Re: When is high not high.

From: Peter Bennett (peterbb_at_somewhere.invalid)
Date: 02/19/05 

Date: Fri, 18 Feb 2005 21:08:52 -0800

On Fri, 18 Feb 2005 21:22:05 -0600, Chris W <1qazse4@cox.net> 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?

For bipolar TTL logic chips, the inputs are effectively emitters of
NPN transistors. Therefore, if the input is not connected, the chip
will see it as a high. You must ground the input for the chip to see
it as a low.

For CMOS parts, an input is the insulated gate on a FET, and, if left
unconnected, will float randomly between high and low. If the input
stays in the "maybe" state (half way between high and low), the chip
is likely to draw excessive current. Therefore, with CMOS parts, all
inputs, whether you use them or not, MUST be connected to either
ground or the positive supply, either directly of through a resistor. 

-- 
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
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