Re: When is high not high.

From: Andrew Holme (andrew_at_nospam.com)
Date: 02/19/05 

Date: Sat, 19 Feb 2005 09:47:37 -0000

Chris W wrote:
> I have my inventory of stuff I got on that ebay auction so I decided
> to build something and see if it would work. I think I learned
> something, but I'm not sure exactly what and was hoping some one
> could tell me. I took my MH7442, a 10 LED bar thing-a-majig, a set
> of dip switches and a resistor. Wired it up to display the LED 0
> through 9, based on the Binary input from the dip switch. At first
> it didn't work. After some experimenting, I found what I thought was
> a high going into th BCD input of the MH7442 was not what it wanted.
> I had the switch set up so a low was the absence of a voltage source,
> what it really wanted, was that a high was the absence of a ground
> connection. My question is, is there some way I could have known
> this? Is there something in the data *** that would tell me that?

The 7442, without letters between the 74 and the 42, belongs to the original
TTL family. What you have observed is characteristic of TTL inputs. LS
devices behave similarly. This might not be mentioned explicitly on the
7442 data*** because it is a feature of the whole family, not just that
particular device. A good book, such as "The Art of Electronics" by
Horowitz and Hill, would explain all.

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