4 wire modems were popular in the UK in the 70s and 80s, the Datapoint system I ran the mid 80s had them for communicating with head office. The advantage of a 4-wire circuit is that it provides full-duplex working, which is not possible on a standard (2 wire) system, so, in theory, data transfer can be faster (in practice, it is usually data one way and handshakes the other, so it makes little difference). Most leased lines in the UK were 4 wire, so we saw a lot of these. We still have a few Racal units at work, on Radar data cicuits (the benefits of plot extraction - a live radar feed goes from needing a 5MHz wide microwave link to a 2400Baud modem, and still has space for remote monitoring to be multiplexed on top, that's real progress!). With the low data rates on most of these units, the will drive a long way, and they have varying levels of intelligence, from the simple FSK oscillator and receiver, to inbuilt error correction, depending on the application. Our old Racal units are very simple, but later units have internal microprocessors to handle the various additional features. Jim.