produce robust and scratch resistant labels then paper may not always be the best material to use.
• Printing one label may result in wasting blank labels – especially likely with Laser printers. So this approach is generally less suitable if you are likely to require varying quantities of labels "on demand".
• Label stationary for office printers are often of the permanent adhesive type, which may not be best for all applications.
• Dot matrix printing of bar codes requires some care. Because each bar is made up of dots it's strictly speaking not possible to produce the dead straight dark edge that a well printed bar code ideally requires. However with care it is often quite possible to produce perfectly readable codes on even a 9 pin dot matrix printer, 24 pin printers give even better results. Beware also that worn printer ribbons and damaged pins are a common cause of badly printed and unreadable bar codes!

b. Using "on demand" Thermal Transfer printers.
These are special printers designed to print bar codes (and other data) onto labels or tags supplied in rolls. Most thermal transfer printers can print onto a variety of label types of differing sizes and materials. It is usually possible find a label type that meets a given need. Indeed, if you require large enough volumes it is sometimes possible to have special label sizes custom produced.

Other benefits of thermal transfer printers include:

• On demand printing - giving the precise number of labels you need at a given time without wastage.
• When used with suitable label stock and ribbons thermal transfer printers can produce very durable scratch resistant labels. The ribbon ink is effectively "burnt" onto the label giving a robust and well printed code.
• Generally speaking thermal transfer printers produce very high quality bar codes of good definition.
• Speed - thermal transfer printers can print labels very fast, often as much as 10" of label material per second, making them ideal for high volume applications.

The biggest down side to thermal transfer printers is the higher initial cost. Because these are specialist printers that are not made in the same mass produced quantities as office printers, they do cost more. However it is often worth considering the overall running cost in terms of cost per label over the expected pay-back period. The higher initial cost can often be significantly offset by a lower overall cost per label. Generally speaking though if volume label production is required, then thermal transfer is usually the best and most cost effective option.

If you go the thermal transfer route you will also need to give consideration to software to drive the printer. The two main approaches here are either to use a PC package that supports the appropriate printer, or to drive the printer directly from your application software. Driving the printer directly is not necessarily as complex as it might first appear. Thermal transfer printers are usually controlled by sending the printer special control commands in a similar fashion to the way ordinary printers are software controlled. If you are producing your own software for the main application (or having it produced for you) then including the correct sequences to control the printer should be quite viable.

Can 1 use pre-printed labels?

The answer here is very dependant on the nature of the application and what you are trying to achieve. Clearly if you need variable data, or unique and specific information that is not previously known represented in your bar code, then pre-printing may not be a viable option.

On the other hand if you are only intending to use the bar code as a from of identifier then buying pre-printed labels from a supplier capable of producing individually numbered labels (e.g. 1000 labels numbered 000 to 999) can be highly cost effective. For example in an asset identification application you might assign the bar coded number 4567 to the office photocopier by attaching the bar code label to the copier and recording that this number is now the identifier for this item. Your asset database will now know that the number 4567 now represents the photocopier - however the number assigned could have been any number, the key thing being that the number once assigned does not change and remains unique to that photocopier.

If your application requires a once off assignment of non-critical numbers for identification purposes only then buying pre-printed variable labels can be a very cost effective approach. One note of caution though, make sure you keep a record of the ranges of numbers you have ordered from your supplier and make sure that if you re-order you specify and receive a number range different to any previous labels - otherwise great confusion may ensue!

Naturally if your application requires that "all of x have the same number', then identical preprinted labels would be required.

NON LABEL PRINTING
Can I print bar codes in a document to be printed on an ordinary printer?

With the increase in office automation and the growing use of applications such as document tracking and workflow monitoring, there has been a steady growth in users wishing to incorporate bar codes into documents they are printing. Sometimes the easiest solution is to literally stick a previously printed label onto the document, but this is not always possible or desirable.

Using Windows Bar Code Fonts
The widespread use of Microsoft's Windows graphical user interface has led to bar code TrueType TM fonts for Windows becoming available, either separately or as part of labeling packages. Using these fonts can be an apparently easy way to incorporate bar codes in documents that are being produced in Windows applications. However all is not quite as simple as it may at first seem and some care needs to be taken.

