# 7400 Chip

Discussion in 'Electrical Engineering' started by Ace, Aug 23, 2005.

1. ### AceGuest

Hi

Forgive my ignorance, but I'm a new at this and haven't much of a clue. I'm trying to build a circuit I've designed with NAND gates, but am slightly confused as to the operation of the 7400 quad 2-input NAND gates chip. I need to connect pin 7 to 0V, but does pin 14 (Vcc) need to be supplied with 5V? And then the inputs to the NANDs should be supplied with 5V for high I presume... And for low do they need to be connected to 0V, rather than left disconnected?

Logic design programs are all fine and dandy, but only when you are faced with chips in the flesh you realise how little you know.

Many thanks IA.

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2. ### John GilmerGuest

That's OK. I'm just amazed that you can actually FIND a 7400 around.
slightly confused as to the operation of the 7400 quad 2-input NAND gates
chip. I need to connect pin 7 to 0V, but does pin 14 (Vcc) need to be
supplied with 5V? And then the inputs to the NANDs should be supplied with
5V for high I presume... And for low do they need to be connected to 0V,
rather than left disconnected?

OK.

In TTL logic, Vcc is 5 volts.

Usually, anything under around 0.8 volt was considered a LOW and anything
over 1.2 volts was considered high.

In practice, LOW was on the order of 0.2 volt and high was from 3 to 4
volts. With OC (open collector) stuff, the high could well be 5 volts.
with chips in the flesh you realise how little you know.

Have fun!

You should know, however, that even in the early 70s, 7400 (quad NAND) was
just "glue" that was used to tie stuff like latches and counters together.

But there were some fun tricks with 7400 stuff:

Tie the output to one of the inputs and when the other input goes from low
to high, the output is just a narrow spike.

Take two NAND gates and take the outputs of each one to one input of the
other and you have your basis R-S flip/flop. Sometimes used to "debounce"
push button switches.

With a little controlled feedback, you can turn 7400 gates into amplifiers
or oscillators. As the old joke says: when the only tool you have is a
hammer all problems look like nails.

If you get to using counters and such you may find out the hard way that
decoding a value from a counter can generate glitches.

Let us know what you are doing. Several of us would be glad to help just
for "old times sake."
You are quite welcome.

3. ### ehsjrGuest

John gave you some good stuff in his post - be sure
to note it. Most often you can find stuff on line with
Google, by putting the part number and the word "datasheet"
(without the " marks) in the Google search box. Here's
one helpful site that search found:

http://www.priory.bromley.sch.uk/students/electronics/reference/7400.htm

Enjoy!

Ed

4. ### krwGuest

I have a few drawers full of 74xx parts (I think), complete with '73ish
date codes. ;-)
At the receiver VL <.8V VH > 2.4V. At the driver, I believe it's .4V
and 2.8V. An input of 1.2V is right in the threshold (two Vbe above
gnd).

Yes, the no-load Voh was about two diode drops below Vcc (3.6ish
volts).

<snip>

operation of the 7400 quad 2-input
I worked with these things in 1966 ~ 1970 designing ICL 1900 mainframes.
The 7400 were £5 a go in very large quantities then, but it was a
super technology compared with silicon transistor boards. CMOS might be
my choice nowadays I think.
There were two schools of thought on some of your questions: that of
the people using them to 'make things' and that of the circuit
design/quality team who were more formal.
yes. We also put a 0.1 microfarad capacitor between 5v and ground for
every 3 or 4 chips to keep the supply clean.
Well, our circuit design boys didn't like that because if the supply
voltage went a bit high they claimed it would fry them. We had a
dropper resistor pair giving about 2.4v I think. However, lots of
designs get away with connecting directly to the power rail.
I later used one of our computers for 10 years - in that time the only
electronic failure in the processor was one of those dropper resistor chips!
Oh yes, definitely, but remember that will force the output high
irrespective of the other input. If you leave them open circuit they
float up, giving a high. We could rely on them to stay this way and
just follow the other input, but the circuit boys said it would slow
them down a bit. With clock cycle times of 300ns (wow!) we could not
afford that.

With 7400 NAND (actual 7400, not the family!) though, if you really want
an unused input (ie you are using it as an inverter) you can connect the
two inputs together - no need to connect to high or low.

Voltage limits on these things are 5v + or - 0.25v, but I was never
happy with a system unless I weeded out anything which didn't work down
to the lower limit of the supply at 4.2v.
yes, it's a steep learning curve.

The devices were almost indestructible. No static problems; shorts in
the wiring to ground, power, each other never seemed to harm them.
That's until a colleague poked about in the wiring with a main hand lamp
and managed to connect +24volts to a reset line visiting lots of places!!

Oh the nostalgia!

Phil

6. ### Guest

I agree with Phil. I like CMOS better than TTL. The power requirements
are a bit less demanding too.

My spa controller is all 4000 CMOS and SSRs. It has survived
hurricanes, thunderstorms and a fire. I don't know what would break
it. I suggest wirewrap sockets for your prototype. Changes are easier.