Abstract: An automatic control system includes a host, a network gateway operatively connected to the host, and a plurality of programmable logic controllers (PLC) operatively connected to the network gateway. The network gateway includes a controller and registers. The controller backups data from specific addresses in the PLC to the registers sequentially for each predetermined time period. When the controller receives an accessing command from the host and the accessing command designates the specific addresses, the controller sends the data, which is corresponding to the designated address and already stored in the registers, to the host. The accessing efficiency can be enhanced because part of data in the PLC is cached in the network gateway.