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Any OR gates with good noise immunity?

Hi all,

I made the foolish mistake in a recent instrumentation design of
routing a copper track from one board to another via a common 'mother'
board and associated edge connectors, straight into the input of a 4071
OR gate. The output of this gate clocks a 4015 4-bit shift register.
The circuit 'works' fine in the lab but the problem lies in the
equipment installation, where a number of noisy mains-powered solenoid
relays manage to falsely trigger the shift register for around one in
every ten actuations. I suspect that the long copper route is acting as
an ideal aerial for this noise, even though the entire circuit is
housed in a grounded metal box.

Are there any 4071 plug-in equivalents with schmitt-trigger inputs, or
other variants with better noise immunity? A less eloquent option would
be to insert two series connected schmitt-trigger NOT gates (of the
40106 variety), but space is already at a premium for such a
'piggy-back'. Any suggestions appreciated.

Thanks,
John.
 
J

John Fields

Jan 1, 1970
0
Hi all,

I made the foolish mistake in a recent instrumentation design of
routing a copper track from one board to another via a common 'mother'
board and associated edge connectors, straight into the input of a 4071
OR gate. The output of this gate clocks a 4015 4-bit shift register.
The circuit 'works' fine in the lab but the problem lies in the
equipment installation, where a number of noisy mains-powered solenoid
relays manage to falsely trigger the shift register for around one in
every ten actuations. I suspect that the long copper route is acting as
an ideal aerial for this noise, even though the entire circuit is
housed in a grounded metal box.

Are there any 4071 plug-in equivalents with schmitt-trigger inputs, or
other variants with better noise immunity? A less eloquent option would
be to insert two series connected schmitt-trigger NOT gates (of the
40106 variety), but space is already at a premium for such a
'piggy-back'. Any suggestions appreciated.
 
N

Noway2

Jan 1, 1970
0
Your problem sounds an awfull lot like a variation of ground bounce
issue that I experienced in a design. In our of our products, we were
using a board that suffered from intermittant flase triggers when there
was a disturbance in the mains power, such as when electromechanical
devices would switch and would occur about 1 out of 10 times.

The problem in my case was due to a lack of a proper ground plane and
adequate bypass capacitors. The board designer believed that putting
one 10uF capacitor with a .1uF in parallel with it about 1 inch from a
logic device with four power pins was adequate bypass. Similarly, he
did not feel that it was necessary to put a bypass capacitor on any of
the logic ICs. The problem was resolved with a redesign of the board
that utilized a solid ground plane, on a two layer board, added proper
bypass caps, and switched the design from through hole to surface mount
which has a much lower lead inductance on the packages.

This brings me to the questions regarding your design. Are you using a
ground plane and what do you have for bypass capacitors on the driver
and receiver devices?

Another thing to consider is how long are the copper runs. You said
that you are running from one board to another through edge connectors,
which suggests that they may be of fairly decent length. It is likely
that the design will require proper termination of the transmissions
lines. Your runs will act as a transmission line, as opposed to a
lumped circuit, if the run length is more than about 1/6th of the
signal's propegation delay, which is a function of the PCB geometry and
material. It is a common misconception that a "high speed" circuit
means fast clock rate, when in fact it is a function of the signal rise
times. This brings me to my final questions, what do you have for
termination on the lines, how long are the copper runs, and what is the
driver rise time (either from a datasheet or from measurement, though
you will need to measure with something other than a capacitive scope
probe). If you don't know these paramaters, I would guess that based
on typical PCB construction if your lines are more than about 4 inches
they will require termination.

I appologize if you are already familiar with the topics I descibed
above, but I figured it would be less offensive to tell you something
you already know that to leave something out that you needed to know.
 
D

Dan Hollands

Jan 1, 1970
0
Hi all,

I made the foolish mistake in a recent instrumentation design of
routing a copper track from one board to another via a common 'mother'
board and associated edge connectors, straight into the input of a 4071
OR gate. The output of this gate clocks a 4015 4-bit shift register.
The circuit 'works' fine in the lab but the problem lies in the
equipment installation, where a number of noisy mains-powered solenoid
relays manage to falsely trigger the shift register for around one in
every ten actuations. I suspect that the long copper route is acting as
an ideal aerial for this noise, even though the entire circuit is
housed in a grounded metal box.

