> From: Byron A Jeff <byron@CC.GATECH.EDU>
>
> The design I hit upon after thinking a bit revolved around multiplexing
> the 6 LEDs utilizing a common pin on one side. Something like:
>
>  Port pin---+-[220ohm]--+---[LED]------  Port pin
>             |
>             +-[220ohm]--+---[LED]------- Port pin
>             |
>             +-[220ohm]--+---[LED]------- Port pin
>             |
>             +-[220ohm]--+---[LED]------- Port pin
>
> And so on. One led can be selected by tristating all but one port pin
> on the right side. The selected LED's color can be picked by twiddling
> with the left port pin and the one non-tristated port pin.
>
> The question that popped into my mind during this design discussion was
> the relevance of the position of the resistor. Does it matter if the resistor
> is on the anode or the cathode side of an LED. Is putting the resistor on
> the anode side simply a convention or is there some other reason that
 justifies
> its placement. Should a bi-polar have resistance on both sides (I've done this
> in previous designs)?
>

It makes no electrical difference which side the R is on.  Two 2-pin
devices in series may be used in any order.

BTW, the above circuit can also drive any 2 LEDs to arbitrary polarity
so long as consistent brightness isn't a problem.  How about 3 or more?
Have to think about that...

Regards,
SJH
Canberra, Australia