The forward and reverse circuit of the motor driven by Darlington tube

The circuit in the figure below drives the forward and reverse rotation of the motor based on the Darlington tube. It consists of two completely symmetrical parts. When one of the two input terminals A and B is at a high level and the other is at a low level, the motor rotates forward or reversely; when both input terminals are at a high or low level, the motor stops: If pulse width modulation is used, the speed of the motor can be controlled. Therefore, there are four combined input states in the figure, but the motor can produce five operating states. The addition of clamping diodes D1 and D2 here plays an important role. It prevents Darlington tubes BG2 and BC3 from losing control, which is safer when running at high power. Another feature of this circuit is that the input control logic level has nothing to do with the DC working voltage of the motor, and it can be reliably controlled using the TTL standard level.