在数据通信中，一种控制多点式或群集数据网络上终端的方法，其中每个终端均由计算机轮流询问，以确定它是否准备好接收或发送数据。数据传输只由计算机启动。

In data communications, a method of controlling terminals on a multi drop or clustered data network where each terminal is interrogated in turn by the computer to determine whether it is ready to receive or transmit data. Data transmission is only initiated by the computer.