Questions on 7400 Series Gates
LAB
I'm wondering if you need to connect PIN #7 to ground to use 7432 as OR
gates. How can you check if the chip is broken or not? I'm applying 5V to
PIN #1 & #2, but I'm getting something like .2V in PIN #3. I should get 1
out of PIN #3 instead of 0. Thanks.
Steve
>
> Hi again,
> I'm wondering if you need to connect PIN #7 to ground to use 7432 as OR
> gates.
YES, it must be powered via Vcc pin 14 and GND pin 7 and must share
ground with anything it is interfaced to!!!
> How can you check if the chip is broken or not? I'm applying 5V to
> PIN #1 & #2, but I'm getting something like .2V in PIN #3. I should get 1
> out of PIN #3 instead of 0. Thanks.
----------------
It will work if you power it as above. These are machines, they must
have POWER AND SIGNAL, both of which share a common ground reference.
Also, tie ALL unusued chip inputs to Vcc or else to GND, or it may
oscillate and heat up.
-Steve
--
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Alan Hackett
7432. Then it should work!
Alan
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Jenal Communications
Manufacturers and Suppliers of HF Selcall
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William L. Bahn
5V (Vcc) to pin 14 and 0V (GND) to pin 7.
If this is TTL (as opposed to CMOS) then, in theory, unconnected inputs are
pulled high internally. Don't rely on that. Explicitly establish a valid
logic level on all inputs. Leave unused outputs floating.
If it is TTL, then applying a logic HI to pins 1 and 2 should result in a
logic HI on pin 3. On many TTL families the open circuit output voltage for
a logic HI is well below 5V (but well above Vohmin).
LAB wrote in message <9m4l86$bas$1...@charm.magnus.acs.ohio-state.edu>...
John Fields
>
> All logic gats must be powered. For this particular chip that means applying
> 5V (Vcc) to pin 14 and 0V (GND) to pin 7.
>
> If this is TTL (as opposed to CMOS) then, in theory, unconnected inputs are
> pulled high internally.
---
Not true. 74XX or 54XX devices provide uncommitted emitters as logic
inputs with the corresponding bases pulled up to Vcc. In order to
conserve power, the unused inputs should be pulled up to Vcc through a
resistor or to 4.5V (the 4.5V is TI's recommendation). They can also
be pulled down to GND, the chip will just draw a little more current.
---
> Leave unused outputs floating.
>
> If it is TTL, then applying a logic HI to pins 1 and 2 should result in a
> logic HI on pin 3. On many TTL families the open circuit output voltage for
> a logic HI is well below 5V (but well above Vohmin).
>
---
John Fields
Professional circuit designer
John Fields
> In order to
> conserve power, the unused inputs should be pulled up to Vcc through a
> resistor or to 4.5V (the 4.5V is TI's recommendation).
---
Oops! I forgot about the decrease in noise immunity if the inputs are
left floating.
Robert
John Fields wrote:
Are you even reading the relevant applications literature? The emitter input
transistor is alternately used in forward and inverse mode. This means that
when the input is L , the transistor operates as the usual NPN. But when the
input is H, the transistor is now inverse with the emitter acting as a
collector. The alpha-I is about 0.02 for standard TTL- don't know about 74S.
Nearly all of the current drive for the succeeding stage comes from the base
circuit- and this will remain negligibly the same whether the input floats
or is pulled active H.
John Fields
---
I believe You missed the point.
The assumption is that an input should be terminated. Either to Vcc or
to GND. If it's terminated to Vcc the current draw will be smaller than
if it's terminated to GND.
Robert
John Fields wrote:
Oh- okay- thanks for the clarification. Now I can see that your advice is
definitely wrong. The succeeding stages are placed in a variety of states that
could mean more or less power depending upon the specific logic circuit. You
should only say "current draw from the input".
David
"John Fields" <jfi...@austininstruments.com> wrote in message
news:3B865452...@austininstruments.com...
> ... 74XX or 54XX devices provide uncommitted emitters as logic
> inputs with the corresponding bases pulled up to Vcc. In order to
> conserve power, the unused inputs should be pulled up to Vcc through a
> resistor or to 4.5V (the 4.5V is TI's recommendation).
As a matter of interest, why do TI specify 4.5V, John?
Do they mean directly to a 4.5V rail (no resistor)?
