RS-232 Tx / Rx monitoring LEDs?

[Mark J. Blair]

> On Aug 20, 2015, at 06:27, Paul Koning <paulkoning at comcast.net> wrote:
> 
> Totem pole outputs have comparable drive strength in both directions, that's precisely their purpose (to provide symmetric rise/fall times when driving capacitive loads).

That's true for CMOS outputs. TTL outputs pull down much more strongly than they pull up, which is why older designers are still in the habit of driving LEDs with active low outputs even though active high outputs work just fine with modern CMOS logic. Look at nearly any TTL datasheet, and note that VOL is much closer to ground than VOH is to VCC.


-- 
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/