Firstly not all bar code symbologies lend themselves to being adequately or correctly represented as individual characters in a font. The Interleaved 2 of 5 Symbology for example encodes two digits in each of its elements, so it's strictly speaking impossible to represent a single digit character in an ITF font. Windows based bar code labeling packages know about this and handle the ITF font accordingly - but your word-processor and spreadsheet package will not! Similarly fonts that use special start and stop codes and those capable of representing non-printable ASCII characters can also cause problems. If you want to use Windows fonts the best advice is to stick to the symbologies that do encode individual characters and do not have complex start and stop code sequences - probably the best Symbology in this respect is Code 39.

Poor and incorrect use of fonts is probably now becoming one of the most common reasons for the apparent "failure" of bar codes to read. However software packages are now becoming available that are especially designed to overcome these difficulties. Rather than function as labeling packages or just simple fonts, these packages know the rules about constructing valid bar codes. Once the software has designed the bar code it can simply be pasted into the main Windows application (via the Windows clipboard). More advanced users can also use these packages via Windows DDE to automate the process.

In-line Bar Code Printer Controllers
Sometimes referred to as 'labeling controllers", these in line devices are probably one of the easiest and most effective ways of enabling the printing of bar codes on conventional laser or Dot Matrix printers (provided the printers are compatible with the de-facto standards - i.e. Hewlett Packard LaserJet and Epson Dot Matrix).

An in line controller is a small box like device that is installed between the printer and the host output device (i.e. desktop PC or mainframe). The printer continues to work as normal but the controller device effectively expands the character set and fonts available to include Bar Codes. Bar Codes are printed by sending a printer control escape ( ESC) sequence in a format that the controller can interpret. Hence it is possible to send a control sequence that the in line controller will interpret as - "print the following characters in the specified Symbology at the specified location on the page". If your application can control and easily modify the ASCII characters being sent to the printer, then you can easily insert and control bar code printing. Probably the most common use of in line controllers is to add a bar code to an existing document that are already being printed, for example document numbers or invoice numbers can also appear as bar codes.

One fundamental limitation of in line controllers is that they can only work where the printing is being done by sending ASCII characters to the printer. Labeling controllers will usually not work with Postscript laser printers or with Windows applications where the printer is being controlled with a Windows printer driver - these work by sending graphics commands to the printer and won't know about the labeling controller. If your Windows application happens to print by spooling ASCII directly to the printer though this would be OK.

Checking and Validation

We are bar coding our products and some customers are having trouble reading the codes, but they read OK with our wand - we have been told we need to verify the codes. What does this mean?

Requirements for the checking and verification of bar codes vary depending on how the Bar Codes you have printed are going to be used. If you are printing bar codes purely for your own internal use and there will be no requirement for anybody outside your company or organization to read the bar codes (so called "closed" systems) then just checking that the bar code encodes the correct characters and that these can be correctly read by your chosen reading equipment may well be perfectly adequate.

However, if you are producing bar codes that will be used by third parties, maybe on a variety of potentially unknown readers and scanners (so called "open" systems), then you will need to go a stage further. Here the requirement is to verify that the bar code has been printed in line with the "rules" of the Symbology you are using and that the print quality meets specifications. Verification is a more complex process than simple checking and requires special equipment capable of scanning your bar code and analyzing the compliance of the code in line with the rules of the Symbology. Usually it is necessary to purchase a specific bar code "verifier' to do this job. Verifiers can usually provide lots of very technical information about a bar code (e.g. contrast ratio rating, wide/narrow bar ratio etc.). Normally the verifier will summaries all these technical measurements to inform the user if the code is within or outside the acceptable ranges of tolerance for the Symbology concerned - sometimes this is further summarized to a simple "good" or "bad".

It is important to understand the difference between checking and verifying. It's not uncommon for those printing bar codes to actually be doing what amounts to just checking but be incorrectly calling it verification. In reality the difference boils down to this - the process of checking with a given reader does not necessarily mean that the bar code is correctly printed and can therefore be read by all scanners operating within acceptable tolerance ranges. However for many the additional cost of validation can be prohibitive and if you are confident that you are using a good quality printing source then just checking may be adequate - if you adopt this approach though beware of the possibility (however remote it may seem) that one day someone may turn around and say “we cannot read your bar codes” - responding that they work OK with your reader is not the same as having properly verified the bar code

TYPES OF BAR CODE
We are planning to put in a new bar code system -which Symbology should we use?

There is a range of different bar code symbologies available and in common use. Generally speaking if you stick to using the most popular symbologies you are unlikely to have any problems sourcing readers and scanners capable of decoding the bar codes. Beyond this general advice the choice of Symbology depends on a variety of factors - certain symbologies are more common in some industries than others, some can only encode numbers but others letters and numbers, some give very compact codes capable of encoding more characters - others less so, some have built in security or check digit systems - some don't!

