Hello, 

I am attempted to use the PLC native Analog Current Inputs to function as IDC fire circuit. I have been discussing with folks that the circuits they have designed will not work correctly.  One sensor would be a heat detector and the second would be a smoke detector. If we are measuring current in think the current would stay constant from the perspective of the PLC. also the second zone would just short out the PLC Sensor as we would be placing the 24VDC to ground. I believe i am evaluating these basic circuits correctly. i am not sure there is a way to set the PLC up to measure like an IDC Fire circuit. 

Regards,

Hello.

The built-in analog measures between 0 and 10Vdc.
So already with that, there is a problem as 24Vdc is used.

All the resistors are in parallel.
That means every branch receives 24Vdc. 
This also means the analog receives 24Vdc.

A serial connection would be better suited.
Pin-pointing the source of a break is difficult.

Is this an academical exercise?

~

Hello.

Seeing that it is an academic exercise
I am going to give you some materials to study: 

A Guide to Fire Alarm Basics – Initiation

Current Sensing Techniques using Different Current Sensors

Look at the image under the heading "Ionization smoke detectors" of the first web link. This represents what you are aiming for, where the meter represents the PLC. Here the current is measured, but the PLC only has analog voltage measurement. Take note on where the meter is placed.

In the old days we only had dial type voltmeters. We had to build circuits to make those meters do measurements beyond measuring volts. The easiest for your case is to add to your circuit a current sensing resistor, also called a shunt resistor. The PLC will measure the voltage on the shunt resistor which would be an indication of the current flow.

Be sure to apply some math on what the size of the resistor should be. Be watchful for specifying a high enough wattage! else the resistor will burn out.

The PLC has a 10bit voltage measurement which will give you an indication of how sensitive you can make the circuit. You do not have to aim for measuring the full 0 to 10V measurement range.

I leave this discussion vague enough for you to figure out some details.

SIDE NOTE 1
In the future.. Please do not use abbreviations. IDC is not known to everyone for example. But if you say "an initiating device circuit" it gives some clue as to what to expect.


SIDE NOTE 2
Notice that the initiating device circuit always have a fixed resistor at the end of the cascade connected devices (see web link 1). Why do you think that is needed?

Are these notes of help?