Testing a 74141

[Craig Garnett]

I bought a 74141 on Ebay and I had a bit of trouble with the delivery but it arrived eventually so I'm trying to test it on the breadboard.

I've hooked up LEDs from +ve to all the outputs and pulled the 4 address lines down to ground with 100k

When I power it up 6 of the LEDS are on and raising any of the address lines does nothing.

It doesn't look good does it?

Craig 

[gregebert]

74xx TTL devices have a rather high input-low current, on the order of 1mA, so you should use a much smaller pulldown resistor, say 100 ohms, or perhaps none at all. Logically, it should not be possible for more than 1 output to get driven low on the 7441, but that only applies if all parameters in the datasheet are met. I suspect that the 100K pulldown isn't really pulling-down to a logic-zero, and the IC has gone to some intermediate logic state which is non-deterministic.

For the LEDs, I would recommend having a 2K series resistor and driving from +12 VDC. That works out to around 5mA  of current which is enough to make many LEDs visibly glow and  well-below the 7mA max current rating of the 7441.

[Craig Garnett]

Thanks,

I'll give those suggestions a go.

[Craig Garnett]

I tried those suggestions and nothing changed, there's around 4ma total through the LEDs now so it shouldn't be in meltdown mode.

[Nick Sargeant]

I'd agree with the above. To make sure you are avoiding any indeterminate states, pull all of the inputs up to +5v with a resistor of about 2k2 to 4k7. Then, for the inputs you want low, jumper those inputs to ground. The pull-up resistors will mean you get a good solid '1' without the jumper, and a good '0' with a jumper to ground. 

When designing with TTL,back in the day, we tended to work with negative logic as a preference, because of the nature of the inputs being connected to the emitters of bipolar transistors. 

[Craig Garnett]

Thanks Nick, I'll try that.

Back in my day I mostly used 4000 CMOS, I didn't do much with TTL.

[Craig Garnett]

That didn't work either, I think I've been sold a duff one.

Craig

[Nick Sargeant]

I think I'd agree. 

I've used the Russian K155ID1 drivers from eBay which seem to work OK (after finding out the first Eagle library I used had the power supply pins swapped so VCC was on pin 12 - reminded me to check new library symbols every time now) but it's clearly been a long time, since they seem to be silly money now. I repaired my HP 5326A with one of those on a small daughter board plugged in to where the HP custom chip lived. Not that I use it as my daily driver any more, since the fan is too noisy. 

If you lived in UK, I could post you one for grins - assuming I can find one in the parts bins. 

[Craig Garnett]

Yes VCC on 5 and ground on 12 does seem a bit odd.

I am in the UK but I need two of them.

I'm toying with the idea of getting two from Cricklewood Electronics or using 4028's and MPSA42's which seems like a better idea but it means finding space for 20 transistors on the board.