Learn it from an experienced technician.
I consulted BMW ICOM A2 from the professional technician mainly because I read much information in BMW forums telling that the only difference between ICOM A2 & ICOM is the hardware.
Actually, ICOM A is the first generation BMW ICOM device.
ICOM A is the basic component. In physical terms, it would provide the vehicle diagnostic interface OBD II interface, and provide adaptive integration to the repair shop network interfaces required. Voltage is provided by KL30 by vehicle interface. ICOM A requirement of the electrical operation of a minimum voltage of 8V. Only the supplied voltage exceeds the minimum limit in order to ensure a stable function.
ICOM B is the external ICOM A MOST (Multimedia Transmission System) interface. It is connected with ICOM A through the USB cable. Voltage is provided by ICOM A through USB. Only when the lowest of the ICOM A supply voltage of 8V, ICOM B would only be ready to run the electrical side of the vehicle through a standard ICOM B MOST interface.
ICOM C is an intelligent interface that allows OBD II socket and BMW round match. As the lengthening of the auxiliary module, it will help to connect ICOM A and non-OBD II vehicle interface. Voltage is provided from the vehicle side of the circular socket by KL30 provided. ICOM C electrical operation provides for a minimum voltage of 8V. KL30 on the voltage sent to the OBD II device on the inside, and thus to ICOM A power supply. Only when the vehicle’s battery voltage exceeds the minimum limit in order to ensure stability of the two devices interact.
Compared with ICOM A+B+C, BMW ICOM A2+B+C has the some special features. The modified ICOM A2 for vehicles with OBD access receives a significantly more powerful processor and more storage capacity. Robust aluminium case will help to avoid damage when using in harsh environments. To protect the MOST connector, the ICOM B for vehicles with optical MOST access will be equipped with a soft cover made of rubber instead of the former plastic cap. The ICOM C for old series with a diagnostics socket will be additionally equipped with an approx. 20 cm long cable, which facilitates the connection of the ICOM A2 and makes storage in the engine compartment easier.