Newbie 74141 help

[Sean]

I'm trying to figure out how a 74141 IC works, but it's making me feel stupid. Please keep in mind that i am completely new to electronics and circuits. Help?!?

I've got a 74141 on a breadboard. Based on the datasheets and nixie clock schematics I've been studying I put 5vdc at pin 5 and pin 12 to ground. Right now I'm just trying to light up an LED (some nixies are coming in the mail). So I connected the LED to the 5v and to the 0 output at pin 16. Since all four inputs are at no power shouldn't this light the LED. Please tell me I'm just doing something stupid.

[Michel]

Not sure if inputs A, B, C and D are '0' on default, you better
connect them to GND. You also need a series resistor with your LED (at
least 680 ohm) and keep in mind that an LED has an anode and cathode.

Wish this helps.

Michel

[Shane Ellis]

You need to bring the binary pins high or low. You've got them
floating. Get a datasheet for the chip, and add voltages to the ABCD
pins, in the proper configuration. There is a binary chart, as well
as corresponding binary to pin layout, on that datasheet.

On Sep 24, 2012, at 9:42 PM, Sean <sean...@gmail.com> wrote:

> I'm trying to figure out how a 74141 IC works, but it's making me feel stupid. Please keep in mind that i am completely new to electronics and circuits. Help?!?
>
> I've got a 74141 on a breadboard. Based on the datasheets and nixie clock schematics I've been studying I put 5vdc at pin 5 and pin 12 to ground. Right now I'm just trying to light up an LED (some nixies are coming in the mail). So I connected the LED to the 5v and to the 0 output at pin 16. Since all four inputs are at no power shouldn't this light the LED. Please tell me I'm just doing something stupid.
>

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[Sean]

Thanks! Didn't realize I needed to pull the inputs down to ground. Guess I was thinking since the chip was grounded at pin-12 that the inputs would default to low. All works as expected now. Thanks again!

[David Forbes]

On 9/25/12 6:07 AM, Sean wrote:
> Thanks! Didn't realize I needed to pull the inputs down to ground. Guess I was thinking since the chip was grounded at pin-12 that the inputs would default to low. All works as expected now. Thanks again!
>

In case you are wondering why this is so, it's because TTL logic is
based on NPN transistors. They are good at pulling a signal down to 0V,
but not good at pulling a signal up to 5V. So a resistor does the
pulling up, and the transistor does the pulling down.

The result of this is that TTL inputs default to a 1, not a 0, when left
floating.

Also, the TTL threshold voltage between 1 and 0 inputs is quite
asymmetrical, being <0.8V is 0 and >2.0V is 1. And TTL '1' outputs do
not venture above 4V.

The newer CMOS logic families are symmetrical in their high and low
drive capabilities, but some of them such as the 74HCT series have the
asymmetrical TTL input level threshold voltage for backwards compatibility.


--
David Forbes, Tucson AZ

[taylorjpt]

In case you are wondering why this is so, it's because TTL logic is
based on NPN transistors. They are good at pulling a signal down to 0V,
but not good at pulling a signal up to 5V. So a resistor does the
pulling up, and the transistor does the pulling down.

You can see this in the schematics:  The inputs are emitter followers that are pulled up to VCC-Vbe and likewise the outputs are also emitter followers that can't go above VCC-Vbe at best and 2 Vbe for the original TTL.

Bottom line: Unconnected inputs are logic-1.

[threeneurons]

General rule of any kind of logic chip, is to terminate ALL inputs. All the inputs on a chip should be either tied to gnd, or a logic-1. I say a logic-1 because some older TTL chips didn't like the inputs tied to +5V. A static logic-1 can be made by using a resistor in the range of 1K to 10K. Tie one end to +5V, and the other to the intended input. 4000 and 74HC CMOS, as well as 74xx TTL need their inputs tied either way. +5V (if that's the supply voltage) can be tied directly to CMOS chips.

As mentioned, several times before, non-terminated TTL inputs will float to a logic-1. But its a marginal logic-1. ~1.6V. So, if its in a circuit, where signals are bouncing around, there maybe some cross-talk, that sporadically brings that input to a logic-0. This may give undesired effects. Therefore, remember the rule of terminating all inputs.

Oh yeah, an input that's tied to the output, of another chip (or even itself), is terminated. 

[chuck richards]

Yes, John is right about that.
TTL inputs tend to float high.

You can see that if you take a 74141 and power it
up and look at each of the four inputs using a
high impedance DVM. They will all be at some voltage
just over the threshold that makes them high.

That floating high business is really not to be used
in a circuit, but for quick tests, leaving a TTL input open
usually results in it being interpreted as being high.

Chuck

>
>
>---- Original Message ----
>From: j...@tayloredge.com
>To: neoni...@googlegroups.com
>Subject: Re: [neonixie-l] Newbie 74141 help
>Date: Tue, 25 Sep 2012 10:00:47 -0700 (PDT)
>
>>
>>
>>> In case you are wondering why this is so, it's because TTL logic
>is
>>> based on NPN transistors. They are good at pulling a signal down
>to 0V,
>>> but not good at pulling a signal up to 5V. So a resistor does the
>>> pulling up, and the transistor does the pulling down.
>>>
>>
>>You can see this in the

>schematics<http://www.tayloredge.com/reference/Electronics/Logic/74xx
>00.pdf>:

>>The inputs are emitter followers that are pulled up to VCC-Vbe and
>likewise
>>the outputs are also emitter followers that can't go above VCC-Vbe
>at best
>>and 2 Vbe for the original TTL.
>>
>>Bottom line: Unconnected inputs are logic-1.
>>

>>--
>>You received this message because you are subscribed to the Google
>Groups "neonixie-l" group.
>>To post to this group, send an email to neoni...@googlegroups.com.
>>To unsubscribe from this group, send email to
>neonixie-l+...@googlegroups.com.
>>To view this discussion on the web, visit

>https://groups.google.com/d/msg/neonixie-l/-/k6WP08KBh7IJ.

>>For more options, visit https://groups.google.com/groups/opt_out.
>>
>>
>>

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