The January 1981 issue of The Hewlett-Packard Journal has a couple of articles giving some pretty good detail of the operation of the bar-code wand made for the HP-41. The first article mostly addresses the optics and the sensor electronics, and the second article addresses things like matters of timing. Log in at hp41.org and go to http://www.hp41.org/LibView.cfm?Command=View&ItemID=172
Obviously timing can't be super critical like it is with RS-232. You have to allow the user reasonable variation in scan speed. The bar-code reader allows a 10:1 ratio in scan speed range, and the speed can vary within reasonable limits during
the scan of a single line of bar code. The threshold for how long a 0 or a 1 is is derived from what was found in the last previous line and space (if I understood it right with a very quick read). In supermarkets where the beam rotates at a constant rate, you still have to allow for the bar code to be at different distances from the window, and for the product being scanned to be moving across the window (ie, not motionless) during the scan. Plenty of other things will be seen by the sensor, but false-start detection will keep it from trying to process garbage data, and checksums and other error-checking will make the register beep if there was a bad-enough smudge or scratch on an otherwise-valid bar code.
There are lots of kinds of bar code, and I think the wand made for the HP-41 only read one kind. The one made for the HP-71 read many more kinds of bar code. I seem to remember that one was made by Zengrange in England, not Hewlett-Packard.
A nice thing about bar code for the HP-41 was that back before the existence of the internet, magazine articles could have the accompanying programs in bar code printed there for the subscriber to enter a program quickly and without errors, instead of having to key it in. At user-group meetings or in mailings, all you needed to distribute programs was a photocopier. The generation of the original barcode came from the printer attached to the HP-41 calculator.
I have probably said almost more than I know, but hopefully it helps. Wikipedia has an article on bar code at http://en.wikipedia.org/wiki/Bar_code
(which I have not read). Please note however that this forum is about the HP-41 calculator and its peripherals, software, etc., and discussions should be kept in that context. If the rather simple bar-code wand for made the HP-41 helps you get started in understanding what has to happen in cash registers, terrific.