Michael Rigby-Jones wrote:

>>                      | < Connect to negative terminal of PS/Battery
>> ---------+           |     +-------------+
>> modem A  |Red -------+-----| modem B     |
>>          |Green -------+---|             |
>> ---------+             |   +-------------+
>>                        | < Connect to positive terminal of PS/Battery

> This surely can not be correct, the presence of a nice low impedance
> battery or PSU would effectively short the lines for any signal,
> making comms impossible.  I thought the the supply was effectively in
> SERIES with the line?

  Well PICked!  I let that one pass when I saw it, but now that you
mention it, the answer is that it should have an inductor or "virtual
inductor" in series with the power supply.  "Real" transmission bridges
use a balanced inductor.


  The virtual inductor circuit was referenced a couple of weeks back at
http://209.1.238.246/1st_pages/A1208.htm (it's Q1 in the circuit) and
has some desirable characteristics over a real inductor including - no
inductive "kick".
--
  Cheers,
        Paul B.