I am embarrassed to report that after trying all the suggestions etc, I then realised that I had connected the enable signal to pin 9 and not 19. I knew it was 19, I knew where 19 was, but for some unfathomable reason I connected it to 9 instead, despite checking multiple times. The original chip works.
Sorry for the noise. Regards Rob > -----Original Message----- > From: Guy Dunphy <[email protected]> > Sent: 01 May 2022 01:22 > To: [email protected]; Rob Jarratt <[email protected]>; General > Discussion: On-Topic and Off-Topic Posts <[email protected]> > Subject: Re: Testing a 74S240 > > 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 >
