TBM1.2-040-W1假設(shè)波特率為9600bps，就是每秒9600位。也就是傳輸一位（1bite）的時(shí)間是1/9600=0.000104s。在這個(gè)時(shí)間內(nèi)電磁波的傳輸距離（也就是光速為299792458m/s ）為31228米。如果電信號的電磁波延時(shí)達(dá)到0.000104s（大約0.1ms），那么就會延時(shí)到錯(cuò)位一個(gè)數(shù)據(jù)位，這樣就會出現(xiàn)誤碼。怎么理解？在某一時(shí)刻主機(jī)收到的最遠(yuǎn)的節(jié)點(diǎn)的數(shù)據(jù)會與大約0.1ms前最近的節(jié)點(diǎn)發(fā)送的數(shù)據(jù)重疊一位，這樣就有誤碼了。TBM1.2-040-W1也就是RS-485的無誤碼通信的理論最遠(yuǎn)距離在9600bps時(shí)只有大約31KM。假設(shè)每1200米進(jìn)行一次中繼，31228/1200=26.02，也就是說要達(dá)到RS-485的理論極限，需要至少26次中繼延長。
以上可以看出，RS-485通信距離的理論極限與波特率成反比，波特率越高極限距離越短。當(dāng)波特率為115200bps時(shí)（=12?9600），無誤碼傳輸?shù)睦碚摌O限距離為31228/12=2602米。只有大約2.6km！這也難怪RS-485的遠(yuǎn)程通信只說9600時(shí)傳輸多遠(yuǎn)，幾乎不提115200bps。
5、其它介質(zhì)和其它總線的理論極限
TBM1.2-040-W1 以上的RS-485距離極限343512米以及31228米（9600bps）與傳輸介質(zhì)無關(guān)，就是說用光纖傳輸RS-485最遠(yuǎn)也是這么多，無線也是一樣。 以上的無誤碼RS-485距離極限31228米（9600bps）與協(xié)議無關(guān)，就是說用CAN、PROFIBIUS最遠(yuǎn)也是這么多，原理是一樣的。以上的通信完全失敗的RS-485距離極限343512米（9600bps）與數(shù)據(jù)位數(shù)有關(guān)，而且成正比例，就是說用CAN2.0（29位）比CAN1.0（11位）的極限更加大，大一倍以上。
到這里，TBM1.2-040-W1大家一定會驚訝RS-485通信距離極限之短?；ヂ?lián)網(wǎng)、手機(jī)、GPS是如何傳輸幾乎無限遠(yuǎn)距離的呢？互聯(lián)網(wǎng)和手機(jī)通信幾乎無法實(shí)時(shí)，而GPS對延時(shí)的計(jì)算及其精確。以后將另外專文討論。
串口232/485通信的理論極限及其實(shí)現(xiàn) 串口232/485通信的理論極限及其實(shí)現(xiàn)
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TBM1.2-040-W1
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