Re: Water heater eating X-10 signal

[ddl@danlan.*com (Dan Lanciani)]

In article <04YUh.314346$5j1.274986@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>, JeffVolp@xxxxxxx (Jeff Volp) writes:

| That's an interesting thought regarding 240V modules.

Incidentally, it isn't my thought; I'm sure I read it somewhere though
I can't remember where.  So I wasn't surprised when I saw how the
CR230 was built.

| Of course, the
| percentage of those is miniscule compared with 120V modules used on
| split-phase systems.

The percentage of 240V modules in my home is exactly 0; hence my
interest in a repeater that drives the legs in phase for less
contention with the blocker/coupler.

| Since most X10 modules work fine down to 100 mV, or even lower, it would
| take almost perfectly balanced attenuation in both legs for in-phase signals
| to null enough at the 240V device to cause a problem.

I'm not sure it's that simple.  The imbalance required depends on the
strength of the signal at the receiver.  If it is already near the sensitivity
threshold then a 50% imbalance could still be insufficient to avoid trouble.
This is basically the generic carrier interference argument where if the
signals are strong enough to begin with then even their difference is likely
sufficient to operate a module.  It's why you typically don't see a carrier
interference problem with multiple synchronized transmitters collocated at a
receiver.  For a simple protocol, X10 can lead to some pretty complicated
analysis...

| With the small
| percentage of 240V devices in service, it would seem to make more sense to
| use in-phase drive so 240V resistive loads would not attenuate the signal.

I certainly can't see any downside, but then I don't have any 240V receivers.
Possibly the repeater designers know something we don't.  I suppose if I
were building a repeater product I'd be tempted to make it a switchable option.
There's something unaesthetic about driving what amounts to a differential
line pair in phase and hoping for random impairments to make it work. :)

| You are correct in your assumption that the XTB-II drives 120KHz in-phase to
| both legs.

Not an assumption; I asked you before. :)

| The commonly used .1uF passive coupler will drive both legs in-phase, but
| the second leg will always be lower in amplitude.  That again would provide
| sufficient signal to a 240V module bridged across both legs.

It depends.  What if the sensitivity of the 240V module is 100mV, it is
currently seeing 100mV on one leg and 0V on the other, and the passive
coupler causes 50mV to appear on the previously-0V leg?

I keep meaning to check whether the official passive coupler flips the
phase.  It uses transformers on both sides so it certainly could.  My
brief experience with it was quite negative with previously working
transmitter/receiver pairs failing once the coupler was switched in.
Something funny was going on and since there was already significant
passive coupling I wonder if it was somehow producing cancellation.
Of course, this was at a time when I was lucky to have 50mV levels
at some locations.

I'm pretty sure that the blocker/coupler cannot flip the phase because
of the way it blocks, and it was with that device that they started
warning about combination with a repeater.

| I have a couple of the old Leviton 6201s kicking around.  If I get a chance,
| I'll scope it to see what it does.

I have the original (two black wires and one white; does not support
extended codes).  There was a second rev that supported extended codes
and could drive a third phase but had an absurd power-on sequence
requirement.  They were both the same part number, right?

				Dan Lanciani
				ddl@danlan.*com