The bits of the message interrupt control byte are used to define the criteria by which the message is identified. The start of message and end of message criteria are defined. To determine the start of a message, either of two sets of logically ANDed start of message criteria must be true and must occur in sequence (Sequence means idle line followed by start character or break followed by start character). To determine the end of a message, the enabled end of message criteria is logically ORed. The equations for start and stop criteria are given below:

Start of Message = il * sc + bk * sc
End of Message = ec + tmr + maximum character count reached
你好象没有定义接收的起始条件吧？