The following is a brief (and by no means comprehensive) summary of the more popular symbologies and their principal features and applications.

CODE 39

Code 39 is probably the most popular general purpose Alpha/Numeric code. Very widely supported and is considered by many to be the standard for general commercial and industrial use. Encodes individual characters and theoretically has no limit to the number of characters that may be encoded. Use of start and stop codes gives good security of reading. Main drawback is that the code is not very compact so in real terms the number of characters that may be encoded is relatively limited. An enhanced version of C39 is capable of encoding the full ASCII character set and is usually referred to as "Full ASCII Code 39" or "extended code 39". Support for the extended version is not quite as wide as the standard version but still good. Check digits can be optionally added to Code 39 codes.

EAN-13/EAN-8

EAN stands for "European Article Numbering association". EAN-8/13 is a numeric only code and is the standard for the retail use of bar codes. The familiar bar code that appears on 1 grocery items and is scanned at supermarket check-outs are EAN codes. If you wish to use EAN codes for open systems then you must join the Article Numbering Association ( ANA) and be allocated a supplier number that needs to be represented in the code - if you are supplying large retailers or supermarkets they will generally oblige you to do this. EAN codes have good security; the 13th digit in EAN codes is actually a check digit. A shorter version, EAN-8 is for small items and leaves out elements of the full code. EAN is very similar in structure and algorithm to its American counterpart UPC, some readers and scanners effectively treat them as being the same. EAN-13 codes can also have "extension" codes added, these are most commonly used for newspaper and periodical identification, support for EAN extensions is not quite as wide as that of the core EAN codes, so if you want to print or read EAN code extensions it's usually a good idea to check for support when ordering printing or scanning products. EAN 8/13 codes uniquely identify a product but contain no information about the product.

 Interleaved 2 of 5 (ITF)

Interleaved 2 of 5 (ITF) is a popular numeric only code, used mainly for industrial applications. The principal feature of ITF is that each element of the code actually encodes two digits - hence the code is quite compact and good for encoding longer sequences of digits. Two main drawbacks however is that there is no inbuilt security and the structure of the code means that it's sometimes possible to get partial reads. For these reasons it's recommended that some form of checking or fixed length reading be implemented within the application (or intelligent scanner) if using ITF. So called "bearer bars" bars running along the top and bottom of the code are sometimes also added to help prevent misreads. The only other real limitation is that ITF must always encode an even number of digits, the convention being to add leading zeros to odd digit lengths.

ITF is the Symbology often used on outer cases of consumer goods (often in a fixed length 14 digit format). These are sometimes referred to as Case Codes.

Code 128

As its name suggests, Code 128 is capable of representing the full range of 128 ASCII characters. Code 128 is an increasingly popular alternative to Code 39 for Alpha/Numeric codes. A key feature of code 128 is that is capable of encoding ASCII functions (e.g. Carriage Return) as well as printable characters. To employ these facilities fully Code 128 uses three character sets (sometimes called tables) to fully define the conversion of each bar code symbol to an ASCII character. Some printing packages automatically select the code set according to the data required in the bar code whilst for others you will need to know and specify which table set variation you wish to use. Code 128 table set C is a numeric only table set and can give very compact numeric only bar codes.

The two examples shown below are Code 128 bar codes; one with 4 numeric characters; the other with 4 alpha-numeric characters (both with the same scaling and ratios). Code 128 codes can be of variable length and check digit encoding is standard. Code 128 can also encode special function codes for certain specialist applications.

1)

2)

EAN128

Standard EAN-8/13 product codes identify the product but contain no information about it. EAN-128 codes allow for information about the product to be included in the bar code. For example, batch number, serial number, expiry date, etc. The EAN-128 bar code is based on the Code-128 Symbology with a leading special function code. Numbering schemes and data content of the EAN-128 is controlled by the ANA Organization.

Other Symbologies
The symbologies listed above are probably the most common and most popular for general bar code applications, other symbologies tend to be used for more specific applications or industries. These include CODABAR and MSI/PLESSEY, which tend to be used in the UK mainly in libraries. DISCRETE 2of5 and CODE93 are variants of ITF and Code 39 respectively and are relatively rare.

BAR CODE READING TECHNOLOGIES
I need to read bar codes, but I'm confused about readers. There seem to be these pen type things that seem relatively inexpensive - or gun scanners that cost hundreds of pounds! What are the differences and what's best for me?

