RS-232 Tx / Rx monitoring LEDs?

Mark J. Blair nf6x at
Thu Aug 20 12:53:35 CDT 2015

> On Aug 20, 2015, at 09:54 , Paul Koning <paulkoning at> 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.

Again, CMOS totem pole outputs are pretty much symmetrical, but TTL totem pole outputs aren't even close. Looking at a Fairchild 74LS04 datasheet, I see a 20x difference in recommended drive-high current (Ioh = 0.4mA) vs. recommended drive-low current (Iol = 8mA). And the voltages are correspondingly different, too: at rated drive current of 0.4mA, Voh = 2.7V (min) to 3.4V (typ) with a 5V supply, while Vol is 0.35V (typ) to 0.5V (max) at 8mA drive current. So that's > 1.6V drop from Vcc while sourcing a mere 0.4mA, vs. < 0.5V rise from GND while sinking 8mA.

Totem pole outputs just mean that the output driver actively drives both up and down, with two stacked drive transistors. It does not imply that the drive strengths are even close to being equal, particularly when we're talking about TTL logic in a vintage computer.

Incidentally, TTL inputs also present asymmetric loads for high vs. low inputs at about the same ratio (18x input current ratio in the 74LS04 example when driven at the input thresholds), and have asymmetrical input threshold voltages. So unloaded TTL output voltages aren't relevant if we assume that the output is driving a TTL input of the same logic family. Even if a TTL output appears to drive all the way up to Vcc with no load, it won't once it's driving a typical load.

So you might think of those TTL totem pole drivers as being symmetrical when they're strictly driving TTL inputs of the same family, since the TTL inputs and TTL outputs are designed to work together. But they're very strongly asymmetrical when driving things other than TTL inputs, such as the LEDs in question here.

Mark J. Blair, NF6X <nf6x at>

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