Are there any 4071 plug-in equivalents with schmitt-trigger inputs, or
other variants with better noise immunity? A less eloquent option would
be to insert two series connected schmitt-trigger NOT gates (of the
40106 variety), but space is already at a premium for such a
'piggy-back'. Any suggestions appreciated.

Thanks,
John.

Since you have a grounded metel enclosure, the best solution would be to
determine how the noise is getting into the enclosure and a stopping it from
getting in. You don't say if the solonoids are driven by circuits inside the
enclosure in which case filtering or snubbing the drive circuits might help.
A low pass filter at the input of the OR circuit might stop the external
noise but could lead to false triggering of the shift register due to the
slow rise time at the input of the OR causing multiple pulses as you go thru
the threshold. Keeping noise out is always better than patching the
internals.


--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com
 
J

Jon

Jan 1, 1970
0
If you are not using a ground plane on the motherboard, you should
provide a separate ground path for the logic circuit, so that none of
the current through the noise-making relays can flow through the ground
that supplies power to the logic.
 
P

Paul Mathews

Jan 1, 1970
0
If the signal(s) involved are not fast, series resistance can be added,
possibly with shunt capacitance. This works wonders, and it reduces ESD
damage susceptibility at the same time. However, start by reducing
spike magnitudes at their source, if possible. Shunt diodes across
solenoid and relay coils can help a lot.
Paul Mathews
 
A

Ancient_Hacker

Jan 1, 1970
0
You could try an old trick-- put a R/C network right at the input pin.
100K and 1nF will damp any spikes roughly shorter than 10usec.

I know, not good digital design to add analog elements, but sometimes
you just have to.
 
Thanks to all for your suggestions so far. The circuit is comprised of
through-hole components on double-sided boards that are flooded with
ground planes, except for the motherboard. I use 100nF bypass
capacitors. The copper line in question is approximately 7 inches long
in total, of which an inch is on the motherboard. The solenoids are not
driven by or connected to my circuit. Switching speeds are very low,
with the pulse being generated by a monostable (duration of almost 1s)
through a diode, so that a separate circuit can latch the state (using
a FET switch with feedback). A 22K resistor to ground enforces the low
state of the line when a different pulse switches off the FET. Should
this resistor be bypassed with a 100nF cap (thereby bypassing the OR
gate input to ground), or would the suggested RC network at the OR gate
input be better?

John.
 
A

Ancient_Hacker

Jan 1, 1970
0
Sounds like that whole area is already made up of MML (Mickey Mouse
Logic). Sorry.

The *best* way would be to redesign it to be all digital clocked logic,
but it's probably too late for that.

Take some small comfort, you're not the first engineer to have this
problem.

Back around 1974 our department bought some $4000 Crown tape decks,
with logic controlled push-buttons. The logic consisted of one small
board with the old Fairchild 9xx RTL logic chips. In case that was
before your time, RTL was a very early IC logic, like about the first
available to anybody other than the military. It ran off +3 volts, and
each input went directly to the base of a transistor, through an
internal 400 ohm resistor.
That made the noise immunity about 0.5 volts, maximum!

The tape decks turned out to be unusable. Just the action of the tape
rubbing against the reels in rewind or fast-forward was enough to set
off tiny static discharges, usually too small to hear or see, but
enough to trip the logic from rewind to FF or Play! This happened
about every 10 seconds! Days of my playing with extra bypass
capacitors was to no avail.

In your situation I'd try the same thing, start with a large (100nF)
capacitor, if that squelches the problem, try one ten times smaller,
until it fails again, then go up to the next power of ten.

You don't want to use really large capacitors, as it's not a good
idea to hold the gate in any state between 1 and 0 for very long. It
could oscillate, or overheat, or fool the next stage.
 
