说明：  FREESCALE 16位单片机片MC9S12DG128的串口驱动程序,环行缓冲区大小可设置,支持半双工RS485通信,可调用用户事件
(SCM MC9S12DG128 Serial Driver, Ring buffer size can be set up to support half-duplex RS485 communications, users can call the incident)