RTL Logic

From: der Mouse <mouse_at_Rodents.Montreal.QC.CA>
Date: Tue Jan 11 00:51:32 2005

[top-postings repaired manually -dM]
>>> Except that totem-pole outputs usually have just a transistor to
>>> ground, but a transistor and resistor to Vcc. This means that the
>>> one driving the signal low will win, and, provided not too many
>>> outputs are wired together, it will sink the current without
>>> damage.
>> well, I design by specifications. The TI specs say that the short
>> circuit current for 74xx series is 18ma which is also not within
>> specs for the general output current capability of 16 ma.
> The 16ma is for sink of the NPN to ground. The pullup is limited to
> around 100ua as I recall.

Limited to 100?A? That disagrees with my experience, which says that
the pullup on an output is 100-200 ohms - if you drive nothing else
with it, you can connect a LED from one to ground, and it's about as
bright as using a switching transistor and a 100-ohm pullup to Vcc,
meaning it's sourcing some 30-40 ma. While the output voltage is
pulled low enough that it won't reliably drive an input high, meaning
this is well outside its design range, the pullup isn't really
_limiting_ the current to sub-milliamp levels.

So I actually hauled out the book. Values given are from "The TTL Data
Book for Design Engineers", "Second Edition", from Texas Instruments,
copyright date 1976.

The chip I looked up the data for is the '00. There are family
differences; 54/74, 54H/74H, 54L/74L, 54LS/74LS, and 54S/74S all have
different columns in the table, and the 54 and 74 families have
different rows. Mostly the figures are the same for 54 vs 74, but
there are some cases where this is not so.

I_OH, the current sourced by a high output level, in ?A, maximum:
54/74?400; 54H/74H?500; 54L?100; 74L?200; 54LS/74LS?400; 54S/74S?1000.
(The values given in the book are actually negative, because the
current is sourced rather than sunk; I changed that for here.)

I_OL, the current sunk by a low output level, in mA, maximum:
54/74?16; 54H/74H?20; 54L?2; 74L?3.6; 54LS?4; 74LS?8; 54S/74S?20.

The schematics given do show the output with a transistor to ground and
a transistor and resistor to Vcc (with a diode in series with the
latter portion, for the 54/74 and 54L/74L). The nominal resistor value
shown is, in ohms, 54/74?120; 54H/74H?58; 54L/74L?500; 54LS/74LS?120;
54S/74S?50.

Now, _input_ pin pullups are much higher resistance. The resistances
shown for those are never less than 2.8K and in one case as high as
40K. Correspondingly, the input current values are much lower: I_IH
(input sink current, input high) is (?A) 54/74?40, 54H/74H?50,
54L/74L?10, 54LS/74LS?20, 54S/74S?50; I_IL (input source current, input
low) is (mA) 54/74?1.6, 54H/74H?2, 54L/74L not given(!), 54LS/74LS?.4,
54S/74S?2.

So yes, according to the spec, two outputs driving the same wire (to
different logic levels) is well outside the spec. Which doesn't mean
it won't "work", of course. :-)

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Received on Tue Jan 11 2005 - 00:51:32 GMT