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K3: Added protection for RS-232 port

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Pete Smith N4ZR

K3: Added protection for RS-232 port

I've been thinking about how one might go about adding some protection
for the apparently fragile (and expensive) RS-232 port on the K3. It
might be possible, if the components were small enough, to add them on
the KIO3 I/O daughterboard, or perhaps on a little external board
mounted at the port.

But the question is, what components? I have seen small gas tube units,
but suspect that by the time the potential reaches 90 volts or so and
the tube flashes over, the RS-232 transceiver is probably toast anyway.
Would it be feasible to use fast diodes to shunt the data lines to
ground at .7 volts or so, or would that introduce other problems (stray
capacitance, etc.)? Anything else that might make sense?

--
73, Pete N4ZR

The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000

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W8JI

Re: K3: Added protection for RS-232 port

> I've been thinking about how one might go about adding some protection
> for the apparently fragile (and expensive) RS-232 port on the K3. It
> might be possible, if the components were small enough, to add them on
> the KIO3 I/O daughterboard, or perhaps on a little external board
> mounted at the port.

The ideal solution is to use a shielded cable, the shortest possible cable,
and have the frame or cabinet of the computer grounded to the same station
common point as the radio and all other gear.

Nearly all of the solution is in how you wire the desk and the radio room
antenna cable and power entrance.

73 Tom

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Don Wilhelm-4

Re: K3: Added protection for RS-232 port

In reply to this post by Pete Smith N4ZR

Pete,

I am not certain why you believe the K3 RS-232 port to be "fragile". I
have not seen a large number of failures identified on this reflector.
There have been quite a number of users who have had problems with USB
to RS-232 adapters, but that is not a failure of the K3 RS-232 port.

Can you give me some examples of RS-232 port failures on the K3?

73,
Don W3FPR

Pete Smith wrote:

> I've been thinking about how one might go about adding some protection
> for the apparently fragile (and expensive) RS-232 port on the K3. It
> might be possible, if the components were small enough, to add them on
> the KIO3 I/O daughterboard, or perhaps on a little external board
> mounted at the port.
>
> But the question is, what components? I have seen small gas tube units,
> but suspect that by the time the potential reaches 90 volts or so and
> the tube flashes over, the RS-232 transceiver is probably toast anyway.
> Would it be feasible to use fast diodes to shunt the data lines to
> ground at .7 volts or so, or would that introduce other problems (stray
> capacitance, etc.)? Anything else that might make sense?
>
>

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Jim Brown-10

Re: K3: Added protection for RS-232 port

In reply to this post by Pete Smith N4ZR

On Sun, 18 Jul 2010 20:59:43 -0400, Pete Smith wrote:

>I've been thinking about how one might go about adding some protection
>for the apparently fragile (and expensive) RS-232 port on the K3.

I strongly suspect this is more pin 1 stuff. Connect the shield, if there
is one, to the CHASSIS, not the so-called signal common (pin 5 on DB9s).
I also strongly recommend using CAT5 for serial cables, using one pair
for each communications circuit, and tying all the signal returns to the
chassis (via the DB9 shell), NOT pin 5. Most DB9s support that. Details
on my website, both in the RFI Tutorial (Solving Problems in the Shack),
and in the Ham Interfacing power point.

Another point. Study the material on my website about shunt mode and
serial mode surge suppression. Most lightning-induced failures of
interconnected equipment is caused by shunt mode surge suppressors
(MOVs).

73, Jim Brown K9YC

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Pete Smith N4ZR

Re: K3: Added protection for RS-232 port

In reply to this post by Don Wilhelm-4

Don, I have read several reports on this list of RS-232 failures
(probably implicating U1 on the KIO3A) apparently due to induced
voltage. My comment was based more on my having used radios with serial
ports for some 15 years and never had any problem with the ports,
despite leaving serial cables connected all the time. Elecraft advised
me always to disconnect any cable from the RS-232 port on my K3 when not
in use, from which I inferred fragility.

Anyway, the point is that I'm ready to add some protection, if I can
figure out how to do it.