G

Genome

Jan 1, 1970
0
Hi all,

I made the foolish mistake in a recent instrumentation design of
routing a copper track from one board to another via a common 'mother'
board and associated edge connectors, straight into the input of a 4071
OR gate. The output of this gate clocks a 4015 4-bit shift register.
The circuit 'works' fine in the lab but the problem lies in the
equipment installation, where a number of noisy mains-powered solenoid
relays manage to falsely trigger the shift register for around one in
every ten actuations. I suspect that the long copper route is acting as
an ideal aerial for this noise, even though the entire circuit is
housed in a grounded metal box.

Are there any 4071 plug-in equivalents with schmitt-trigger inputs, or
other variants with better noise immunity? A less eloquent option would
be to insert two series connected schmitt-trigger NOT gates (of the
40106 variety), but space is already at a premium for such a
'piggy-back'. Any suggestions appreciated.

Thanks,
John.

Errrrr..... Wot if you stick a 1MEG resistor from the output to the noisy
input and hack the track at the input to insert a series 10K resistor in it.

That makes a schmitt trigger, doesn't it?

DNA
 
J

John Fields

Jan 1, 1970
0
Errrrr..... Wot if you stick a 1MEG resistor from the output to the noisy
input and hack the track at the input to insert a series 10K resistor in it.
 
A

Ancient_Hacker

Jan 1, 1970
0
An excellent idea, making that OR gate into a schmitt trigger!

One might want to adjust the 10K resistor upwards. The 1Meg / 10K
combo is fine for like a scope trigger application, where you want a
sensitive trigger zone, but for rejecting noise you want a much wider
hysterisis zone. 1Meg / 220K would give a lot more noise rejection.
 
F

Fred Bloggs

Jan 1, 1970
0
Switching speeds are very low,
with the pulse being generated by a monostable (duration of almost 1s)
through a diode, so that a separate circuit can latch the state (using
a FET switch with feedback). A 22K resistor to ground enforces the low
state of the line when a different pulse switches off the FET.

You need to tell us more about this "FET switch with feedback" latching
"the" state and what this has to do with the OR gate and CLKing the SR.
After low power consumption, CMOS is mainly used for the largest
possible noise immunity attainable by a logic family- making it unlikely
that noise pickup is the real problem.
Should
this resistor be bypassed with a 100nF cap (thereby bypassing the OR
gate input to ground), or would the suggested RC network at the OR gate
input be better?

Who knows- capacitor bypass does no good when charged from a low
impedance source. Why don't you sketch out this masterpiece and post it
somewhere you can link us to.
 
The MOSFET is used to latch a transistor that drives a 30mA IR LED.
Instead of taking a shortcut by tapping off this, I should have wired
the monostable directly into the S input of an S/R latch IC and
connected the Q output to the OR gate and the transistor that powers
the LED. Lesson learned. In any case, a 10nF capacitor across the 22K
resistor solved the problem. I've tested over 200 solenoid actuations
since without trouble.
Thanks.
 
F

Fred Bloggs

Jan 1, 1970
0
The MOSFET is used to latch a transistor that drives a 30mA IR LED.
Instead of taking a shortcut by tapping off this, I should have wired
the monostable directly into the S input of an S/R latch IC and
connected the Q output to the OR gate and the transistor that powers
the LED. Lesson learned. In any case, a 10nF capacitor across the 22K
resistor solved the problem. I've tested over 200 solenoid actuations
since without trouble.

That fix doesn't mean anything, it is strictly experimental and blind.
You could be talking about anything and the logic should have achieved
steady state long before the solenoid actuated if that's what it's
doing. You could be talking about anything:
View in a fixed-width font such as Courier.