(I'm assuming the TTL supply rail is nominal and therefore >4.5V)
From an old Analogue Design Engineer (who still thinks of TTL as 'state of
the digital art' much to the amusement of his micro-processed,
field-programmable colleagues and can even remember those 1960's RTL simple
logic blocks that looked like giant 74XX DIL i.c.'s)
Mark Zenier
John Fields <jfi...@austininstruments.com> wrote:
>William L. Bahn wrote:
>>
>> All logic gats must be powered. For this particular chip that means applying
>> 5V (Vcc) to pin 14 and 0V (GND) to pin 7.
>>
>> If this is TTL (as opposed to CMOS) then, in theory, unconnected inputs are
>> pulled high internally.
>
>---
>Not true. 74XX or 54XX devices provide uncommitted emitters as logic
>inputs with the corresponding bases pulled up to Vcc. In order to
>conserve power, the unused inputs should be pulled up to Vcc through a
>resistor or to 4.5V (the 4.5V is TI's recommendation). They can also
>be pulled down to GND, the chip will just draw a little more current.
Er, not to pile on or anything, but those chips have been obselete
for 25 years. Where do you even get 74xx chips outside of mil. spec.
or off some pegboard where they've been hanging for 20 years or so?
Last new 74xx I saw was at Radio Shack and they had to go to the Soviet
Union (Sovtek brand) to get them.
74LS and 74F chips, with a few exceptions, are really a form of DTL and
have different rules. Motorola, last I looked, specs their 74LS diode
inputs as good for up to 15 volts and they can be directly connected
to Vcc. (Warning: some chips use other input structures so you have to
look at the spec sheet Abs. Max. ratings).
74xx Multi-emitter inputs will have an Abs. Max. input voltage of
5.5 volts. If you raise an input to too high a voltage, it decides it
would rather be one of the input transistor's collectors which doesn't
work so hot. So to prevent noise spikes on the Vcc exceeding the limit,
you need the resistor or a seperate lower supply that give some noise
margin.
Mark Zenier mze...@eskimo.com Washington State resident
Richard Crowley
> > ... 74XX or 54XX devices provide uncommitted emitters as logic
> > inputs with the corresponding bases pulled up to Vcc. In order to
> > conserve power, the unused inputs should be pulled up to Vcc through a
> > resistor or to 4.5V (the 4.5V is TI's recommendation).
"David" <ran...@hotmail.com> wrote ...
> As a matter of interest, why do TI specify 4.5V, John?
> Do they mean directly to a 4.5V rail (no resistor)?
> (I'm assuming the TTL supply rail is nominal and therefore >4.5V)
I seem to recall that the digital ICs of that era (TTL and especially ECL?)
were significantly prone to "latch-up". Any excursion of the pull-up voltage
> 5.00V (VCC) would latch up the device. This was especially dangerous
during power-up, depending on the design of the power supply. Thus the
conservative recomendation to do pull-up through a resistor (current
limiting) to 4.5V
Robert thinks he can figure out TTL theoretically from the "equivalent
schematics"(*), but he apparently has not used them in the real world. No
engineer of any reputation would leave TTL inputs flapping in the breeze.
The TTL grandchildren of today are orders of magnitude more robust and
tolerant than the original 7400 series.
(*) Remember that they are called "equivalent" schematics for a reason. They
DON'T behave like discreet components connected in the same configuration!
Nearly everything behaves differently when it is created in monolithic
silicon. They may be characterized and documented as digital/binary devices
to the end-users, but they are very much analog inside and are designed as
such! The most complex microprocessor with hundreds of millions of
transistors is nevertheless analog at the lowest circuit level.
RC
Dave
news:3b8670c0$0$20912$7f31...@news01.syd.optusnet.com.au...
One reason is that it is good practice to use pull up resistors because if a
circuit is designed to be tested then the logic inputs need to be
controllable.
An In-Circuit Test (ICT/ATE) machine can force drive logic components and
test them if pull up/down resistors are used but would be unable to do so if
the component inputs were directly shorted.
1198
require pull-up resistor is:
There will be current flowing from a input that attached to 5 volt without
pull-up to another input from the same multi-emittor transistor, drastically
reduce the fan-out capability of the driver to this input. Since the margin
for low input is rather small in old TTL this turn out to be a good practise
because using a pull-up resistor will reduce such problem.
Dave <Fl...@changing.career> wrote in message
news:9mfv2e$stt$1...@newstoo.ericsson.se...