With an Rx and Tx arrangement, special serial cables are used to connect the new server to both the Rx and Tx lines at the existing server’s connection. The new server contains the configuration for the relevant outstations, points, and so on, as well as the direct or PSTN channel that represents the connection. The channel has the eavesdropping feature enabled so that the new server can identify that the input (the data from the existing system’s Rx line), and the output (the data from the existing system’s Tx line) is unsolicited.
The Tx lines of the new server’s connections to the existing system are disconnected. With these lines disconnected, the new server cannot send data to the outstations, and therefore cannot affect the operation of the existing system.