73, Pete N4ZR

The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000

On 7/18/2010 10:31 PM, Don Wilhelm wrote:

> Pete,
>
> I am not certain why you believe the K3 RS-232 port to be "fragile".
> I have not seen a large number of failures identified on this reflector.
> There have been quite a number of users who have had problems with USB
> to RS-232 adapters, but that is not a failure of the K3 RS-232 port.
>
> Can you give me some examples of RS-232 port failures on the K3?
>
> 73,
> Don W3FPR
>
> Pete Smith wrote:
>> I've been thinking about how one might go about adding some
>> protection for the apparently fragile (and expensive) RS-232 port on
>> the K3. It might be possible, if the components were small enough,
>> to add them on the KIO3 I/O daughterboard, or perhaps on a little
>> external board mounted at the port.
>>
>> But the question is, what components? I have seen small gas tube
>> units, but suspect that by the time the potential reaches 90 volts or
>> so and the tube flashes over, the RS-232 transceiver is probably
>> toast anyway. Would it be feasible to use fast diodes to shunt the
>> data lines to ground at .7 volts or so, or would that introduce other
>> problems (stray capacitance, etc.)? Anything else that might make
>> sense?
>>
>
>

hb9ari@bluewin.ch

Re: K3: Added protection for RS-232 port

Hi Pete,

I will look around U1 on the KIO3A!
Never had an RS232 failure since >30y
and never disconnect them when not in use...
only when connecting an other device...
I always thought it was a "relatively" robust
interface!

Thanks for info!

73 QRO,

Rudolf, HB9ARI (K3 #1212)

Pete Smith wrote:

> Don, I have read several reports on this list of RS-232 failures
> (probably implicating U1 on the KIO3A) apparently due to induced
> voltage. My comment was based more on my having used radios with serial
> ports for some 15 years and never had any problem with the ports,
> despite leaving serial cables connected all the time. Elecraft advised
> me always to disconnect any cable from the RS-232 port on my K3 when not
> in use, from which I inferred fragility.
>
> Anyway, the point is that I'm ready to add some protection, if I can
> figure out how to do it.
>
> 73, Pete N4ZR
>
> The World Contest Station Database, updated daily at www.conteststations.com
> The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com,
> spots at telnet.reversebeacon.net, port 7000
>
>
> On 7/18/2010 10:31 PM, Don Wilhelm wrote:
>
>> Pete,
>>
>> I am not certain why you believe the K3 RS-232 port to be "fragile".
>> I have not seen a large number of failures identified on this reflector.
>> There have been quite a number of users who have had problems with USB
>> to RS-232 adapters, but that is not a failure of the K3 RS-232 port.
>>
>> Can you give me some examples of RS-232 port failures on the K3?
>>
>> 73,
>> Don W3FPR
>>
>> Pete Smith wrote:
>>
>>> I've been thinking about how one might go about adding some
>>> protection for the apparently fragile (and expensive) RS-232 port on
>>> the K3. It might be possible, if the components were small enough,
>>> to add them on the KIO3 I/O daughterboard, or perhaps on a little
>>> external board mounted at the port.
>>>
>>> But the question is, what components? I have seen small gas tube
>>> units, but suspect that by the time the potential reaches 90 volts or
>>> so and the tube flashes over, the RS-232 transceiver is probably
>>> toast anyway. Would it be feasible to use fast diodes to shunt the
>>> data lines to ground at .7 volts or so, or would that introduce other
>>> problems (stray capacitance, etc.)? Anything else that might make
>>> sense?
>>>
>>>
>>
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>

Don Wilhelm-4

Re: K3: Added protection for RS-232 port

In reply to this post by Pete Smith N4ZR

Pete,

Since you are concerned, I would suggest adding some MOVs between the
RS-232 port pins 2, 3, 4, and 7 and ground.
MOVs rated at 25 volts should be sufficient.

I have read a few (but very few) reports of lightning surges taking out
the RS-232 port, but in all cases that I have known about the computer
was also damaged, so it is difficult for me to say that the K3 RS-232
port is any more "fragile" than any other component.

There may be valid concerns if one lives in a high lightning incident
area and do not have extensive lightning protection installation at your
station, every piece of equipment in the hamshack may be at risk, but
there are ways to minimize that risk.

I would rather suggest that you do some work on lightning protection.
Do a search on the ARRL website for "lightning protection" and look
especially at the 3 part series authored by Ron Block. That is good
information about how to make your station (and your family) more safe.
For good protection, the installation of several grounding radials
around the tower (or mast) with driven ground rods at periodic intervals
and a perimeter wire around the house, all tied into the utility
entrance ground will go a long way toward affording some lightning
protection. Those ground radials help to dissipate the energy in a
lightning surge over a large area of the earth - the wire should be as
large as you can get - #6 is the smallest I would consider.

