Testing a 74S240

[Guy Dunphy]

At 11:25 PM 30/04/2022 +0100, you 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.

As others have mentioned, the supply current is normal. These are fast, high power devices.

When you say 'breadboard', what do you mean? Is it one of those blocky things with rows of
holes with metal connecting fingers inside? 
Those have a lot of capacitance between rows, and with fast 74S logic and an inverting buffers
chip like the 74S240 that can be a problem. Also I bet you didn't bother with a supply decoupling
0.1uF ceramic directly between Vcc and Gnd at the IC.

So be aware that you may have a circuit oscillating at something above 20MHz, and your multimeter
will just be showing averages.
To avoid this, add the supply cap and tie all unused inputs directly to Gnd.
With the input you are interested in, tie to Gnd or to Vcc via a 1K resistor.
All with _short_ wires. Also with your multimeter (on Volts range) it's a good idea to
have a 1K resistor in series with the probe tip AT THE TIP. Otherwise your meter lead
is a nice radiating antenna, and can cause oscillations with that less than ideal breadboard.
The resistor won't affect voltage readings.

Old 74xx logic (mostly) doesn't have these problems, and people used to that get confused when
much faster logic seems to be behaving weirdly.

At least the IC won't blow up. I had an interesting learning experience the first time I got hold
of a 74AC series 20 pin buffer chip. I blithely breadboarded it with just the power rails and turned on +5V.
BANG! the die exploded. Blew a nice big crater in the plastic.
Turns out with the fully CMOS inputs, they will float around in the zone between 0 and 1,
which causes the very powerful output drivers to draw huge current as both the upper and
lower drivers turn somewhat on. Times eight... Instant silicon vaporization.

After tying all the inputs to valid logic levels, no more explosions.

Guy