I have been looking at current loops, interesting history behind them. Here's how I think it could be implemented on Master V2. Looking at the schematic http://www.solarvan.co.uk/bms/Master20082009.jpg
you would need to remove the 5 volt supply from J11 pin one (well don't actually remove the 5 volt feed from J11, just connect the wire to the current source) and instead feed it with a device like a LM134/LM234/LM334
. Set this device as a 5ma current source. Then remove VR2, VR3, C9 (should already be removed), R27 and D9. Note, we leave R8 (1k) and replace R34 with a jumper/0 ohm link. Now what happens is the LMx34 will supply 5ma current whenever one of the slave master bus optos conducts and 5ma across a 1k resistor (R8) generates 5 volts that the pic will see as a pulse. Or you could set the LMx34 for 1ma and R8 to 5k, this also gives 5 volts.
Nice thing about this is the slaves remain unchanged.
I won't have time to try this out anytime soon, but if anyone else wants to see if it will work, please let us know.