This really depends on the RTU brand, but in "the old days", many long-range modems were 4-wire because they were simple analog circuits that mapped RS-232 to something else. Thus 4-wire allowed a free-flowing bi-directional path.
However, now days most modems include small micro processors which can pack even a full-duplex comm path into two-wire (even half-duplex) However, it WILL affect the timing. So that's why the protocol is important. Some protocols won't care - if the RTU starts to talk before the 'master' is done, and the response is delayed, some protocols will not care. Others will, and treat this as failure.
Sure, there are alternative modem choices that can handle full-duplex communication on 2-wire connections! There will be trade-offs regarding speed but in many applications this is not an issue. Check out our modem products at http://www.miille.com and if you have questions or need a quote, please contact us from there. Our most popular modem is our 366-101 that is DIN rail mounted and supports many older modem communication standards; both 2-wire and 4-wire connections.
