[sdiy] LED current source needed for 'digital' outputs?

[Phil Macphail]

On 01/03/2014 15:08, "Tom Wiltshire" <tom at electricdruid.net> wrote:

>
>Other chips sometimes specify different ratings for current sink vs
>current source, so be sure to check that. That's why you sometimes see
>LEDs hanging off the +5V rail and giving an inverted output.
>
>T.
I think that is a TTL legacy, which only had NPN transistors. Current
sourcing was via an emitter-follower (or darlington), while current
sinking was with a grounded-emitter. With CMOS the current drive is pretty
symmetrical to ensure equal rise and fall times.

Phil.
>