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Old 12th Oct 2021, 6:55 am   #1
Radio1950
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Default Interesting Vintage TTL Logic IC Problem

Interesting Vintage TTL Logic IC Problem

I was asked to design a circuit to provide a 5 volt pulse train at 3000 and 300 pulses per minute, to “calibrate” a counting device.
There was a possible later need for 1500 and 150 ppm, so I factored that in to the design.

In a whim of nostalgia, I decided to use vintage TTL, and to meander down familiar paths from the 1970s through to 2000 or so, where, probably like a lot of us, we had many equipments with heaps of TTL on boards.

I had lots of maintenance experience to board level, with repair, on Thomson MSSR radar processing equipment, etc, et alia, with hundreds of fully populated dual DIN41612 PCBS. Some 54xxN, but mostly 74xxN, and some 74LSxxN.
Never saw a single 76xxN.

I had virtually no design experience on TTL per se.

But ...
Should be easy.
What could go wrong?

So I used 12 V AC mains plug pack and a 5.1V zener to generate a 5V 50 Hz (3000 cycles per minute) drive signal, one lead at 0V the other 5Vpeak, with a Zs of 1000R or so.
Mains frequency was good enough accuracy for this project.

I then used a decade counter and divide by 2.
Rummage through my Logic IC bins to find some suitable TTL.
No 7490N to be seen there, so I had to use a few 74LS90N, a decade counter.

Wired up on veroboard, plenty of bypass caps, ICs on sockets, used a tight 5V DC supply.

SET and RESET lines to 0V; IC jumpers set to give divide by ten.

The 3000 cpm source signal went to the LS90 counter clock input via a 1000R “isolating” resistor.

Power up, check DC OK, check counter OP with CRO – zilch.


I expected to see a pulse train of period 20mS.
Occasionally, the OP went to HIGH, then to LOW, but mostly stayed LOW.

Many, many, hours later, I could not fault the circuit. New IC, all possible checks carried out.

Photo of project board and roughie circuit attached.

CRO photos of input signal and decade counter output attached. Probes are 10x.
Vert channels are DC coupled, 2V/cm, bottom trace is LS90 input, top is LS90 output.
Photo 3 Horiz is 10mS/cm, SYNC CH1.
Second photo is signal source with Horiz at 1mS/cm.

I thought the fault was so interesting, that I thought I might post it for veteran TTL users and designers.

I was not aware of the problem, for a good reason.

I have since fixed the underlying problem, but for fun and your intrigue, I ask for your possible solutions please.

A very nice Kewpie doll will be handed to the first person with complete correct (I think) answer, but must be picked up personally from my workshop.
I am currently impecuniously disadvantaged and cannot afford postage.

I hope that I am not the only person in the TTL world to know of this problem.
But have a feeling that I may even learn some more from your answers.

No clues yet.
Probably easy for experts.
Good luck.
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Old 12th Oct 2021, 8:38 am   #2
Neutrino
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Default Re: Interesting Vintage TTL Logic IC Problem

I assume that the decoupling capacitors are hidden on the back of the Veroboard.

I think that a fast edge, such as produced by SN74LS14 inverters with Schmitt-Trigger inputs, is needed to ensure that the flip-flops in the decade counter are clocked simultaneously, especially with the input applied to CKB to get a symmetrical output at QA.

I would have used a lower voltage Zener diode to ensure that the input voltage cannot exceed the supply voltage.

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Old 12th Oct 2021, 8:39 am   #3
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Default Re: Interesting Vintage TTL Logic IC Problem

Without diving in too far, I would have started with a schmidt gate on the input - e.g. 7413. TTL can become a bit touchy when presented with slow changes. Does the input go close enough to 0V just using that zener?
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Old 12th Oct 2021, 9:20 am   #4
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Default Re: Interesting Vintage TTL Logic IC Problem

Yes, caps on board rear.
If I posted a photo of the rear of the PCB, most readers would need a sedative and a good lie down.
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Old 12th Oct 2021, 9:39 am   #5
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Default Re: Interesting Vintage TTL Logic IC Problem

I think that 10K input resistor is too big. It could prevent the input going to a logic"0".
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Old 12th Oct 2021, 9:44 am   #6
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Default Re: Interesting Vintage TTL Logic IC Problem

There's a nice little cafe upstairs in Waterstones St Albans.
Great to peruse radio books.
The resistor is just a holdown, "just in cases".
Shouldnt affect anything?
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Old 12th Oct 2021, 9:50 am   #7
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Default Re: Interesting Vintage TTL Logic IC Problem

The sink current for CKB is -3.2mA and low voltage is 0.4V. You really need a transistor buffer with pull-up resistor to drive this.
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Old 12th Oct 2021, 9:56 am   #8
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Default Re: Interesting Vintage TTL Logic IC Problem

Quote:
Originally Posted by PJL View Post
The sink current for CKB is -3.2mA and low voltage is 0.4V. You really need a transistor buffer with pull-up resistor to drive this.
I did try a PN100 emitter follower with no improvement.
It had 1000R emitter resistor, no collector resistor, 5V supply, DC coupled to clock input.
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Old 12th Oct 2021, 10:08 am   #9
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Default Re: Interesting Vintage TTL Logic IC Problem

Those slow edges aren't going to help. I'd be very uncomfortable driving the counter's clock input from them without a Schmitt trigger to add some hysteresis and sharpen up the edges.

