说明：  用DEBUG的O命令将一数发送到串行口（2F8H/3F8H）；然后在另一台机上用I命令读入串行口的内容。如果发送的内容与接收的内容一致，说明两机已经连好线，否则必须检查连线，直到两机可通信为止；
(with DEBUG O ordered sent to a number of serial I (2F8H/3F8H); then another plane with orders I read serial port content. If the sending and receiving of consistency that the two planes had not even good line, or it must examine connections until two machine communications;)