直连线：用于连接网络中计算机与集线器（或交换机）的双绞线，它又分为一一对应接法和100M接法。
通过两个节点的串口，使用一根直连线缆将它们连接起来。
Use a null modem cable to connect the two nodes through their serial ports. Now test the serial connection, as follows.
两个系统的串口之间的直连线缆用来在两个节点间传输heartbeat。
A null modem cable connected between the serial ports of the two systems is used to transmit heartbeats between the two nodes.
一根 IBM 串口直连线缆（serial null modem cable）。
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