Use a NO contact in front of a MOV block that moves your next AO value to the terminals (where the AO value is held in a holding DB tag waiting to be moved to the analog output.

---| |--------[MOV]

Then you can trigger the activation of the AO value for short periods of time based on how long each ---| |--- is true for each MOV call, like

MOV 0->A.O.
MOV [0-10V] -> A.O.
MOV [27648] -> A.O.
MOV [-10-0V] -> A.O.
MOV [-27648] -> A.O.

eng_dave

alanTI

the 1215 has 2 0..10V Analog Inputs on board. you could make a voltage divider with a known resistor and your test piece e measure the voltage drop across the test piece.

There are modules that can directly measure resistance, the are called RTD modules, typically used to measure temperatures trough varible resistors, but htey will also read "raw" resistance.

this is the product number 6ES7231-5PD32-0XB0 for the first one I found for s7-1200

Be careful to properly check the input range and resolution, these modules are very often surprisingly limited.

Another solution could be to use the analog output to generate a known current across the test piece and then use the analog input to read the voltage.

 Hi Alan, Thanks for your response 

So I've not now hooked up my 2 wire test lead into the analog side of my 1215 with 1 wire in the analog output side and the other into analog input side, My trouble i seem to be having now is i need to turn on the analog output to be in either a constant on or turn on via trigger from the digital input (switch) im using to start my program with but i can only seem to trigger the analog output from a analog input. 

Make sure every +DC supply as the same PE and -terminal reference.

On my projects, every +DC supply's - or common output is connected to PE.  That keeps the voltages all referenced to the same voltage.

That prevents 18VDC from appearing on a 10V AO - at least in my case.

Can you pencil sketch up the various wires for the 2 DC supplies, AI and AO and post a picture of that sketch?  I say sketch, so we will not judge how it looks.  If you have a single schematic drawing, you can post a screenshot of that, but that is not likely the case, and why I asked for a small quick hand-made sketch.

eng_dave

Ok so im now having another issue with this 

I have 1215 dc/dc/dc 2m is 24- and 3m is 24- and 1 wire of the test lead into the output and 1 wire into the input 

However my output analog voltage is 18v+ so this is causing the input channel to clip at the 11.something 

Is there a way to set the analog output voltage to just send out 10V because i did as described to output a NO input to a move block with 27648 in and out to the analog output. even changing the input value doesn't affect voltage. 

If using an ext 12v power supply then i can get the voltage to see its actual value as in the picture tuned down to 11.4v to make sure program sees this. so program works for monitoring but unsure how to get the 10V output into our tester and through the circuit.

Also are we right in thinking the minimum resistance the analog input can see is 100ohm also? so we may need to add a pull up resistor of 100ohm into the circuit so it can detect the 0.8-1.8ohm and 9-11.7 ohm ranges 

Use 2 Resistors, the one shown in your sketch, and perhaps another R of similar value but much lower wattage rating.

Wire the 2 R's in series.

Apply the AO across the 2 series R's - creating a voltage divider, something like this:

     HTR         R known
o---v^v^v^---o---v^v^v^---o
|            |            |
+-[AI-]    [AI+]          |
|                         |
+-[AO-]-------------------+ 

AO across both R's in series.  AI only across R-HTR 

If R-known ~ R-HTR, AI ~ 5V (if AO = 10V).

You can use Ohm's Law to calculate any value once you accurately measure R-known and verify output voltage and AI input - using a calibration technique to map AO values to actual voltages and actual voltages to AI values.

You will of course need to periodically install another R-Known in place of R-HTR to verify proper operation to account for environmental and other factors causing drift.

There are other ways to accomplish the same task.  That said, you may want to use other methods that improve accuracy and repeatability.  This is to triage whether R-HTR is ~within expected tolerances.

eng_dave

Please see attached pictures of my setup, 

I know the figures for the 0-10v resistance are wrong its just to show what are looking/expecting to see

So we output the 0-10v (when we get 0-10v) we should then get a different voltage return depending on the resistance 9-11.7ohm into the input via the circuit and we can measure the resistance and decide if its a pass or fail