To start with, this current source has NOTHING to do with the power that any LocoNet® unit needs to get working.
Communication in LocoNet® and most LocoNet devices is done via one single wire.
This wire is carrying between 10 and 15 Volts, and will be short circuited to ground when communication is taking place.
Some available products feed this 10-15 V via a simple resistor.
But the original way of achieving this, is to feed this 10-15V via a so called current source.
It is basicly doing the same as a simple resistor, but it limits the current in the short circuit situation
to 15 mAmp maximum.
This will avoid that any wire resistance over the used cable will cause communication flaws.
This of course between certain limits.
There is a technical more important reason to do it this way, but there is no need to go that far in explanation.
A problem could arise, when -for example- GCA85 or GCA101 or CAN_GCLN are connected to the LocoNet® together with any central unit
like Twincenter, Intellibox and/or any of the available Digitrax units.
All these units contain some way of feeding this 15 mA to the LocoNet communication line.
These currents are considered to add to each other, so the total current will be much higher than needed.
That is why GCA units all provide the option to shut off this current source.
When GCA85 or GCA101 or CAN_GCLN is the only main LNet unit, its current source should always be activated.