When is high not high.

[Chris W]

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 sheet
that would tell me that?

--
Chris W

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
http://thewishzone.com

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

 

[Peter Bennett]

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 sheet
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
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Robert Monsen]

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 sheet
that would tell me that?

Yes. If this is the same as a 74HC42, then the line over the Ys means
they are 'active low'. That means that an active output is 0 rather than 1.

Thus, if you input a number b0010, then the /Y1 output will be 0, the
rest will 1.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[Andrew Holme]

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 sheet 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 datasheet 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.