Mr Slim problem

Hello everyone, got a problem with a Mr Slim puh6 cassette system R407C. System is on group control with 4 systems in total. The unit has the fault E6 - (miswire/com fault) and is not allowing control of the other units. I have checked the interconnect wiring through the indoor isolator and disconnected condensate pump and high level pump switch but still the same. I'm getting 21ish volts DC between S2 & S3, is this normal? Also getting two solid red LED's on indoor board.
Killing power to this system takes out the controller & therefore control of the other systems, so I presume this was the master.

If I ever get this system running again, the unit that has the problem is to be switched off - apparently it ruins the bread, great eh? So can the master/slave settings, be altered electronically on the controller ? or is it a dip switch type setup, if it is, how?

I suppose the simple way of doing this is putting another controller on this system only but don't really fancy doing this as the false ceiling is one of those metal-tiled wedge-in type + the main A/C control panel is miles away on the first floor.

Any thoughts or help greatly appreciated

Andy

Sorry, it's ages since I last worked on Mr Slims.

But I seem to remember that there are dip switches in the OUTDOOR unit which has to be set master/slave/multi and so on.