RS-232 Tx / Rx monitoring LEDs?

Paul Koning paulkoning at
Thu Aug 20 11:54:51 CDT 2015

> On Aug 20, 2015, at 11:58 AM, Mark J. Blair <nf6x at> wrote:
>> On Aug 20, 2015, at 06:27, Paul Koning <paulkoning at> 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.

Ok, but when you refer to "drive strength" I assumed you were talking about current, not voltage.  By that measure totem pole outputs are pretty much symmetrical.


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