Roaming Cyclic
Not 100% sure as I've nit had to install one but I think the I/O approach plus is mainly Arinc 429 (digital)interfaces with the aircraft systems, for the more modern aircraft whereas the I/O are analogue interfaces (synchros etc)for the older retrofits. There may also be a channel number difference for the GPS (2/8/12 blah blah). . .Basically I think the differences are more noticeable for the installer rather than the operator.
Regards
NigD
PS Trimble have a website if you really want to know