The 42C OC's (blue version) signal(En/Stp/Dir) drive current requires more than 7mA. if the host mcu's I/O pin high level is 5.0V(such as ATMEGA2560),it can work well. but if the host mcu's I/O pin high level is 3.3V(such as STM32F407VET6),it can't work,please order the green version.