View Full Version : Input on 74LS04 won't go all the way to Ground

September 23rd, 2014, 09:16 AM
Hi all;
I am working on the ZFDC (Z-80 Floppy Drive Controller) Board from Andrew and John..
And on the Input to the 74LS04 pin 13 (U32) it will Automatically go to about 1.75 volts, I can Ground it with a wire and it does go to 0 volts.. Also, there are (at present no other Ic's) plugged in that are connected to that Line..
I have tried multiple different Ic's, 74LS04, 74LS14, 74AS04 and 74F04.. They all do the same thing..
Also when this Ic is out the pin goes to ground, and does not float at the higher value..
I have cleaned the Board and checked for shorts and momentarily thru a resistor pulled it to +5 volts..
With no Ic's in at all I have both pulled this pin and those next to it and those next to the pin on the other Ic's that are connected to it, Both High and Low..
No change in its behavior in spite of doing all this..
I guess, I could short this pin and those next to it to ground and +5 Via my 400 Amp Batteries, but I am NOT to Excited about doing this.. While not in the system and ALL Ic's Out of the Board.. I did this on a Wire-wrap DEC Board and I got Lots of Feedback on that..
ANY SUGGESTIONS ????????????????????


Dwight Elvey
September 23rd, 2014, 10:07 AM
TTL input will float high if not driven. This is normal.
I think that is what your asking?

September 23rd, 2014, 10:29 AM
Hi All;
Thank You Dwight for Your answer, I will check Both the Schematic and the Board itself..
But, I think it even did this when the 74126 was driving it Low, it would not go all the way down..
But, let me confirm this before saying for sure..
Which was "why" I was looking into this in the First place..


Dwight Elvey
September 23rd, 2014, 10:42 AM
If a TTL is driving it and it is not going to less than about 0.10 volts,
it may have a bad input or the source TTL is not pulling it
down enough.
A floating LS input should actually float to around 3 volts but
your meter of scope may be loading it down.
If you want to see if the input is within range, use a 1K resistor
to ground for an undriven LSTTL. It should have a voltage of
about 0.2 volts but should be below 0.4 volts worst case.

September 23rd, 2014, 10:51 AM
Hi All;
Thank You Dwight, I will try those things, I have been using a scope and not a meter.. I will try the 1K resistor and see If it helps.. That did the trick, I also pulled the input to the 74LS126 high, and the Output went High as it should, I took off the Resistor and it went back to as it was before, when at a low level (not pulling it high).. I have the 1K Resistor on a Clip for now..
I think Just in case Next time I put in an Order for some Ic's, I will include some new 74LS126's..
I just looked at the other outputs for the 74LS126 and they All do the same thing, So for now I can look thru my stash for a better 74LS126 or use the 1K pull downs.. Until I can Order some New ones..
Thank You, Dwight..

THANK YOU Again, Marty