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Rich Webb Rich Webb is offline
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Default A cordless phone question

On Sat, 22 Oct 2011 11:56:39 -0500, Ken wrote:

I have two AT&T model 1475 cordless phones. (I know they are quite
old, but I like the feel of the phone even though it is a 2.4GHZ phone.)
I acquired the second one for a few dollars primarily for parts if the
first one crapped out. Both of the units work just fine in all respects.

Since they are the same model and the phone from one does NOT work on
the other base unit, I assume there is a different frequency that each
is running at. I opened up one unit and I did notice a crystal tacked
on the back of the PWB that I assume is unique for each pair.

My question: Just out of curiosity, what do the manufacturers do to
make their same model cordless phone communicate with only the base it
belongs? My guess is they change something simple like the crystal on
both the base and handset so that there is a distinction between units.
Anyone know?? Thanks.


It's almost certainly handled in the digital realm rather than a single
change in the base frequency. The handset and base are paired together
by something like both sharing the same seed value for a pseudo-random
number generator. The PRN is used as the basis of digital spectrum
spreading of the RF signal, frequency hopping or direct sequence, so
that only units that share the same sequence can inter-operate.

Some base units and handsets can be "registered" together so that the
base can operate with more than one handset. Since that capability isn't
mentioned in the 1475's manual, I'd guess that the pairing is done once,
at the factory.

*If* the pairing key in the handset is stored in an identifiable chip,
say, a serial EEPROM, then it might be possible to get at the key and
change it.

--
Rich Webb Norfolk, VA