What type of bar code reader to use for a given application is dependant upon a number of considerations?

• The environment that reading will take place in.
• The nature of the host computer if applicable - e.g. Desktop PC or Handheld Computer - and whether portable or fixed position.
• The required throughput or volume of reads required.
• The quality of the codes being read.
• The level of training / capability of the likely users.

Most types of bar code reader described below are available in variants to output the bar code data using any of the main types of output format: Wand (Emulation), RS232 or Keyboard wedge. Note that the output format must normally be specified when the reader device is ordered and the internal reader electronics usually vary between the different output methods.

Wand (Emulation) outputs the bar code as an analogue signal which requires decoding prior to being input to the host computer system.

RS232 output bar code readers include decoder systems, outputting the bar code data as an RS232 signal (sometimes in an eavesdrop mode - see later section).

Keyboard Wedge output bar code readers include a decoder and connect between the keyboard and the host PC. The bar code data is sent to the host as key presses (the keyboard continues to operate normally). There are three general bar code reading technologies, and there are even variations of product type within these:

Wand or Pen Readers
Wand readers are the lowest cost "entry level" technology used to read bar codes. For the most part Wands look very much like ordinary pens, designed to be held in much the same way. Wands usually have no moving parts and rely on the user moving the optical head of the wand over the code to produce the scanning signal for decoding. Most wands are of the so called "dumb" type, meaning that their only function is to convert the optical signal seen at the tip into an analogue electrical signal that is then fed (usually via the attached cable) to the host device for decoding.

An extension of the basic wand reader is to place some decoding and / or memory storage in the wand. Devices such as these are usually described as "Smart", or "Intelligent" wands. These devices process the signal before transmitting to the host (e.g., converting to an RS232 Serial output signal). "Memory" wands serve a similar function but have on-board memory to store the data for later downloading to a host device. The common features of all wand devices though are that they require the tip or head of the device to contact with and move across the bar code or label.

This provides a number of obvious and immediate limitations:

• The surface being read must be suitable for having the wand passed over it. If a code is to be read frequently in this way it must be robust. Poor reading owing to codes printed on paper based materials effectively "wearing out" can be a problem.
• The user must be able to get into close proximity to the code in such a way that they can easily "swipe" the code with the wand.
• Very small or very large codes can be difficult for users to scan.
• Some practice and a certain "knack" needs to be developed to get a good first time read rate; novice users sometimes experience difficulty at first.
• The surface bearing the code must have adequate light/dark reflectance. In an attempt to protect delicate codes users sometimes place some form of clear plastic protection over the codes. However if the plastic itself is too reflective (or too thick) the wand will not be able to get an accurate image from the code.
• Wands are not really suited to "high volume" applications. Apart from wear and tear on the wands, the wear and tear on the users can be a bigger problem!
• Wands really need to be used on codes that are on a flat surface, reading around the curve of a cylindrical object is either very difficult or impossible. Despite these limitations

Wands are a very popular choice for low volume / close proximity type applications. The biggest overall benefit of a Wand of course being the relatively lower cost compared to the alternatives.

CCD Scanners

Of the two "non-contact" technologies, CCD scanners are generally the lower cost. CCD scanners basically use similar technology to that used in modern video cameras, they "scan" the bar code by registering a complete image of the code at once. CCD scanners are usually characterized by their wide heads and the way that they appear to illuminate the bar code in - usually red - light. Many people will have seen scanners such as these in use at checkouts in places such as DIY stores.

Although technically a non-contact technology, most CCD scanners usually only have a depth of field - i.e. reading distance - of one or two centimeters. Recent advances in optics though are now enabling some CCD scanners to work at longer distances, around five centimeters and in some cases more. For the most part though CCD for the time being at least remains largely a close proximity reading technology CCD scanners can come with a variety of output and interface types depending on the individual model and manufacturer. Because CCD scanners already contain a built in microprocessor the output from a CCD scanner is usually in a format that has already been decoded.

Common output types are RS232 Serial, TTL Serial or Keyboard Wedge Interface (see later section on Wedges). In some cases CCD scanners can emulate the analogue output usually associated with a Wand to enable compatibility with existing equipment or applications usually designed for Wand input referred to as Wand Emulation.

Apart from the need for close proximity the only real drawback to CCD scanners is the need to match the maximum reading width of the scanner head to the bar codes to be read. Hence a CCD Scanner with a maximum head and reading width of 60mm cannot read a bar code that is 7Omm wide. Generally the widest head width for regular style CCD scanners is around 8Omm. It would seem logical therefore to always select the maximum head width scanner available, however there is sometimes a very marginal trade off between greater width of CCD head and first time read rate, narrower heads sometimes having a very slight performance advantage. Hence care should be taken in selecting a CCD scanner to ensure that the correct head width is selected.

