Testing a 74S240

[Paul Berger]

On 2022-04-30 19:25, Rob Jarratt via cctalk wrote:
> 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
>
According to the TI TTL data book 100mA is not out of line.  I just 
tried one and my power supply indicated about 125mA. Grounded pin 19 and 
with pin 13 open pin 7 measures 0.176 (as I am sure you know an open 
input on TTL is a 1) grounding pin 13, pin 7 rises to 3.815V.

Paul.