why 74xx series ic used in electronic boards

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hello
in control boards i have seen many 74xx series IC it may be HC,LSAC etc and some where CDxx ic.

my question is why do hardware designers use driver IC even Microprocessor and Micro controller is able to drive the pulse or signal.
please help me :-x
 

Sometimes it is for historical reasons, the 74 seies has been availables since approx 1974 and they were the only mass produced logic devices at the time, microcontrollers and microprocessors were not available then.

They are usually cheaper than an MCU and in some circumstances it is commercially better to use them, for example, if you want a single AND gate, why bother to source and program an MCU when a 7408 costs almost nothing and drops straight in the board.

Also consider that some 74 series devices can handle higher voltages or high currents than an MCU alone could not manage.

Brian.
 
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    abc_de

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Also sometimes you really want pure combinatorial logic, with no or minimal amounts of stored state, this makes failure analysis very much easier.

Further the discreet logic will often be faster then a micro, and have better defined timing.

Regards, Dan.
 
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Hi,

the delay time from input to output for a 74xx device is just some ns.

A microcontroller needs at least microseconds....


Klaus
 
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    abc_de

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The answer for this question could be somewhat imprecise without a scpecific circuit to check, but there could be many reasons for that, varying from the convenience to simplify the manufacturing process requiring no programming tool, could also be due to reluctance of designer of changing something that had never failed before, and could be even due to someone being little comfortable with the programming universe.
 
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I was using TTL in 1969. Its very rugged, can handle a reasonable amount of power, the latest series are very fast, some series are power frugal. And they are common with a vast array of special purpose decoders/encoders dividers etc. Even now some special purpose chips are not made in CMOS.
Frank
 

I remember TTL ICs drawing a lot of power supply current even when they were doing nothing. Cmos ICs draw almost no current when they do not work very fast. Cmos also works from a supply from 3V to 18V but TTL needed a regulated 5V supply. Cmos has much more noise immunity.
 
Audioguru said:
Cmos has much more noise immunity
Click to expand...

Interesting information, I always thought that due to the high impedance of the gate, this would turn that technology more susceptible to noise. Perhaps I was thinking about EMI interference.
 

I think audioguru is referring to the logic threshold where the inputs recognize the input is switching between logic H/L. In CMOS it's balanced around between the H/L voltage rails, unlike TTL which is a diode drop above the low side and less than half the voltage (@5V) for the high.

For some pretty pictures of this...
https://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/

You get a lot more noise margin on the logic low side in CMOS. Hence the reason for the legacy active low signaling, when using TTL, for things like resets (you don't want something that will reset your logic if it has very little noise margin in the inactive state).
 
abc_de said:
my question is why do hardware designers use driver IC even Microprocessor and Micro controller is able to drive the pulse or signal.
please help me :-x
Click to expand...
Logic gates and discrete counters are much faster than any micro controller can ever be.
If you want speed, and the task is a simple one, discrete logic is the way to do it.
 

Hi,

Some examples of external logic:
* address decoder for external devices on a bus
* hardware for in_system_programmability during reset of microcontroller

Klaus
 

Sure, a microcontroller "can" do many things. And some people
will use a uC for things it's not needed or suited for.

Using buffer SSI logic often comes from peoples' bad experience
with abnormal conditions. Such as, sure, a uC can take an amp
for a microsecond on one pin (ESD / EOS). But it will probably not
take an amp for a microsecond on all of its I/Os simultaneously
without blowing out the ground (VSS) bond wire. You don't put
high pin count devices right up to the off-box connector if you're
smart. Too much "frontal area", to catch too many bullets. So
send in the infantry, which are tougher.

You would also be ill advised to try driving a long multiconductor
cable from a uC's outputs. While individual outputs may be
capable enough, simultaneous switching of a heavy load apiece
can bounce VDD / VSS enough to cause malfunction. A small
close-in gate load, and let the SSI logic deal with the ground
bounce 4-6 pins at a time, is cleaner where it counts. The
octals with their 24mA drivers can be a problem in this way
if you give them all the load they're capable of driving on each.
and your board power / ground distribution network is not up to
the spikes.
 

ok ...
so many things matters like, circuit design overhead, flexibility, speed, power consumption etc are matter during design of board.
in most of old board made in Germany which i have seen they have use TTL IC AC series but now days engineers use HC series CMOS ics.

if i design a board then i will use TTL of 'LS' series or CMOS 'HC' series IC.
 

Hi,

if i design a board then i will use TTL of 'LS' series or
Click to expand...

* I personally can´t find a benefit in using old, slow, power consuming LS logic family anymore. It´s at least 20 years ago, when I used LS.
* I personally select the logic family by the specifications: supply voltage, power consumption, drive capability, speed, 5V_input_tolerance (while supplied with lower voltage), price, availability...

Klaus
 

KlausST said:
* I personally can´t find a benefit in using old, slow, power consuming LS logic family anymore. It´s at least 20 years ago, when I used LS.
Click to expand...

LS can be handy for quickly testing an idea on a breadboard if you already have the chips.
Yes its slow and old and power hungry, but you don't have to tie down all the unused pins, and its pretty static safe.

The final PCB will always use one of the higher performance 74 series CMOS families, but I have here a vast selection of ancient TTL, and LS, many hundreds of them ! So its very convenient.

Just put together a very basic home brew prototype super VGA graphics card built out of a Frankenstein selection of old mixed technology chips. Works fine.

Now I have seen on screen the pictures it creates, I am more confident to lay out a PCB with a fair chance of it working first attempt.
 

dick_freebird said:
Sure, a microcontroller "can" do many things. And some people
will use a uC for things it's not needed or suited for
Click to expand...

Something that is no often considered in consumer projects where durability is not so relevant, is the MTBF factor. In critical designs such as life support, or other classified categories, perhaps the use of logic gates can brings some benefit since they have less area of silicon to fail, although once spreading the gates over the board, the amount of solder spots will increase the points prone to failures. This is just another aspect that eventually could be acounted, case by case to determine whether to use a solution based on gates or uC. Someone somewhere must have done this analysis, but to be honest the result would make no difference to me, I would continue using microcontrollers, it is much more flexible.
 

In my past experiences I was repairing power control board in which I found faulty 74AC**IC I have replaced ic with same no. But it was 74HC** then power control board start malfunctioning.
Can any one suggest what would be cause
 

abc_de said:
Can any one suggest what would be cause
Click to expand...

Components from lots of unreliable manufacturers...In the past it was very common to deal with situations like this, particularly with port logic ICs or discrete components from certains suppliers.
 

Ok
Can use suggest me which of logic ic best for microcontroller input and output .... HC/HCT cmos or AC/LS TTL
thanks
 

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