Digital quantity expansion board applied to rail transit vehicle braking system

The utility model relates to a digital quantity expansion board, and specifically relates to a digital quantity expansion board which is applied to braking instruction receiving and braking state feedback of a rail transit vehicle braking system. The digital quantity expansion board comprises a micro controller, a bus module, a digital quantity input/output module, a relay passive contact switch output module and a protection module. According to the utility model, the provided digital quantity expansion board applied to the rail transit vehicle braking system can be widely applied to city rail transit vehicle braking systems, can receive a braking instruction in real time, transfers the instruction into the braking system for processing through the bus, and outputs a braking state signal to a vehicle to monitor the braking state information in real time; the digital quantity expansion board applied to the rail transit vehicle braking system has the characteristics of small input current, wide applicable temp