For much larger bar codes it is usually necessary to go up to Laser scanners. One further limitation concerns curved surfaces, whilst CCD scanners can cope with modest curvature much better than a Wand can, CCD readers are not usually designed to cope with the optical effects of curvature CCD Scanners probably represent the best "value for money" option for close proximity scanners and a good number are sold as Wand upgrades. If getting close to the code is not a problem and the throughput requirement is medium volume, then CCD scanners are a popular choice.

Laser Scanners

Laser scanners usually take the form of the characteristic "gun" type shape, usually being trigger operated by the user and apparently "firing" a slim beam of red laser light at the bar code. The beam effect is in fact an optical illusion, what is actually happening is that a pinpoint of laser light is being scanned across the code at rate around 30 scans per second, the speed and intensity of the scan giving rise to the apparent line effect. The very practical and useful effect of this though is that the user can see exactly where the laser is being targeted and "shoot" the scanner at the bar code correctly.

The major benefits of laser scanners are twofold; firstly they generally have a much greater depth of field than CCD scanners. Hence the distance of an "ordinary" laser scanner from the code will be around six to eight inches, but this can vary significantly as a function of the actual code size. Some Laser scanners can be special " Long Range" versions that can operate even further away, however these usually use the much higher powered Class II lasers and are both more expensive and require higher power to drive them.

The second major benefit is that of speed and accuracy of reading. Because the laser is scanning the code at a high speed, it quickly has a lot of "goes" at reading the signal; hence the decoding will often appear to the user to be virtually instantaneous. As a double check though the scanners are often set to have "redundant read" option which means that the scanner will not confirm and process the scan until it has received the same reading two or three times, only then will it be happy that the code has been correctly read. Redundant reading can be so fast with laser scanners that two or three times redundant scanning is often a default setting and many users will be completely unaware that this additional error checking is even happening!

The relatively low amounts of energy used in laser scanners do lead to limitations of use in conditions of high ambient light - the most common high ambient light condition being outdoors in good daylight. In these cases the background light energy can be higher than the reflected light from the laser and the scanner is effectively "dazzled" by the background light. For this reason laser scanners are generally not suitable for use outdoors. CCD scanners and even Wands can suffer from similar problems although usually to a lesser extent.

Laser scanners also exist in other forms apart from the familiar gun style. The most common form is the fixed head laser scanner. Modern supermarket check-outs are generally equipped with fixed head scanners. Instead of the side to side scan of the Laser gun, fixed scanners fire the laser beam in special patterns with the objective of intercepting and reading a passing bar code presented at any angle. The same approach is applied to the industrial use of fixed scanners where for example a scanner may be mounted to read codes on items passing along a production line.

Output signals from laser scanners generally take the same form as that of CCD scanners, i.e. Decoded Serial, Wand Emulation or Keyboard Wedge output. In addition some scanners transmit a raw laser signal (Laser output) and must be attached to suitable decoding equipment. Specialist EPOS (Electronic point of sale) terminals sometimes require an output type known as OCIA.

I thought Laser beams were dangerous - are Laser scanners safe?

The short answer is yes; in proper use they are quite safe. Certain international standards define the safety considerations for different power levels of laser light used in electronic products. The details are very technical, however Class I lasers - the lowest power - are very low power. The level of light energy that the human eye would receive from a Class I laser is (as a rough guide) less than the light the eye would receive on a bright sunny day! In any event the normal human eye blink action would provide more than adequate protection if a class one laser should happen to hit the eye. Class II lasers are higher power, Class II laser products carry a warning label about the possible effects of looking directly into the laser beam, however these products are still safe in general operation for two reasons, firstly the normal blink reaction will normally protect the eye in adequate time to avoid any damage and secondly with bar code scanners the beam is constantly moving. As a rough guide the intensity of a Class II laser beam is roughly analogous to looking directly at the sun on a bright sunny day.

I have been told that I should buy a 'medium resolution' reader - What does this mean?

Resolution refers to the ability of a given scanner to 'see' different sizes of bar code elements. If you have very small bar codes where the size of each element (i.e. the smallest bar) is very narrow, then you will need a reader that is capable of seeing very small bars - this would require a high resolution reader.

