RS-232 Tx / Rx monitoring LEDs?

[Eric Smith]

On Thu, Aug 20, 2015 at 10:54 AM, Paul Koning <paulkoning at comcast.net> wrote:
> 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.

Not for true (bipolar) TTL.  See the specs for the 7400, 74LS00, 74S00:
    http://www.ti.com/lit/ds/symlink/sn74ls00.pdf

          Ioh(max)  Iol(max)
7400      -0.4      16
74LS00    -0.4      8
74S00     -1        20

CMOS for comparison:
    http://www.ti.com/lit/ds/symlink/sn74hct00.pdf
    http://www.ti.com/lit/ds/symlink/sn74act00.pdf

          Ioh(max)  Iol(max)
74HCT00   -4        4
74ACT00   -24       24