In addition, bringing all lines into the station (including AC mains
power, telco, LAN as well as antennas, rotor cables and control lines)
through a single point grounding panel which has protective devices
installed on each and every line, and connect every chassis in the shack
area to that single point entry panel.

A station ground system of the type I have described is much better
insurance for containing surges than doing something to beef up
protection on any particular device. It is not easy, nor is such
protection inexpensive, but I deem it important.

73,
Don W3FPR

Pete Smith wrote:

W8JI

Re: K3: Added protection for RS-232 port

This is absolutely the single most important thing to do....

> In addition, bringing all lines into the station (including AC mains
> power, telco, LAN as well as antennas, rotor cables and control lines)
> through a single point grounding panel which has protective devices
> installed on each and every line,

I would also pay attention to what Jim Brown said about the pin 1 problem.
That is at the very top of important things to worry about.

http://www.w8ji.com/station_ground.htm

Pete, being on the second floor, has some additional wiring "tricks" he must
do.

73 Tom

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George & Jan

Re: K3: Added protection for RS-232 port

In reply to this post by Pete Smith N4ZR

If you are "home brewing" protection remember that RS-232 data signals go
from minus to positive. Levels between -3 & +3 volts are undetermined. If
you try to protect the RX & TX lines to ground then make sure that you can
pass at least -5 volts to insure a good low.

In my 30 years experience interfacing broadcast equipment in South Florida
(the lightning capital of the US) to tall towers & Florida Flash & Flicker:
First is massive grounding - large conductors with many ground rods
Second is keeping lightning away from the towers (dissipation arrays),
Reduce all inductances in the ground system (including ground rods,
tower bonding, primary power bonding and equipment bonding)
Then surge protection on primary power and interconnecting lines.

Our experience found that RS-232 UARTS were were not particularly
vulnerable. Audio distribution amplifiers failed most with power supplies
second while RF, Video, RS 485 & Ethernet failures about equal to RS-232.

George
AI4VZ

>> I've been thinking about how one might go about adding some
>> protection for the apparently fragile (and expensive) RS-232 port on
>> the K3. It might be possible, if the components were small enough,
>> to add them on the KIO3 I/O daughterboard, or perhaps on a little
>> external board mounted at the port.

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Bud Morin

Re: K3: Added protection for RS-232 port

In reply to this post by Pete Smith N4ZR

I hope this isn't true. Which end of the cable? For over a year I've
never disconnected it. I'm too old to reach around, under, or over the
shelves to try disconnecting and all that is involved in that.

On 7/19/2010 6:08 AM, Pete Smith wrote:
> Elecraft advised
> me always to disconnect any cable from the RS-232 port on my K3 when not
> in use, from which I inferred fragility.

73,
Bud, K9ZT

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KENT TRIMBLE

Re: K3: Added protection for RS-232 port

The RS-232 port on my K3 has been controlling both a Palstar AT-AUTO
and a Quadra amp continually for over two years with zero problems.

Only time that line is disconnected is when I do K3 updates with the
KUSB adapter cable or logging during Field Day and other events.

73,

Kent K9ZTV

> On 7/19/2010 6:08 AM, Pete Smith wrote:
>> Elecraft advised me always to disconnect any cable from the RS-232 port on my K3 when not
>> in use, from which I inferred fragility.
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Don Wilhelm-4

Re: K3: Added protection for RS-232 port

In reply to this post by Bud Morin

Bud,

The "end of the cable" is not significant. Lightning does not respect
any particular "end of the cable".

The situation extends well beyond the RS-232 connection to the K3. It
includes everything in your hamshack. The real answer is that in the
event of a surge, everything in the shack should always be at the same
potential - even if that potential is many kilovolts above normal ground
potential (I do not exaggerate) - but achieving that requires providing
some protective measures.

If you have done nothing in your hamshack installation to handle
lightning surges. AND you in a high lightning incidence area, the best
protection is to disconnect everything when not in use - do that until
you have a good surge protection scheme installed..