A larger bar code can of course be read with a high resolution reader; the main risk here though is that larger codes can be more susceptible to imperfections that a high resolution scanner might detect. Hence a bigger or poorly printed bar code is often best read with a lower resolution reader.

For the vast majority of day to day applications though a medium resolution scanner represents the best compromise between these two extremes - most users normally only have to concern themselves about resolution if reading very small or very large bar codes.

Resolution is normally expressed as the width that a given scanner can resolve down to medium resolution usually being viewed as a resolution of around 0.15mm-0.19mm.

CONNECTING READERS and SCANNERS
I want to connect a bar code reader to my PC - what do 1 need?

Connecting a bar code reader to a desktop PC is actually a lot easier than many people would first think. By far and away the easiest and most popular way to connect Wands and scanners is to use a device called a Keyboard Wedge interface, generally abbreviated to "Wedge" interface. A Wedge interface is small device that is attached to the keyboard socket on the PC base unit. The keyboard is plugged into the wedge, as is the bar code wand or scanner. The Keyboard continues to operate in the usual way and its operation should be completely unaffected by the presence of the Wedge.

When a bar code is scanned the wedge receives the signal from the wand, decodes it and then sends the decoded output to the PC by emulating the appropriate keyboard strokes. In most cases the end of the bar code string is indicated by the wedge sending a Carriage Return (CR) keyboard stroke.

The major benefits of wedges are:

• Ease of connection and interfacing.
• No need for any special device drivers or software programmers to be loaded onto the PC.
• Existing application software can be used with no need for modification.
• Most wedges and scanners can usually be adequately powered from the keyboard line power coming from the PC, so no special provisions need to be made to power the scanner.

There are, however, some potential downsides/limitations that may need to be considered in some cases.

• The scanner/wedge combination will always be on and available, so if the user scans at the wrong time or the wrong place in the application, the bar code data will still appear as if typed in at the keyboard.
• Wedges are generally not suitable for laptop or Notebook PCs. Many Laptop PCs will disable their built in keyboard if the unit senses any devices attached to the external keyboard port.

As long as these two considerations do not pose a problem, then a keyboard wedge is a simple and cost effective way to connect a wand or scanner. A further option that does sometimes offer potential cost savings if you are planning to use a CCD or Laser scanner is to purchase a scanner that has the PC wedge built into it already (instead of having a separate wedge). These devices usually terminate in a Y type connector to plug directly onto the PC socket on one side and an in line socket to take the keyboard cable.

 I not only want a bar code reader for my PC, but 1 also want to plug one onto a handheld computer, can 1 buy just one reader and swap it between them?

The answer will usually depend on exactly what the plugs and interfaces involved are, and the type of wands or scanners to be connected. Complication with different plugs and connectors and the possibility of having to set scanners to different configurations for different devices are just too complicated to enable an easy and practical "swap" from one device to another. The problem is exacerbated by the fact that there is no universal industry standard for bar code connectors. Over the years different manufacturers have adopted different types of plugs and connectors with varying pin connections and configurations. Gradually though the 9 way D style connector is beginning to emerge as a rough (but by no means certain) standard - but pin connections still vary widely! If you are ordering a Wand or Scanner to connect to a host device you already have, then you may need to give your supplier details of the connector and pin-outs on the device - look in the product's manual or technical guide if necessary.

I have an RS232 terminal, what's the best way to connect a Wand or Scanner?

It is possible to purchase keyboard wedges that will work with terminals; the major problem here is again the lack of standards. Even within the same range and model of some makes of terminal there can be variations of the keyboard scan-codes for different characters (scan-codes being the electronic codes the terminal sees when a key is pressed on the keyboard). Providing the terminal is connected to its host device via an RS232 serial connection then the optimal solution is to connect the scanner not to the keyboard but to the serial line. The idea is much the same as that of the PC wedge, except that this time an RS232 Serial Wedge - sometimes called an Eavesdrop wedge - is connected between the host and the terminal.

Eavesdrop wedges decode the wand/scanner input and converts it into an RS232 signal that is then sent to the serial line, as far as the host is concerned the bar code string has come from the terminal (remote echoing causing the bar data to also appear on the terminal screen). Eavesdrop wedges do cost a little more and take a little longer to set up, however their real benefit is flexibility and compatibility, because the scanner and wedge are independent of the terminal there is no need to replace the serial wedge should the terminal be changed or moved.

CONCLUSION
Hopefully this document from Belgravium will have covered your initial “thirst” for Bar Code information and you will now be in a position to plan and have some idea as to what products you will require.