Could we use one PLC (or  one tranduser) to turn the 4..20 mA to 2..10 Vdc and use the analog voltage parallel to all controlers ???

1.  I seriously doubt that there is a Multiple analog output device out there.  Most of these multiple signal applications are handled with digital protocols nowadays.  There are splitters that take one 4-20mA and reproduce it as an identical signal on two or three outputs. 

You might want to look at the Carlos Gavazzi Dupline, as mentioned in this thread:  http://www.plctalk.net/qanda/showthread.php?t=80366&highlight=Modbus+master

2.  A number of paralleled voltage outputs will 'load' the source because the input resistances are in parallel so the law of parallel resistances come into play to lower the source resistance.   If Excel's math is accurate, then twenty 1Mohm inputs in parallel will reduce the 500 ohm dropping resistance to 495.05 ohms for about a 1% error (495/500) in the voltage across the paralleled resistances.

3.  I have a project that needs 24 4-20mA analog outputs, but each one is unique, a separate process variable, not duplication of a single value.

There are I/O modules that are Modbus input (most integer, one that handles floating point) and have multiple analog outputs, usually four or six that cost about $300-$400 US each.

I'm using the Moore Industries NET NCS series to write Modbus floating point values to the Moore 'controller' which feeds the values to six 4 channel AO modules to get the 24 AO's needed.   $4k USD for 24 channels, $3.4k for 20 AO's

Dan
 

Hello AutoGen_22989,

you can us e.g. transducer from Phoenix Contact.

This is the link for a "standard" - 
Signal conditioner - MINI MCR-2-UI-UI-PT - 2902040
and this with a signal duplicator - 
Signal duplicator - MINI MCR-2-UNI-UI-2UI - 2905026

With both you can transduce your current signal to a voltage signal.

You have different possibilities

1. transduce your signal from current to current
   the maximum output load for the current output of this duplicator is 600 Ohm
   If you wire 10 analog inputs with a input resistance of 50 Ohms in series than you can do it with 10 (12) channels per output of the duplicator
2. transduce your signal from current to voltage
   you can wire the voltage inputs in parallel
   the minimum load for the voltage output is 10 kOhms.
   in my opinion you can use e.g. 20 analog inputs with 1 MOhm in parallel
   your load is then 50 kOhms and high enough for the signal duplicator

This both suggestion don't need a big effort.

Hope this helps

regards
CaOd