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为什么用串口模拟的MT命令,要发送两次才有数据的返回

如题,为什么用串口模拟的MT命令,要发送两次才有数据的返回。模拟的数据是FE 0A 25 00 00 00 00 00 00 00 00 00 00 00。说明下,我把FCS功能关掉了,返回的数据是FE 01 65 00 00 64 。可是必须要发送两次数据才有返回,发一次就没有。如果我第一次发没有返回,等5s以上,再发一次就有数据了。能给解答下吗?谢谢了!