[General] Not clear if a 74LS145N device is working

[HectorRedal]

Hi,

I am not sure if I am posting this new thread in the right forum.
Let me know if I need to move this thread to any other forum.

I am trying to repair a old personal computer.
I have traced the signal in the motherboard. I have found an strange behaviour in a 74LS145N IC.
The datasheet for this device is the following: https://elcodis.com/parts/5801911/M74LS145P.html

The working voltage is 5V for this IC.
As input voltages I found 1.2V in pins 12, 13, 14, 15.
According to the description of the specification, V_IH = 2 V and V_IL = 0.8V.
So, the input voltage for this IC is neither high nor low. Is my supposition right?

But, as output I see all the voltages at low level at pins 1, 2, 3, 4, 5, 6, 9, 10 and 11.
Pin 7 is 5 V.

According to the description of the specifications, only one single ouput is set to low:
"When inputs D_A, D_B, D_c and D_D are designated with a BDC code in this decoder / driver, the Y_0 -> Y_9 output corresponding to the number is set low while the other 9 outputs are set high. When binary number of 10 or more is applied to D_A -> D_D, all the outputs are high".

Is the observed behaviour correct? The datasheet does not specifiy what output values appear when the input voltage is neither high, nor low.

Any comment / suggestion / critics is welcome.

Thanks.

BR,
Hector.

 

[KlausST]

Hi,

As input voltages I found 1.2V in pins 12, 13, 14, 15.

You need to check this with a scope. A voltmeter won´t work.

But, as output I see all the voltages at low level at pins 1, 2, 3, 4, 5, 6, 9, 10 and 11.

Quite unusual. This is an indication to replace the IC.

Klaus

 

[HectorRedal]

Hi,


You need to check this with a scope. A voltmeter won´t work.


Quite unusual. This is an indication to replace the IC.

Klaus

Hi Klaus,

I forgot to comment that I am using an scope, not a voltimeter.
Sorry for not mentioning before.

In the scope I can see that input pins have a constant value of 1.2V.
I can also see that output voltages have a constat value of 0V except pin 7 with a constant value of 5V.

Best regards,
Hector.

 

[FvM]

The datasheet does not specifiy what output values appear when the input voltage is neither high, nor low.

Behavior is undefined and can't be predicted.

Illegal input level refers to a problem of the driving circuit, not the driven IC.

 

[KlausST]

Hi,

Illegal input level refers to a problem of the driving circuit, not the driven IC.

In most cases.

But I just had a repair where defective 74HCT244 pulled down their input signals (about 3V instead of 5.0V with new devices). They became hot and the driving ICs also.
From the circuit I assumed this was an ESD failure.

Klaus

 

[HectorRedal]

Hi,


In most cases.

But I just had a repair where defective 74HCT244 pulled down their input signals (about 3V instead of 5.0V with new devices). They became hot and the driving ICs also.
From the circuit I assumed this was an ESD failure.

Klaus

The driving IC for this 74LS145N is a an uPD8255A that has:
/RD (5V), /WR (5V) and CS (0V), that according to the datasheet, it means that the IC provides a High Z state in the data bus (input to the 74LS145N).

I don't know to to test / debug the uPD8255A, so as to pin point if the problem is the uPD8255A or the 74LS145N.