Selecting a Microchip

You basically want to implement something like PoE (power over ethernet) over 1200m and with up to 60 (or is it 127?) cable powered devices. I am definitely not an expert in this area but this sounds like trouble. Just take a look at PoE specification (http://en.wikipedia.org/wiki/Power_over_Ethernet), where max 13W is guaranteed for a single device at the end of up to a 100m cable. Maybe you will need to have a separate power supply for each repeater, separate shielded cable for power supply and separate shielded cable for the signal wires (or use CAT7 cable where each wire pair is shielded, as is the outer sheath, if you really think you can get away with 100/120 ohms difference). Using 1 wire pair for signal and 3 wire pairs for power in CAT7 might help, but all that is still optimistic since I am worried about how much current will go through these wires (especially when all relays are switched on if you don't use latching relays) and how will the current at it's sudden extreme peaks interfere with the signal (although protocol limitation of accessing single relay card at once helps in this case since you can't switch them all on at once). You must estimate how much max current each relay card will draw and do your math to see if all this is possible or modifications need to be made because of too much current or too much power loss over distance. Maybe your calculation tells you that in this case solid state relays can justify their price by consuming less current then standard relays.