1756-M13

1756-M13

1756-M13

 也就是RS-485的理论传输的最远距离在9600bps时只有大约343512米。假设每1200米进行一次中继，343512/1200=286，也就是说要达到RS-485的理论极限，需要至少286次中继延长。

 4、无误码的RS-485通信距离的理论极限
       并非只有当整个10位数据完全重叠时才无法通信，实际上只要有1位错位存在误码就不好。虽然有时候软件有一定纠错功能，容许存在一定程度的误码情况下也可以传输数据，但是我们还是要弄清楚无误码的极限。

假设波特率为9600bps，就是每秒9600位。也就是传输一位（1bite）的时间是1/9600=0.000104s。在这个时间内电磁波的传输距离（也就是光速为299792458m/s ）为31228米。如果电信号的电磁波延时达到0.000104s（大约0.1ms），那么就会延时到错位一个数据位，这样就会出现误码。怎么理解？在某一时刻主机收到的最远的节点的数据会与大约0.1ms前最近的节点发送的数据重叠一位，这样就有误码了。也就是RS-485的无误码通信的理论最远距离在9600bps时只有大约31KM。假设每1200米进行一次中继，31228/1200=26.02，也就是说要达到RS-485的理论极限，需要至少26次中继延长。

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