WebIO had been supporting X10 powerline technology since year 2000 with the first prototype of WebIO v1.
X10 is an old home automation technology dating back to the 1970s that provides a means for devices to "talk" over the AC powerline. For more information on X10, start with the wikipedia page on X10 automation.
X10 was a great technology for home automation due to its simple quick setup and being easy to understand. Not known for its reliability, X10 did work great as long as your powerlines were free of interference/noise and your power phase were bridged. X10 for a time was also widely available, even sold at local Radio Shack and other stores or a very low price compared to many home automation protocol devices sold today. Although today with no manufactures making X10 devices, used or old inventory devices have significantly risen in price compared to the $5-$10 per device often found back in 2005.
WebIO7 supports X10 powerline transmit only to control your "legacy" X10 equipment. WebIO7 does not support X10 wireless as did WebIO v4.
WebIO7 provides a 3 position terminal block for connection to an X10 transmitter/transceiver. WebIO7 connects to an X10 transmitter so that it can send X10 messages over the powerline to other X10 devices. Unlike older models of WebIO which used an RJ12 module jack, the terminal block used in WebIO7 allows for easier connection support of more X10 transmitters. WebIO7 only supports X10 transmitters and does not receive X10 signals, due to this X10 transceivers will work except for the receive functions.
WebIO supports multiple X10 power line transceivers:
There are also a number X10 transmitters that are not supported. Basically the transmitter must have a serial interface and use the TW523 protocol. The CM11 transmitters are not supported.