Of course, that is really not always practical - so if you wish to
protect your K3 correctly (along with all the other equipment in your
hamshack), install some good lightning protection - single ground point
entry point for ALL lines coming into the hamshack - NO exceptions -
ethernet, telco, antennas, rotator, control lines, and yes even the AC
mains entry to the hamshack should enter through a single panel
employing MOV or other protection devices present at that panel for each
and every line - this is the single most important point of lightning
protection. Look at all the equipment in your hamshack area, and
consider everything within a 4 foot radius from each piece of equipment
as something which should be within your "area of protection" - all
lines connecting to those pieces of equipment should come through this
"grounding window". If you can touch any piece of equipment while at
your operating position, it should be included in the protected
equipment region.

That is a minimum - I would also recommend that you also have a
perimeter ground wire around your house, and from each tower or mast
(and vertical antenna), you have a good surge dissipating ground system
installed consisting of at least 4 ground radials at least 30 feet
long. These ground wires should be #6 or larger wire and be connected
to DRIVEN ground rods at intervals not exceeding 2 times the length of
the ground rods. The principle is to allow the surge to dissipate into
"mother earth" as easily as possible.

Take a look at the information n the 3 part series on Lightning
Protection that was pubilshed in QST in 2002?. Go to the ARRL website
and do a site serach on Lightning Protection. The 3 part articles by
Ron Block are my personal "bible" for lightning protections.

One important note - any grounding system (unless more than 150 feet
from the house) must be connected to the house Electrical Utility ground
wire - NEC requires it - ignore this fact at your own peril - failure to
do that can destroy equipment and place you and your family in danger
should a lightning surge occur (consider fire hazards).

73,
Don W3FPR

Bud Morin wrote:

> I hope this isn't true. Which end of the cable? For over a year I've
> never disconnected it. I'm too old to reach around, under, or over the
> shelves to try disconnecting and all that is involved in that.
>
> On 7/19/2010 6:08 AM, Pete Smith wrote:
>
>> Elecraft advised
>> me always to disconnect any cable from the RS-232 port on my K3 when not
>> in use, from which I inferred fragility.
>>
>
> 73,
> Bud, K9ZT
>
>

Jim Brown-10

Re: K3: Added protection for RS-232 port

In reply to this post by Bud Morin

It should not be true, but whoever told you that appears not to understand
lightning and bonding. Study what W8JI, W4TV, Jim Lux, and I have written
about grounding. If you do that, you should not have a problem.

73, Jim Brown K9YC

On Mon, 19 Jul 2010 21:02:34 -0500, Bud Morin wrote:

>I hope this isn't true. Which end of the cable? For over a year I've
>never disconnected it. I'm too old to reach around, under, or over the
>shelves to try disconnecting and all that is involved in that.

>On 7/19/2010 6:08 AM, Pete Smith wrote:
>> Elecraft advised
>> me always to disconnect any cable from the RS-232 port on my K3 when not
>> in use, from which I inferred fragility.

>73,
>Bud, K9ZT

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Pete Smith N4ZR

Re: K3: Added protection for RS-232 port

In reply to this post by Don Wilhelm-4

73, Pete N4ZR

The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000

On 7/20/2010 12:01 AM, Don Wilhelm wrote:
> ...so if you wish to
> protect your K3 correctly (along with all the other equipment in your
> hamshack), install some good lightning protection - single ground point
> entry point for ALL lines coming into the hamshack - NO exceptions -
> ethernet, telco, antennas, rotator, control lines, and yes even the AC
> mains entry to the hamshack should enter through a single panel
>
All of my hamshack equipment is on a single outlet. This has me
wondering whether a good quality connection between the entry panel SPG
and the neutral *at the outlet* (it is less than two feet from the entry
panel) would approach or achieve the objective of helping everything to
rise and fall together.
> employing MOV or other protection devices present at that panel for each
> and every line - this is the single most important point of lightning
> protection.
>
Yet other knowledgeable people say that using MOVs in this way is apt to
*cause* damage. I realize that this discussion should probably move to
Towertalk now, but don't want these questions to go unraised, and we
have a bunch of the right people here.

