I'm designing a project based mainly on CD4000 series logic
devices. The finished product will consist of two different
sections linked by a long (~40m) 2-core shielded cable. The
signals are very low frequency pulses (a few Hz at most) and
transition times are not critical.
This kind of vague thinking is the root of much design evil.
You know what you mean, but we don't.
The audio guy and a guy who builds picosecond laser drivers
will have very different perceptions of "critical transition times".
If the time is not critical, make it infinite and you don't need
the cable.
Numbers were invented for a reason. Use 'em.
It's not practicable to let the two sections share a common
power supply. Neither is it possible to ensure that the
separate PSUs will be switched on and off at the same time
or in a specified sequence. Therefore, one unit may already
be sending a signal to the other before the latter's PSU is
turned on.
To protect the receiving unit, I'm thinking of placing 10k
resistors in series with the CMOS inputs (in addition to
parallel terminating resistors) plus Schottky diodes from
the IC input pins to Vdd and ground.
Is this OK? Is it necessary? Is the integrated protection
good enough?
Must be the phase of the moon. Today, there seem to be a lot
of very vague questions with fromthehip answers.
It's like a blindfolded quick-draw contest with live ammo.
You've been given some good advice, but there are a lot of
issues to consider.
I take a lot of flak here for insisting that people define
exactly what they're trying to accomplish.
A typical scenario involves tunnel vision. Some very smart
engineer comes up with a solution that has one little problem.
She asks about that problem and gets a solution to THAT
problem, not realizing that it created two additional problems.
It's often worth a few minutes to rethink the system.
The design goals for intended operation are no more important
than the specs for what happens when some idiot gets their hands
on it and does unspeakable things. Unfortunately, people tend
to skip both.
You don't say what your "product" is, but there's a wide range
of system requirements depending on the target demographic.
A simple catch-all starting point relevant to this
discussion is electrostatic discharge.
What ESD protection do you require and how are you gonna verify
that you meet it? A 10K resistor sounds like a lot of
protection until you start poking a 15KV ESD generator around.
The devil is in the details. And the current doesn't always
go through the resistor like you'd expect if you looked at the
schematic. If you don't believe it, watch a few lightning
strikes up close.
A properly deployed opto-isolator can cure a lot of common-mode
system ills, but it's not a panacea.
It might protect your CMOS input, but now, you have to protect
the opto-isolator from ESD...unless you use fiber for the whole
run. And that will address a number of other issues like EMC.
Diodes are another potential gotcha.
You're gonna need a minimum of two resistors.
One to protect the input from the peak overvoltage allowed by
the diodes and strays. And another resistor to protect the diodes.
For 5V hobby circuits, I've taken to using 5.1V zener diodes as the
protection element...and here's why.
I once built a GPIB interface from a PIC processor. I unplugged the
power to the chip and the GPIB kept right on controlling.
I immediately started writing a paper on how I'd written code that
tapped into zero-point-energy and ordering parts for my water-powered car.
Then, I realized that the GPIB was powering the chip through the
forward-biased input protection diodes on the PIC. Not any part of it
met spec, but it worked just fine.
Depending on the circuit, a diode clamp to VCC can let an
input transient take out every IC on the board. That's probably
not the protection you expected. It's easy to assume that you can
shunt arbitrary current into VCC, but the math might surprise you.
The zener diodes can protect the circuit from power supply injection.
You've identified power sequencing as a potential problem.
Consider when the protection diodes put just enough voltage
on VCC to cause a lot of dissipation in some external power switching device
and let the smoke out.
Then, there's the problem that the robotic welding arm kills two
workers if you turn power on in the wrong sequence.
We haven't even started talking about what happens when your system
clock happens to be just the right frequency to resonate the cable
and makes the FCC very unhappy. Unless your operator has a ham
license and 40-meters is the electrical length. ;-)
Write the specs...ALL the specs including the design verification
procedures and the customer acceptance test procedures and the
third party certification test procedures. Consider customer abuse.
Details matter...that's where the devil lives...right next door to
Murphy.
