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

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.

Hi,

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

Click to expand...

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.

Click to expand...

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

Klaus

KlausST said:

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

Click to expand...

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.

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

Click to expand...

Behavior is undefined and can't be predicted.

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

Hi,

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

Click to expand...

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

KlausST said:

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

Click to expand...

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.