Pulse generator question

[[email protected]]

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

 

[Ross Herbert]

The input current requirements of a DM7486 gate are very small (max
1mA) so this should not load down the pulse gen. Check the operation
of the gate/s in the IC using simple hard-wired logic level inputs on
a bread-board and make sure that the truth table is confirmed. If this
works as expected then perhaps the pulse gen is faulty.

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

SNIP

 

[Jerry Maple]

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.

 

[[email protected]]

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.

 

[DaveM]

--
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
could load it down. Pins 3, 6, 8, and 11 are the outputs, according to
http://www.abcsemiconductors.com/composants/pinout/7486.phtml
--

Jerry Maple
General Dynamics C4 Systems
Scottsdale, AZ

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!!!!

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

Never take a laxative and a sleeping pill at the same time!!

 

[Eric]

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



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

 

[Mark]

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

 

[[email protected]]

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.

 

[Gerard Bok]

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,

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

 

[[email protected]]

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.

 

[Ross Herbert]

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.