Testing a 74S240

[Rob Jarratt]

In trying to fix my M7133 CPU from my 11/24 I thought I had identified a
failed 74S240. However, when I replaced it (while adding a socket), the
fault remained. So, I guess the original chip may not be faulty. I decided
to test the original chip on a breadboard to see if it is OK. This is where
I got rather confused.

 

I used a bench PSU, obviously connected Vcc to +5V and GND to the negative
terminal. I connected pin 19 (the active low Enable ) to GND. And then I
tested the particular pair of pins, 13 and 7. I did not connect any of the
other pins. However, pin 7 seemed to hover around 0.6 to 0.8V, no matter
what I did with pin 13. I tried it with the replacement 74S240 and got the
same result. I tried a second replacement 74S240 which had never been
installed on the M7133 in case something on the CPU board was damaging it,
and got the same result.

 

I looked at the M7133 schematic and saw that pin 19 is connected to GND by a
180R resistor. I don't have one of that value so I tried a 220R. My
understanding is that the resistor isn't completely necessary, but I tried
anyway. However, the results were identical. I added a 220R to the input on
pin 19 just in case, again to no avail.

 

I noticed that the chip (original and replacement) was drawing 100-110mA
from the bench PSU, which seems a bit high.

 

I wondered if I might need a pull up resistor on the output, but my
understanding is that this is not necessary.

 

Is there a flaw in my testing method? Have I misunderstood something
fundamental?

 

Regards

 

Rob