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Old 12th Oct 2021, 11:03 am   #10
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Default Re: Interesting Vintage TTL Logic IC Problem

Besides a Schmitt trigger, you will likely need a low pass filter to get rid of mains transients. My ttl based clock from 1972, ran quite fast without this !
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Old 12th Oct 2021, 12:00 pm   #11
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Default Re: Interesting Vintage TTL Logic IC Problem

Also, the 12VAC ground path could be dubious, dependent on how and if the supply voltage is derived from it.
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Old 12th Oct 2021, 12:10 pm   #12
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Default Re: Interesting Vintage TTL Logic IC Problem

As Maarten suggests if the ac input providing the trigger is the same as the ac used to generate the +5v supply it depends on how you derive the +5 from the supply.
If you used a bridge rectifier then you will not have a reliable 0V reference for the trigger.
Half wave rectification would actually be better and short out the 1K resistor to the 0v.
Better yet a separate supply and make sure that one side of the ac used for trigger is connected to the psu 0V

Peter
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Old 12th Oct 2021, 12:37 pm   #13
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Default Re: Interesting Vintage TTL Logic IC Problem

Hi!

Where 50 Hz or 100 Hz pulses are need from a mains rectifier, a popular method, often found in phase control circuitry, is to insert a single diode between the (+) output of the rectifier and the reservoir capacitor, anode to rectifier and cathode to reservoir capacitor, with the half–wave or full–wave pulses from the anode of the auxiliary added diocde.

If a full–wave or bridge system is in use as the L.T. supply rectifier, an additional ÷2 flip–flop or counter section is needed between the mains–pulses and the input to the seconds/minutes counter chain, because the mains counting pulses are 100 Hz full–wave!

When I did a TTL digital clock project in my student days the method recommended to me to count the mains was to square/differentiate the mains pulses and feed it to a 555 monostable with a time delay of about 15 ms to produce output pulses every 20 ms to feed the first ÷50 divider chain to give a 1 sec count!

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Old 12th Oct 2021, 1:25 pm   #14
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Default Re: Interesting Vintage TTL Logic IC Problem

This is the circuit I used. It went into a PIC microcontroller, but as I was only reading 1 bit it had no problems with 'sloping' edges.

The 7490 types internally have the one 'clk' input connected to 3 or 4 flip-flops, so I don't know how they would switch with 'sloping' inputs, but I would aim for the 5 nS rise and fall times that are used in the datasheet diagrams.
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Old 12th Oct 2021, 2:01 pm   #15
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Default Re: Interesting Vintage TTL Logic IC Problem

All of the PICs I've used have Schmitt trigger digital inputs, so that would be an advantage for Buzby123's application.
PIC Schmitt trigger inputs caused problems with a number of designs I worked on a few day jobs ago, but that's another story.

John
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Old 12th Oct 2021, 4:21 pm   #16
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Default Re: Interesting Vintage TTL Logic IC Problem

Quote:
Originally Posted by Radio1950 View Post
Quote:
Originally Posted by PJL View Post
The sink current for CKB is -3.2mA and low voltage is 0.4V. You really need a transistor buffer with pull-up resistor to drive this.
I did try a PN100 emitter follower with no improvement.
It had 1000R emitter resistor, no collector resistor, 5V supply, DC coupled to clock input.
It's not an emitter follower you need. General purpose NPN emitter to 0V, collector to CKB, 2K2 resistor from collector to +5V and base to your AC source.
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Old 12th Oct 2021, 7:28 pm   #17
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Default Re: Interesting Vintage TTL Logic IC Problem

This circuit has never failed to me. I have used it a lot of times from the 60Hz mains with very good results. On other ocassions I have even used an optocoupler to generate the pulses. The circuit also works for CMOS chips.
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Old 12th Oct 2021, 10:49 pm   #18
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Default Re: Interesting Vintage TTL Logic IC Problem

The +5V DC supply is completely isolated from the clock source.
The trial emitter follower was mostly to do with clamping the clock LOW state more towards 0V.
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Old 13th Oct 2021, 10:29 pm   #19
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Default Re: Interesting Vintage TTL Logic IC Problem

Solution.

When all else fails, get the manual out.
I looked for clues in 74LS90N datasheets.

In only one (Motorola) of many, I found this tiny footnote “tr<100nS” (“tsubscript r<100nS).
Pennies falling.

My clock risetime was many times this.

Severe and ongoing embarrassment. Basic. Old age.

Solution, use a 74LS14 Schmitt to give edge and buffer action.

I also forgot the old TTL rule, “buffer in, buffer out”, so the other LS14 sections now buffer all outputs.

Several forum members were straight onto this risetime and falltime issue, notably “Neutrino, lesmw0sec, and cmjones01”.

Good comments from all. Thank you.

I have cut up the Kewpie Doll into six sections, wrapped all, awaiting personal collection.
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Old 14th Oct 2021, 8:31 am   #20
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Default Re: Interesting Vintage TTL Logic IC Problem

Thanks for the mention - I'll pass on the doll! I recall when 7490s became available. We were amazed that a bit of plastic could divide by 10 - it certainly beat a card full of bistable transistor circuits!
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