Protection for CMOS output?

[Pimpom]

I want to build a simple manual count-up system with a local as
well as a remote 2-digit display. The system will be based on
discrete CD4xxx CMOS logic and the count pulses will be sent to
the remote unit along with a 12V supply over a shielded 2-core
coax cable. The cable is ~20m long.

The remote input will have the usual schottky diode plus resistor
protection. The output from the local unit is buffered with
parallel inverters with a series resistor at the output. Is it
necessary/desireable to provide additional protection for the
output?

 

[Jim Thompson]

On Tue, 4 Feb 2014 12:40:12 +0530, "Pimpom" <Pimpom@invalid.invalid>
wrote:

I want to build a simple manual count-up system with a local as
well as a remote 2-digit display. The system will be based on
discrete CD4xxx CMOS logic and the count pulses will be sent to
the remote unit along with a 12V supply over a shielded 2-core
coax cable. The cable is ~20m long.

The remote input will have the usual schottky diode plus resistor
protection. The output from the local unit is buffered with
parallel inverters with a series resistor at the output. Is it
necessary/desireable to provide additional protection for the
output?

I don't know about ancient CD4xxx stuff, but most modern digital
output sections have ESD protection.

What I'd suggest would be maybe 100 Ohms in series.

...Jim Thompson
--
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[RobertMacy]

On Wed, 05 Feb 2014 06:55:00 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:

On Tue, 4 Feb 2014 12:40:12 +0530, "Pimpom" <Pimpom@invalid.invalid
wrote:

I want to build a simple manual count-up system with a local as
well as a remote 2-digit display. The system will be based on
discrete CD4xxx CMOS logic and the count pulses will be sent to
the remote unit along with a 12V supply over a shielded 2-core
coax cable. The cable is ~20m long.

The remote input will have the usual schottky diode plus resistor
protection. The output from the local unit is buffered with
parallel inverters with a series resistor at the output. Is it
necessary/desireable to provide additional protection for the
output?


I don't know about ancient CD4xxx stuff, but most modern digital
output sections have ESD protection.

What I'd suggest would be maybe 100 Ohms in series.

...Jim Thompson

Depends on how well you did the physical etc at the remote you can
probably 'get away' doing just a resistor back at the source of the
output, however...

Whenever you partition a system, think in terms of sending 'information'
NOT 'energy' only send DC energy. Doing so, will solve many potential woes
ahead of time. For example, if your remote is powered, you can use robust
interface chips, designed for husky ESD that then communicates with your
more delicate digital electronics. That can be a chip at the remote, and a
chip at the base. Plus, you can often simplify the cabling. For one off,
the extra cost is not too justifiable, but if you're making 10
million/year and don't want 0.1% returned for warrantee, can be worth it.

To see if you can take the ESD at the remote, simply 'model' the ESD as a
10pF capacitor [or 100pF] charged to 18kV and be prepared to think in
terms of 1GHz bandwidth, and 'see' where currents flow and what the
resulting voltages go to. If you have everything done in a really good
layout, the voltages can 'pop' really high, but only as a common mode.
With good layouts the important, damaging differential voltages won't be
more than small spikes of 100 volts or so with ESD diodes at the edges of
your board followed by the 100 ohm resistor into chip's outputs with their
'short' to GND or Vdc which might be able to take the joules of energy
hitting that tiny little ESD protection circuit inside the chip.

You can actually calculate [well, more like estimate] all that 'may'
happen and get a handle on when enough is enough.