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Datasheet Confusion

B

Bart

Jan 1, 1970
0
I'm looking at the datasheet for the 74LS193 binary counter and typical "HI"
output voltage is 3.4 volts but the current is listed as a minus 0.4 mA
How can you have a plus voltage with a minus current?
I also see this confusion on the 74LS08 AND gate chip.
I'm sure this confusion stems from my inexperience and hopefully someone can
explain this to me.
Thanks in advance,
Bart
 
J

John Fields

Jan 1, 1970
0
I'm looking at the datasheet for the 74LS193 binary counter and typical "HI"
output voltage is 3.4 volts but the current is listed as a minus 0.4 mA
How can you have a plus voltage with a minus current?
I also see this confusion on the 74LS08 AND gate chip.
I'm sure this confusion stems from my inexperience and hopefully someone can
explain this to me.
 
L

Lord Garth

Jan 1, 1970
0
Bart said:
I'm looking at the datasheet for the 74LS193 binary counter and typical "HI"
output voltage is 3.4 volts but the current is listed as a minus 0.4 mA
How can you have a plus voltage with a minus current?
I also see this confusion on the 74LS08 AND gate chip.
I'm sure this confusion stems from my inexperience and hopefully someone can
explain this to me.
Thanks in advance,
Bart

One thing you should learn early about TTL is that it is a current sink
logic family.
Ouputs are meant to sinking current to ground.

Since you are dealing with a counter, read about debounce circuits for input
conditioning
prior to connecting a push button to the clock input.

Also, learn to decouple your power supply by using small capacitors very
near the
IC power pins.

And finally, an open / disconnected input is not a zero.
 
A

Alan B

Jan 1, 1970
0
I'm looking at the datasheet for the 74LS193 binary counter and typical "HI"
output voltage is 3.4 volts but the current is listed as a minus 0.4 mA
How can you have a plus voltage with a minus current?
I also see this confusion on the 74LS08 AND gate chip.
I'm sure this confusion stems from my inexperience and hopefully someone can
explain this to me.
Thanks in advance,
Bart

The minus sign merely indicates direction. Note that the max LO current is
+8.0mA. When dealing with digital TTL circuits, you'll run into the terms
"source current" and "sink current." The logic "1" output is typically the
"source" state, and indicates that the TTL device must be the source of the
current for the node, and the value of 400uA is the max that the gate
output node can be expected to deliver. When the device is switched to
"0," it can be used as a "ground" or "return" "sink" for a separate source
of current somewhere on the node, and the max value is the maximum current
that the gate can be expected to pass when so configured. I hope that you
can see from this explanation that the current direction will be opposite
for each case.

Again, the sign only indicates a direction, and I'm not sure why the data
sheets determined which direction should be "-" and which should not.
That, at least, is my basic understanding of it. If I've erred on some
small point, Phil Allison, our resident keeper of all things CORRECT AND
PROPER, will be along shortly to call me a twit and a retard, and fail to
help in any way toward correcting the error.

HTH!
 
P

petrus bitbyter

Jan 1, 1970
0
Bart said:
I'm looking at the datasheet for the 74LS193 binary counter and typical
"HI" output voltage is 3.4 volts but the current is listed as a minus 0.4
mA
How can you have a plus voltage with a minus current?
I also see this confusion on the 74LS08 AND gate chip.
I'm sure this confusion stems from my inexperience and hopefully someone
can explain this to me.
Thanks in advance,
Bart

Bart,

Voltage is measured relative to "common" or "ground". As TTLs are used with
5V power supply and connected to GND and Vcc, voltages are always positive.
Currents only flow when an output is connected to an input. Currents also
have a direction. Manufacturers ever decided to define a positive current to
flow into a terminal no matter what terminal (input or output or even Vcc).
So current flowing out of a terminal is considered negative. Schematics of
in- and output are on the datasheet (at least on my - Texas - one) and you
can check out what happens when an output is connected to an input and the
output is driven high or low.

petrus bitbyter
 
R

Rich Grise

Jan 1, 1970
0
Since you are dealing with a counter, read about debounce circuits for
input conditioning
prior to connecting a push button to the clock input.


Triggering a counter with a raw pushbutton might be instructive as
a way to see contact bounce.

Cheers!
Rich
 
L

Lord Garth

Jan 1, 1970
0
Rich Grise said:
Triggering a counter with a raw pushbutton might be instructive as
a way to see contact bounce.
Good point....it won't hurt anything but expect it won't work right either.
 
B

Bart

Jan 1, 1970
0
Bart said:
I'm looking at the datasheet for the 74LS193 binary counter and typical
"HI" output voltage is 3.4 volts but the current is listed as a minus 0.4
mA
How can you have a plus voltage with a minus current?
I also see this confusion on the 74LS08 AND gate chip.
I'm sure this confusion stems from my inexperience and hopefully someone
can explain this to me.
Thanks in advance,
Bart
Thanks to all that replied. I had the Fairchild and Motorola datasheets that
didn't give the schematic for output terminals as did the Texas Instruments
one. After reading the replies to my post I have a grasp on it now.
Thanks again! This group is terrific.
Bart
 
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