BCU1, BCU2, BCU?, whats the difference

BCU versioning is hardware related. Since the BCU is a microprocessor board providing connectivity with KNX BUS media, it holds the KNX System Software (i.e. the protocol stack plus other necessary stuff) in a dedicated ROM. The parametrization given through ETS will instead reside in EEPROM, along with current application software (so application sw is somehow temporary and writable by ETS, and system sw is one time only, given by manufacturer). Refer to the picture given. For only the sake of completeness, the RAM stores current values of group objects and other temporary data. Some of data may be transferred to EEPROM upon voltage failure (to restore load statuses after a black out).

The versions are related to the current System Software Version. So BCU2 holds a newer version of the sw than BCU1, in ROM (that's why sometimes the BCU version is referred to as "mask version").

The versions imply some consequences, especially related to the number and type of objects available (datapoints, to be bound to group addresses). For example, the following applies to the maximum number of group objects that can be allocated and managed on a specific BCU, by the application software

BCU1 -> 12 BCU2 -> 32 BIM M112 -> 255

Other differences exist also, as to the maximum number of group addresses, the support for serial number (for example to put a device into programming mode via bus only), access control and, especially, parametrization.

With regard to ETS, there is no connection between ETS versions and BCU versions, but of course, you may find impossible to program NEW BCU versions with OLD ETS versions. In those cases, much likely, the .VDx file (ETS database for that device) would not simply load into ETS.

There is no restrinction, instead, in making different BCUs work together in a KNX installation. Backward compatibility is always ensured. Also, newer ETS versions can manage old BCU versions without any problem (within their limitations, specified above).

In the picture given, the module on the right is the Application Module (AM), connected to the BCU (in the left, right attached to the bus, of course) via the PEI (the standard serial interface, depicted as a set of parallel lines, in the picture). Most often (even though not always), the AM will simply provide some HW singals only (e.g. switches, buttons...) while the application program will be executed on the BCU uP.

BCU1 - very old one BCU2 - old one BIM M 112 - actual, the application with BIM M 112 usually is more advanced than BCU2/1.

This means what its name suggests.

The BCU (Bus Coupler Unit) is the electronic layer which manages communication over a network EIB/KNX (coding and decoding information). Each EIB/KNX device needs a BCU to transmit/receive telegrams on the bus.