Pulse generator question

The input current requirements of a DM7486 gate are very small (max

1mA) so this should not load down the pulse gen. Check the >I'm trying to help my son with his electronics lab work for school. The

SNIP

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Dumb question - did you double check your pinouts on the 7486?  Trying 
to drive an output pin instead of an input with the pulse generator 

Yes we did double check all the wiring and it is correct. I never considered that the pulse generator could be bad though. I suppose if there was series resistance to the output and it was exceptionally high then the output would display on a scope but might be loaded by the input of a gate. It seems far fetched but I'm not sure what else it might be. Of course I have no schematic for the pulse generator so I can't take a look at the output circuit, and Systrom Donner has been sold / bought out at least twice since this model was made. Lenny.

It's been several years since I used a Datapulse 101 generator, but it certainly should be able to drive a single TTL input. In fact, IIRC, that generator has a 50-ohm output, which, even when loaded with a 50-ohm termination, will drive quite a load. If the generator's output is being killed by a TTL input, there is certainly something wrong.

1) Make sure that you aren't AC-coupling the generator to the circuit under test. 2) Make sure the pulse is ground referenced, with a maximum of +5V peak pulse amplitude 3) Make sure the TTL gate works when you apply jumpers to Vcc and ground.

If all the above are good, then the most likely cause is a faulty generator. THose things are very hefty instruments, but they do fail, especially if someone applies a DC voltage to the output. Cheers!!!!

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Dave M
MasonDG44 at comcast dot net  (Just subsitute the appropriate characters in 

why not just get a LM555 timer IC and generate a 10KHz pulse for your project, that output signal should not disappear.

wrote in message news: snipped-for-privacy@o13g2000cwo.googlegroups.com... I'm trying to help my son with his electronics lab work for school. The project is to set up an exclusive or gate, an SN or DM 7486 and apply a

1.0KHZ square wave at a TTL level, (which I'm assuming is simply 5.0Vpp) to one input while alternating an either high or low level to the other input. We are to observe the output of the gate on the scope and then draw up a waveform display depicting the output corresponding to the different inputs applied. I'm using an old Systron Donner Datapulse 101 pulse generator which has two outputs available: a positive and a negative. They are marked "output into 50 ohms" I set up a 5.0Vpp 1.0ms positive going pulse with a 50% duty cycle as measured on the scope and then attempted to apply it to the input of the gate. As soon as I did this though the pulse disappeared as though the gate's input was loading it down. I next placed a 50 ohm termination on the output and tried it again. I noted that the output (before I connecteed it to the gate had decreased slightly due to the additional loading the 50 ohms provided, so I boosted it back up to the 5.0V level and observed that the pulse once again disappeared a soon as I connected it to the gate input. I am aware that some generators have a "TTL" output. I suppose that I should know this but its been a long time since I've played around with any TTL stuff so I have to ask: a. Is my 5.0V pp signal level correct? b. Would a generator having a "TTL" output provide a higher current drive signal into the gate? And is this what is needed? c. Is this why my generator is being loaded (I suspect) down? d. Can I do anything about this short of trying to find another generator? (Which would be difficult, if in fact it appears that this is the problem). The project is due on Thursday so any assistance anyone may be able to offer would be most sincerely appreciated. Lenny Stein, Barlen Electronics

make sure the generator is set up so the low is around 0 volts and the high is around +5 (Note that is not the same as 5vp-p becuase 3 to 8 is also 5 volts p-p but would be wrong)

If the generator can drive a 0 volts to +5 volt pulse into a grounded

50 Ohm resistor but not into your gate, then you have the gate hooked up wrong or the gate is bad.

Mark

Perhaps I stated this ambiguously. The 1.0KHZ pulse starts at the zero line, or zero volts. One complete cycle is 1.0ms. The pulse starts off at t=0. It takes a positive excursion to +5.0V for .50ms. It drops back to 0V for .50ms and returns to +5.0V at 1.0ms. One complete cycle. Does this sound like what you're describing? In any case I'll reconfirm the signal into a 50 ohm load and if that works then I'll have my son triple check his wiring. Thanks. Lenny.

Are you aware of the 7486 pinout oddities? There are several flavours and they do not share the same pinout!

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Kind regards,
Gerard Bok

Well we discovered the problem. Actually I'm a bit embarrassed to relate this but I will. My son triple checked his breadboard and assured me that everything was ok. So I took him at his word. He is good but after almost 45 years in this business (is the power off?, ... SURE it is....) I should have looked myself. The schematic showed one of the inputs going through a 1K resistor to

5.0V. The junction of the gate and resistor went through a switch to ground so that input could be pulled low when needed. The other input of the gate has a symbol for a pulse generator to ground. He mistook that as a direct connection to ground, effectivly grounding any signal applied to that input. The circuit now works fine and my 15.00 Datapulse generator which I picked up at the surplus store the other day is a keeper. I sure would like to have a manual for it though. Thanks to everyone for all the help and advice. Lenny Stein, Barlen Electronics.

So, if you had taken my advice and tested the ic by itself using hardwired logic levels (ie, without any of the fruit used by the circuit you were building), you would have solved the problem in 5 minutes. Any experienced tech should learn very quickly never to take another persons word on oath when things don't work as they should. They always check the circuit themselves.

Next time I will double check myself. Thanks for the whipping. i needed it... Best regards, Lenny