I have just put up a page about how you can understand an LED matrix and the two ways of going about driving them.
Comments / improvements please from experienced users.
Beginners:- anything you find hard to understand?
Good article, will be definitely useful.
Do you have a datasheet for ULM2008 (I couldn't find this one anywhere)? Is it compatible with the more common ULN2803?
Thanks a lot, I did in indeed mean the ULN2803, I have now corrected it.
Thanks Mike. Very timely, as I'm just now looking at driving a 4-digit 7-segment display (haven't purchased one yet, still looking at options). I've been leaning towards the Maxim 7219, but I will look at those other options.
I do see some layout issues. Likely a result of using iWeb to generate the HTML. I've noticed that software which attempts to do WYSIWYG word-processing type HTML tends to over-specify the markup, and also make incorrect assumptions. An example is this:
I captured only the center of my browser display, because the rest isn't necessary for showing the vertical gap.
So I looked at the source code, and I see that the HTML is using absolute pixel positioning on the DIV elements. And this is typical of what I see from code generators. The problem is that it assumes that font rendering and monitor resolution will be the same on any platform where the page is viewed. I would be happy to help out with simplifying the layout (it would still look the same) if you're interested.
A couple editorial comments:
I would add a comma in With each, data is supplied to one side of the matrix and a walking bit applied to the adjacent side.
I think I would break this into two sentences. These tutorials (one on the official Arduino site) are very bad and will eventually destroy your Arduino. Although you might think it works for some time eventually the Arduino will fail.
Thanks for your time in writing this (and your other tutorials).
Thanks, in that big gap is an animation of a multiplexer. What browser are you using? Have you got the quick time extensions?
The animation worked fine for me with IE9.
I think a comparison picture of Common Anode vs Common Cathode would be helpful too, maybe help keep folks from buying 7219/7221s and the wrong display type, then having to obtain additional parts to make it work.
Thanks but I am not sure I understand the concept of a common anode or common cathode matrix. This is because all the cathodes in one direction are connected together and all the anodes in the other direction. So all you need to do is to rotate the display by 90 degrees.
For non square matrices then you can get a column cathode and column anode.
Yes I will consider adding something on pre built matrices and matrix driver chips.
Thanks for the report on the animation it took hours to get it up on the FTP site, it worked out that there is a media folder generated that I was failing to transfer.
I think the common anode vs common cathode comes into play more when you use RGB matrices.
In that case, an 8x8 RGB looks electrically like an 8x24 - if you have one and bought drivers for the other you're kinda screwed.
Ha yes quite right. RGB LEDs are probably worth a page to themselves. I is amazing how people still want to connect them in series. 
CrossRoads:
I think the common anode vs common cathode comes into play more when you use RGB matrices.
In that case, an 8x8 RGB looks electrically like an 8x24 - if you have one and bought drivers for the other you're kinda screwed.
If you buy 7 [8] segment displays, they come pre-wired one way or 'tother.
http://www.sparkfun.com/products/9932
Grumpy_Mike:
For non square matrices then you can get a column cathode and column anode.
Same thing, I believe, just different terminology. And, unless I'm being dyslexic, assumes the column is common.
Grumpy_Mike:
Thanks, in that big gap is an animation of a multiplexer. What browser are you using? Have you got the quick time extensions?
Oh, well, I normally browse with lots of stuff disabled, because I've had too many cases of auto-start media interrupting what it is I'm listening to while I'm browsing. I didn't see the object reference when I looked at the page source, because I didn't know to look for it, and iWeb is not constructing the page with inline images, etc, occurring in the markup in the spot where the the text flow is.
So, my other suggestion is to include a link to the media along with the embed. 
If you buy 7 [8] segment displays, they come pre-wired one way or 'tother.
Very true but the page is about a matrix not a segment (however many) display.
Same thing, I believe, just different terminology.
Not quite, if you have say a 6 by 7 matrix it defiantly has a "way round", so if it has 6 in the x direction those 6 can be common anode or cathode depending on if you have column cathode and column anode.
With a square matrix you can simply rotate it by 90 degrees.
my other suggestion is to include a link to the media along with the embed
Good idea I'll do that tonight thanks.
Thanks for the site and info Grumpy, I have made it my homepage on my browser so I can read EVERYTHING you have there.
TONS of great info!
Grumpy_Mike:
If you buy 7 [8] segment displays, they come pre-wired one way or 'tother.
Very true but the page is about a matrix not a segment (however many) display.
Same thing, I believe, just different terminology.
Not quite, if you have say a 6 by 7 matrix it defiantly has a "way round", so if it has 6 in the x direction those 6 can be common anode or cathode depending on if you have column cathode and column anode.
With a square matrix you can simply rotate it by 90 degrees.my other suggestion is to include a link to the media along with the embed
Good idea I'll do that tonight thanks.
You're welcome. Too bad it doesn't loop when viewed in YouTube.
I think I understand the distinction you're making. (Work was particularly taxing this week, but I think I've mostly recovered.) So, using your drawing with a column of resistors, and a ULN2803, I would say the each row is wired common anode. But, the terminology of column anode [cathode] is referring to the array as a whole.
Too bad it doesn't loop when viewed in YouTube.
I think I will put more cycles on the You Tube video. Interestingly Vimeo wouldn't load it as it said it was too small to be a movie.
Yes the term column anode / cathode only applies for a rectangular matrix, for a square one a rotation of 90 degrees changes it.