Here is a 4-20 mA In/Out Analog Mux with Cascade option. This is a simple circuit i designed to make a Automation System within a budget.
Mixed Circuits Analog with Digital
This takes 4-20mA from many Transmitters and gives out just one 4-20 mA output. The Mux is done with a digital byte or word. This is a slow scanner as process is slow, that way many analog inputs can be multiplexed and sent into one analog input of a D/A.
In near real time systems a faster mux could be used or mux totally avoided. This was made in some numbers, so the pcb is better than others.
PCB and Schematics for the Analog Mux prototype are here.