Hi. A and B inputs is using with old Hall or inductive sensors, whitch was installed in ignition distributors in old car.
Most users tipycaly dont use this inputs. In the future there will be firmware for vaf to map or maf to map conversion, then it will be useful.
Now to get RPM in software you can use sigal from ECU to tachometer connected also to A. Before you will use it check that it has a square wave form from 0 to 16V(maximum - if higher don't connect it) levels.
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