UPC, Postnet, and Code 39. I've written software for all three
systems in the past.
> > You can buy bar code scanners that sit between the keyboard and the
> >computer. Some require a physical wipe across the code, others move the
> >beam for you and let you read the item without physical contact.
> There are three types of scanners: pen wand, CCD and laser. Pen
> wands are worthless, as they misread often (a retail client
> called me in after their system was condemned by the state of NJ
> for misreading.) CCD read the entire code, but need to touch the
> bar (or be within 2-4 inches). Lasers are the most reliable, and
> some can shoot twenty feet (for inventory).
I have use an HP pen wand type for years without any problems. The
book divides the things a little differently; but yes, in general you
either:
* Move a wand like device with built in illumination over a single
bar at a time,
* Provide even illumination to an area and scan the image
electronically with a CCD or similar array, or
* Measure the general level of reflection as you scan a laser beam
across the code.
Grocery stores to the latter with great success, using UPC or EAN
codes.
The hand wand is actually far more accurate on a Per Scan basis, but
the laser and CCD scanners just keep scanning till they get it right! And
you can see what they are imaging by the line of light on the target,
which provides the critical feedback that a scanner operator needs to do
the job well, better than with a wand reader under favorable
circumstances. Stop and think a moment, and you will realize that in a
grocery store, where the equipment just keeps scanning, most of the reads
are mis-reads that are rejected by the equipment. As you can imagine,
scanners do Much Better when you use checksums and other games such as a
fixed number of bars, etc. to prevent incorrect reads from being accepted.
To provide better rejection for incorrect scans, UPC version A and E
are fixed length codes with a fixed number of code bars. Version D is a
variable length code. The two halves of versions A and E have their own
checksums, with the second part using an encoding scheme that is the
negative of the scheme (not the value) used in the first part. It is
expected that the two will be read independently, and put together by the
equipment into a single code sequence. It is not so much designed to be
easy to read; rather, it is Very well designed to Reject bad reads.
Rejection is something that Code 39 is less good at.
> > Whenever I do a large mailing to clients, I use bar codes below the
> >return address so that I can retrieve the database record from which the
> >envelope or card was generated. That lets me phone them and update or
> >weed out returns rather quickly. It is not that hard to print bar codes
> >using a laser printer.
>
> And that is still another type of bar code (from the USPO). Most
> WP software will print those.
You are thinking about Postnet and Fim bars. I use a code39 stripe
under the return address, not the zip code, as all the offices in a
building will have the same Postnet code, making it useless for
identification.
The USPS has a free book specifying that crap code. No way I can
afford the equipment to read Postnet! The equipment has to scan the
entire letter to decode postnet, as it will never be in the same exact
position on every envelope, yet reading it depends upon determining the
exact height of the bars! Leave it to the government to use something
Very Expensive and Inflexible! I have written software to print Postnet
and Fim bars too. Using Postnet under the TO address often cuts about a
day off the time-to-target. But it is crap as far as ease of decoding is
concerned!
> Printing of bar codes from a laser printer will work for small
> volumes but are not too reliable for mass reading as in a retail
> store. Then you need thermal printers.
Depends upon your laser printer. A good laser printer will produce
excellent bar codes. But you will have to change your toner and drum more
often to maintain that level of excellence, so it is advised to test scan
a few codes from each sheet.
> Gee, you'd think I know something about bar coding and retail <g>.
I did some consulting work for a warehouse that was using bar codes on
everything, and for another company making ink jet based envelope printing
equipment. The warehouse used a lot of wand type equipment, but started to
convert to laser type readers with built-in memory when the prices came
down enough. They used laser printers for all the bar code tags. Although
they did not have many mis-reads, misreads were mostly caused by either
not wanding with an even movement, or by going off the side of the tag.
We were trying to get them to use "bearer bars", a kind of guard bar on
the top and bottom of the bar code.
As I once said, the web is a side line for me. I earn most of my
living programming. But I do enjoy programming up stuff for the web. It
is the most common user interface in use today!
See http://www.mall-net.com/webcons/code39.html for some simple(?) C,
PERL, and DBase software to print bar codes. Stuff you can modify.
(Stuff you probably will have to modify...)
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