73, Pete N4ZR
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W8JI

Re: K3: Added protection for RS-232 port

> On 7/20/2010 12:01 AM, Don Wilhelm wrote:
>> ...so if you wish to
>> protect your K3 correctly (along with all the other equipment in your
>> hamshack), install some good lightning protection - single ground point
>> entry point for ALL lines coming into the hamshack - NO exceptions -
>> ethernet, telco, antennas, rotator, control lines, and yes even the AC
>> mains entry to the hamshack should enter through a single panel

That is good advice. All you need is one path into the operating desk or
close to the operating desk that is not common to the entrance panel to undo
the whole mess. That bonding, including bonding back to the mains ground, is
more important than anything else.

> All of my hamshack equipment is on a single outlet. This has me
> wondering whether a good quality connection between the entry panel SPG
> and the neutral *at the outlet* (it is less than two feet from the entry
> panel) would approach or achieve the objective of helping everything to
> rise and fall together.

Of course it would. You could use proper MOV's and bypasses on ALL of the
power mains leads to the SPG at the panel or entrance point. (It really does
not have to be a through-hole two-sided panel, it can be a flat sheet things
mount on.)

>> employing MOV or other protection devices present at that panel for each
>> and every line - this is the single most important point of lightning
>> protection.

Bonding and wire and cable routing to avoid ground loops through equipment
is by far most important.

> Yet other knowledgeable people say that using MOVs in this way is apt to
> *cause* damage.

I don't know why anyone would say that. What kind of damage could it cause?
To me, that makes no sense at all. MOV's might not help or might not be
*useful* on all lines, but adding them would certainly not hurt anything.
There would be no harm in peppering the panel with MOV's. It's a case where
adding something might not always help, but adding it would never hurt
anything.

For example.... I do not have a single special lightning protection device
like a polyphaser on any coaxial cable at any entrance but adding the proper
device would not hurt. I rely on my DXE and RCS-8V antenna switches for
differential mode coaxial line protection because they do a much better job.

73 Tom

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Don Wilhelm-4

Re: K3: Added protection for RS-232 port

In reply to this post by Pete Smith N4ZR

Pete,

The only possible reason I can think of for people to say that is that
sometimes the MOVs can damage themselves in the event of a surge.
They become sacrificial devices - if they fail, replace them.
Yes, MOVs should be always be enclosed, so should they explode when they
fail, bits do not go flying just anywhere.

73,
Don W3FPR

Pete Smith wrote:
> Yet other knowledgeable people say that using MOVs in this way is apt to
> *cause* damage. I realize that this discussion should probably move to
> Towertalk now, but don't want these questions to go unraised, and we
> have a bunch of the right people here.
>
> 73, Pete N4ZR
>
>
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Joe Subich, W4TV-4

Re: K3: Added protection for RS-232 port

In reply to this post by Pete Smith N4ZR

>
> 73, Pete N4ZR
>
> The World Contest Station Database, updated daily at
> www.conteststations.com The Reverse Beacon Network at
> http://reversebeacon.net, blog at reversebeacon.blogspot.com, spots
> at telnet.reversebeacon.net, port 7000
>
>
> On 7/20/2010 12:01 AM, Don Wilhelm wrote:
>> ...so if you wish to protect your K3 correctly (along with all the
>> other equipment in your hamshack), install some good lightning
>> protection - single ground point entry point for ALL lines coming
>> into the hamshack - NO exceptions - ethernet, telco, antennas,
>> rotator, control lines, and yes even the AC mains entry to the
>> hamshack should enter through a single panel
>>
> All of my hamshack equipment is on a single outlet. This has me
> wondering whether a good quality connection between the entry panel
> SPG and the neutral *at the outlet* (it is less than two feet from
> the entry panel) would approach or achieve the objective of helping
> everything to rise and fall together.
>> employing MOV or other protection devices present at that panel for
>> each and every line - this is the single most important point of
>> lightning protection.
>>
> Yet other knowledgeable people say that using MOVs in this way is apt
> to *cause* damage. I realize that this discussion should probably
> move to Towertalk now, but don't want these questions to go unraised,
> and we have a bunch of the right people here.
>
> 73, Pete N4ZR
> ______________________________________________________________
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P.B. Christensen

Re: K3: Added protection for RS-232 port

> All of my hamshack equipment is on a single outlet. This has me
> wondering whether a good quality connection between the entry panel
> SPG and the neutral *at the outlet* (it is less than two feet from
> the entry panel) would approach or achieve the objective of helping
> everything to rise and fall together.

