One thing I didn’t pay enough attention to when planning for the electric window blinds was the width of the sliding glass doors. Some of these are more than 4m wide, which is problematic because:

• A more-than-4m-long roller tube would need to be quite a large diameter to be stiff and strong enough
• Very few blind fabrics are available more than 4m wide – and they’re expensive

Therefore, it’s generally better to use two narrower blinds instead of one very wide blind. The small gap between them is mostly blocked by the sliding door frame.

The issue then is that I only specified that a single 230V KNX relay be wired to each door location – and it’s not possible to simply connect two 230V blind motors ‘in parallel’ to one relay channel. The reason is nicely explained in this text from Schalk Steuerungstechnik GmbH:

Roller shutters or louver blind motors with mechanical limit-switches must as a rule not be electrically connected directly in parallel, because due to the different motor running times the limit-switches of some motors may be reached while other motors are still running. The motors that are already switched off would then receive inductive voltage at the counter-winding from the motors that are still running, which can lead to destruction of the limit-switches.

There are two main options to overcome this issue:

• Using blind motors with ‘electronic’ (rather than ‘mechanical’) limit switches, since these don’t suffer from the same limitation
  • This was the approach adopted for the wide sliding door in one of the bedrooms and it has mostly been successful – though the limit settings on one of the motors do tend to ‘creep’ over time and periodically need re-setting
• Multiple motors can be isolated from each other using relays, installed adjacent to the blinds, that block the induced voltage from a still-running motor from damaging an already-stopped motor

Various companies manufacture suitable relays but the products from Schalk Steuerungstechnik GmbH seem pretty good. In particular, their MGR U2 which can take the output from one KNX relay (or a pushbutton or whatever) and drive two blind motors. Variants are available for 1, 2, or 4 motors and for DIN-rail or ‘flush’ mounting (in a regular back-box, behind a blank cover plate). These do not require an additional permanent live supply and are very competitively priced.