Hello

I wonder where is the A1 B1 and A2 B2 in this circuit

And I need explain is it rising edge trigger or what
Thanks for all

 

What does your post have to do with the title of this thread?

 

i assume you saw a1 b1 etc in the datasheet such as:


pins 8 and 16 are power for two identical units.
first unit has inputs pins A1, B1 etc. (pins 1 and 2)
second unit has input pins A2, B2 etc. (pins 9 and 10)

the schematics you posted has first unit marked as U26A and the second as U26B.

also inverter at input A1 is not shown as a separate gate but part of AND gate (See the circle on one of inputs).



Note both representations are the same thing (same IC) so there is no need for substitution

 

What does your post have to do with the title of this thread?

Yes you are true Sir

No link between the title and the question

Here is th link about it

I do have Mc14538 and I want to know where is the AB/1-2 of the 74ls123 and how it's triggered (in the schematic )in order to replace 74ls123 with MC14538
I know that it's less pulse width about 10 us that's enough for my demand and need

Both mc14538 and 74ls123 not pins compatibles
I find hard understanding AB in both

Even in schematic I provided I don't know where is AB

Thanks for your response

 

i assume you saw a1 b1 etc in the datasheet such as:
View attachment 354516

pins 8 and 16 are power for two identical units.
first unit has inputs pins A1, B1 etc. (pins 1 and 2)
second unit has input pins A2, B2 etc. (pins 9 and 10)

the schematics you posted has first unit marked as U26A and the second as U26B.

also inverter at input A1 is not shown as a separate gate but part of AND gate (See the circle on one of inputs).

View attachment 354517

Note both representations are the same thing (same IC) so there is no need for substitution

Thanks for your explanation yes I saw the datasheet but what tricked me the MC14538 when I try to use it in the schematic the circle pointed to B not A

 

74x123 has /A AND B input.
A must be LOW and B must be HIGH to trigger the circuit.
In other words,
falling edge of A while B is HIGH
or rising edge of B while A is LOW.



MC14528/14538 has A OR /B input.
The circuit is triggered on
a rising edge of A while B is HIGH
or a falling edge of B while A is LOW.

 

View attachment 354518
74x123 has /A AND B input.
A must be LOW and B must be HIGH to trigger the circuit.
In other words,
falling edge of A while B is HIGH
or rising edge of B while A is LOW.

View attachment 354519

MC14528/14538 has A OR /B input.
The circuit is triggered on
a rising edge of A while B is HIGH
or a falling edge of B while A is LOW.

Thanks a lot for you

But in simple words if you please in the schematic is it triggered rising or falling triggering
And in simple words gain regardless the values of RX CX how to to use MC14538 to work in place of 74ls123 in schematic I provided in the same manner
Thanks in advance

 

you need to decide which input is to be edge sensitive. then keep the other input in suitable state.
as MrChips already stated:

74x123 has /A AND B input.
A must be LOW and B must be HIGH to trigger the circuit.
In other words,
falling edge of A while B is HIGH
or rising edge of B while A is LOW.

MC14528/14538 has A OR /B input.
The circuit is triggered on
a rising edge of A while B is HIGH
or a falling edge of B while A is LOW.

 

So is my understanding now correct or not

to replace 74X123 with MC14538 in same schematic I need to connect ... A1 to trigger source... B1 to vdd ...rest to vdd same as the other part A2 B2 rest is that true or I am mistake

 

not sure what you are trying to do and why. 74LS123 is active, normally stocked and easy to find product, for example
https://www.mouser.ca/c/?q=74ls123

if you are trying to replace IC on an already made board, this substitution makes no sense (it will be a challenge) since MC14538 is not pin compatible with it. if you are creating new board design and trying to substitute IC before board is made, this could work assuming you check the specs (supply level, logic levels, output fanout, pinouts...).

the way 74LS123 is connected, the AND gate and inverting input (A) are simply bypassed. timer works as if B connects directly to trigger. so doing the same on MC14538 would require bypassing input B, and using input A as trigger.

in other words, going from one IC to another:
1. swap A and B inputs (use B instead of A and use A instead of B)
2. bypass inverted input so that it does not interfere with triggering. on LS123 this was done by connecting B to GND (input A is trigger). on MC14538 this is done by connecting A to Vcc and using B as trigger.

Tip:
when in doubt, always give yourself a plan B (an easy way out). this could be adding jumpers or DIP switches or solder-jumpers so that you can determine and apply correct connection later - after board is made. i am often pressed with time to release design to meet deadlines while many new features are added. so i do just that, add in my designs things that can alter circuit behavior without mess of piggybacking things or wires criss-crossing PCB. this allows me to salvage protoboards and deliver things on time which would be impossible with time wasted on on reordering.

 

not sure what you are trying to do and why. 74LS123 is active, normally stocked and easy to find product, for example
https://www.mouser.ca/c/?q=74ls123

if you are trying to replace IC on an already made board, this substitution makes no sense (it will be a challenge) since MC14538 is not pin compatible with it. if you are creating new board design and trying to substitute IC before board is made, this could work assuming you check the specs (supply level, logic levels, output fanout, pinouts...).

the way 74LS123 is connected, the AND gate and inverting input (A) are simply bypassed. timer works as if B connects directly to trigger. so doing the same on MC14538 would require bypassing input B, and using input A as trigger.

in other words, going from one IC to another:
1. swap A and B inputs (use B instead of A and use A instead of B)
2. bypass inverted input so that it does not interfere with triggering. on LS123 this was done by connecting B to GND (input A is trigger). on MC14538 this is done by connecting A to Vcc and using B as trigger.

Tip:
when in doubt, always give yourself a plan B (an easy way out). this could be adding jumpers or DIP switches or solder-jumpers so that you can determine and apply correct connection later - after board is made. i am often pressed with time to release design to meet deadlines while many new features are added. so i du just that, add in my designs things that can alter circuit behavior without mess of piggybacking things or wires criss-crossing PCB. this allows me to salvage protoboards and deliver things on time which would be impossible with time wasted on on reordering.

Thanks for you and for all

The reason I want to Use the MC 14538 in place 74x123 is that I do have about 1000 chips
Now I understand the logic of both ...no it's not replace of working device I am going to design timing stuff

Once again thanks for you
Thanks for all

 

Final answer to this topic
In this schematic using 74123 with same configuration with MC 13538 the result is inverted result of 74123 due (rising/falling trigger) for each chip






Thanks for all

 

i do not know what you mean.

the two pictured cases show same IC (74LS123A) but using different input as trigger. response of both cases is the same. they just react to a different trigger edge as was already discussed.

 

Here is what I mean both chips If configured as the previous 74123 configuration they going to give opposite rising / falling logic

 

We don't understand what you are saying.

With either IC, you can trigger it however you want, with a rising edge or with a falling edge.
Which one do you want?

 

We don't understand what you are saying.

With either IC, you can trigger it however you want, with a rising edge or with a falling edge.
Which one do you want?

I am saying both has opposite logic and i found how to trigger as I want

Here's the trigger logic for both










Thanks for all

 

I already told you that in post #6.

 

I already told you that in post #6.

Yes sir you said that but I am not expert as you that's why I hardly understand the issue

Thanks a lot for your response