In the U.S., tying the service entry common ground/neutral bond back to the
shack outlet neutral is a code violation. However, I see nothing in the NEC
that prohibits an alternate grounding conductor to be added to the ground
screw on a receptacle. Still, I wouldn't do this unless you have reason to
believe there's a high resistance between the receptacle ground point and
the serving panel.

If you're going through the trouble of adding a ground conductor, I would
studying code-compliant station grounding methods as shown on the K9YC and
W8JI websites.

After establishing an effective station ground and if there's still a
concern relating to the rise and fall potential created by surges on the
grounding conductor in the shack, consider adding a whole-house surge
suppressor only at the main entrance panel and not a sub-panel. Also, the
best method of adding secondary surge suppression on a branch circuit is not
through the use of so-called "three modes of protection" MOV devices.
Instead, consider using only single-mode to neutral protection. The better
units store the surge energy through a high C component and bleed the surge
to neutral, rather than ground. They're expensive units. SurgeX and
Brickwall are two manufacturers using this technology.

Paul, W9AC

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P.B. Christensen

Re: K3: Added protection for RS-232 port

In reply to this post by Joe Subich, W4TV-4

> "The only possible reason I can think of for people to say that is that
sometimes the MOVs can damage themselves in the event of a surge.
They become sacrificial devices - if they fail, replace them.
Yes, MOVs should be always be enclosed, so should they explode when they
fail, bits do not go flying just anywhere."

In the context of utility line surge suppression, MOVs should only be used
at the service entrance (i.e., utility meter and/or service entrance panel).
If MOVs are used on a branch circuit, they should only be used between line
and neutral. Neutral is the grounding conductor actually designed to carry
current -- not ground.

We'll never get past the belief that "all three modes of protection" is a
good thing. I was once a believer myself until recently, having installed
expensive Leviton hospital-grade receptacles in the shack with built-in
"three modes of surge protection." Even IEEE literature continues to
condone these devices. The dilemma becomes clearer when you draw the
electrical distribution on paper and examine the potential surge flow.

Arguably, a more effective method of diverting a surge on a branch circuit
is to *store and bleed* the surge to neutral. These units cost much more
than a single MOV when used from line to neutral but there's also more
propensity of blowing the MOV rather than the bleeding type of single-mode
surge suppressor.

Paul, W9AC

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Jim Brown-10

Re: K3: Added protection for RS-232 port

In reply to this post by W8JI

On Tue, 20 Jul 2010 07:48:24 -0400, Tom W8JI wrote:

> MOV's might not help or might not be
>*useful* on all lines, but adding them would certainly not hurt
anything.

The issue with MOVs and other shunt mode suppressors is that they divert
the strike into whatever ground they are connected to. If all grounds
are properly bonded, as Tom, Joe, and Paul have described, that's fine.
One MAJOR bonding error is the Pin 1 problem. But if the grounds are NOT
properly bonded, a big part of the strike potential can exist between
equipment that is grounded at different points, and when that equipment
has low voltage connections between it, that high potential causes
destructive equipment failure. This is the mechanism that causes
failures with three mode suppression that Paul is talking about. A
primary function of bonding all the grounds is to minimize the potential
throughout the building (and between equipment) in the event of a
strike.

An important way of thinking about this is that the function of an MOV
(or other shunt suppressor) on a signal or control line is to limit the
size of the DIFFERENTIAL voltage. When we mount a typical coax
suppressor on a copper ground window, the outside of the coax is
grounded, which minimizes the common mode voltage of the strike, and the
differential signal is limited by the suppressor.

Things get more complicated with a balanced line, like a rotor cable or
the control lines to a SteppIR, because both sides of the line need
MOVs, and the two MOVs on a line may not conduct equally with a strike.
That gets even more complicated with multiple balanced circuits feeding
a SteppIR. :)

Yet another problem with shunt mode suppression on signal and control
lines (like RS232) is that the suppressor adds capacitance, and the
speed/distance of serial lines is limited by the capacitance of the
circuit. That capacitive limit is another reason that CAT5 makes
excellent serial cable -- it's capacitance is quite small compared to
other cables, typically about 12pF/foot. We've used this in the pro
audio world to extend the useful range of a simple RS232 circuit to
several hundred feet.

73, Jim K9YC

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