..
..
.. 5V
.. |
.. .-------+
.. | |
.. | [R]
.. | |
.. | a
.. | k ~~ IR
.. -| |
.. p |-----+
.. -| |
.. | |
.. | | ____
.. | | \ \
.. | | | >---> to sr clk
.. +-------|-------/___/
.. | |
.. | | 4071
.. [Rd] |
.. | |
.. | |
.. | |
.. | |/
.. >----|>|-----+-----|
.. | |>
.. [22k] |
.. | |
.. --- ---
..
..
..
.. 5V
.. |
.. .-------+
.. | |
.. | [R]
.. | |
.. | a
.. | k ~~ IR
.. >| |
.. |-[Rb]+
.. /| |
.. | |
.. | |
.. | | ____
.. | | \ \
.. | | | >---> to sr clk
.. +-------|--------/___/
.. | |
.. | | 4071
.. | |
.. [Rc] |
.. | |
.. | |-
.. >----|>|-----+-----| n
.. | |-
.. [22k] |
.. | |
.. --- ---
..
..
..
.. 5V
.. |
.. [R]
.. |
.. a
.. k ~~ IR
.. |
.. |-
.. .----------------------+---| n
.. | | |-
.. | | |
.. | ____ | ---
.. off >-------|--------------\ \ |
.. '----\ \ | o--+------> to sr clk
.. | o----/___/
.. on >-------+----/___/
.. | 4001
.. [10k]
.. |
.. ---
..
 
J

Jim Thompson

Jan 1, 1970
0
Hi all,

I made the foolish mistake in a recent instrumentation design of
routing a copper track from one board to another via a common 'mother'
board and associated edge connectors, straight into the input of a 4071
OR gate. The output of this gate clocks a 4015 4-bit shift register.
The circuit 'works' fine in the lab but the problem lies in the
equipment installation, where a number of noisy mains-powered solenoid
relays manage to falsely trigger the shift register for around one in
every ten actuations. I suspect that the long copper route is acting as
an ideal aerial for this noise, even though the entire circuit is
housed in a grounded metal box.

Are there any 4071 plug-in equivalents with schmitt-trigger inputs, or
other variants with better noise immunity? A less eloquent option would
be to insert two series connected schmitt-trigger NOT gates (of the
40106 variety), but space is already at a premium for such a
'piggy-back'. Any suggestions appreciated.

Thanks,
John.

See...

http://www.analog-innovations.com/SED/NoiseBlank.pdf

for one way to handle big-time noise.

I had another version, somewhat simpler, but I haven't located it yet.

...Jim Thompson
 
N

no_one

Jan 1, 1970
0
we used to use a 12V variant of TTL circuits back in the 80's in flash LASER
systems so as to not be upset by the firing of the flash bank.
 
J

Jim Thompson

Jan 1, 1970
0
we used to use a 12V variant of TTL circuits back in the 80's in flash LASER
systems so as to not be upset by the firing of the flash bank.
[snip]

In systems where I know where the noise is going to occur (such as a
car ignition system) you just blank during that interval. In my CD
ignition designs the distributor pickup was blanked while firing was
occurring.

...Jim Thompson
 
Thanks for taking the time to do the schematics Fred. The relevant
portion of my cct is below along with the new cap.
.. ------------<Reset
.. 8V |
.. | |
.. +----------|------+
.. | | |
.. [R] | |
.. | | |
.. a | |
.. k ~~ IR | +---
.. | | | |
.. +----[R]---+---+-> P
.. | |
.. | +---
.. |/ |
..Set>---|>|--+--[R]----| |
.. | |> |
.. | | |
.. | | |
.. | --- |
.. | | ____
.. -----------------------+-----+---Long track--\ \
.. | | | >--->
.. | | /___/
.. New cap --- [R]
.. --- |
.. | |
.. -----+
.. |
.. ---
Best viewed in fixed font again
 
Thanks to Fred for taking the time to draw those schematics. The
relevant portion of my cct is below along with the new cap.
.. ------------<Reset
.. 8V |
.. | |
.. +----------|------+
.. | | |
.. [R] | |
.. | | |
.. a | |
.. k ~~ IR | +---
.. | | | |
.. +----[R]---+---+-> P
.. | |
.. | +---
.. |/ |
..Set>---|>|--+--[R]----| |
.. | |> |
.. | | |
.. | | |
.. | --- |
.. | | ____
.. -----------------------+-----+---Long track--\ \
.. | | | >--->
.. | | /___/
.. New cap --- [R]
.. --- |
.. | |
.. -----+
.. |
.. ---
Best viewed in fixed font again.

My circuit does not actuate solenoids, but it's installed in a machine
with plenty of them. The noise blanking suggested by others is very
interesting, but since solenoids can be actuated at any time, I don't
know if this approach would work in my case.
 
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