The other robust option would be any industrial network: CAN, LIN, Modbus, Profibus, etc. (several of which are a protocol layer that runs on any physical layer, or can tunnel through another..).
CAN is a multidrop network, which would be nice, but it would have to run damn fast (way faster than practical) to serve even a few boxes at once. And they're not described as chained (though that might make installation easier, if changing the topology is an option?), so that's no help.
RS-485 is quite robust, but you may need to provide isolation, and that drives up the cost.
Ethernet would then be the cheapest isolated option, and again, you can use it on a pretty low level, no PC needed -- it may not be as well supported at this level in terms of available tools to work with it (e.g., if you aren't using IP, then you aren't going to get anything into a PC expecting an IP address..), but that's really not much different from RS-485 where you're building your own protocol and stack completely from scratch!
And as Mike's shown, low level Ethernet is quite accessible.
Tim