Testing a 74S240

[wrcooke at wrcooke.net]

> On 04/30/2022 5:25 PM Rob Jarratt via cctalk <cctalk at classiccmp.org> wrote:
> 


> 
> 
> 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

There is a TI datasheet here:
https://www.ti.com/lit/ds/symlink/sn74ls244.pdf
According to it, the typical current is around 100 mA and it could go as high as 150.  So the current sounds fine.

As for the testing method, it seems like it "should" work to me.  However, I would try first tying the other half enable (pin 1) to ground / vcc and see what happens.  Next I would try tying all 8 inputs hi or low.  None of that should make any difference, but I've been surprised before.

I would also try tying the enable inputs high.  It's possible you have a mis-marked 74s241 on which pin 19 is active high.

Will