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Unitronics UAC-CX-01RS4 connection to a sensor

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I wanted to see anyone opinions' on the wiring of my Dissolved Oxygen Sensor to Unitronics UAC-CX-01RS4. This is my first time working with Modbus, So far in testing I have only received a 0 for my read values for DO mg/l, DO Oxygen sat %, DO ppm, and DO Temp Celsius.  And for the Status I have received a -11 for all 4. 

This is how I interpreted the wires are connected from:

  • White wire RS485 (command) to D- on the UAC-CX-01RS4. 
  • Brown Wire RS485 (Data) to D+ on the UAC-CX-01RS4.
  • Pink wire to 12VDC. 
  • Blue wire to Signal Ground (SG) on UAC-CX-01RS4.

I am unsure if the Functional ground needs to be connected to anything from the Unitronics UAC-CX-01RS4. Right now its left unused, but at this point I feel like it should be going somewhere. 


Thank you for any help. 

unitronics UACCX01RS4.png

DO Sensor wiring.png

DO Modbus master.png

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  • MVP 2023

Please post Sensor model # or pdf manual.

Some sensor must be configured by OEM software before connect to Modbus network.

I see (data)  (command) pinout - it is not Modbus standard pinout. Maybe sensor is not Modbus by default (some configuration bit must be set for Modbus enable).

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  • MVP 2014

Functional ground is not likely to be an issue for basic testing. It would normally connect to the physical ground, like the metal enclosure or racking, etc.

Baud rate and other serial parameters need to match. I see you have set 9600 baud. Was that from the sensor datasheet?

It is also common with RS485 that the data wires aren't labelled consistently between different manufacturers. For example A/B or +/- and so on. If you are sure everything else is correct, try swapping the two data wires. Also with termination, whilst everyone says to add it, on very short networks I find it is often necessary to remove termination. So if you are just testing with a short cable between the PLC and the sensor, try with and without termination.

I hope this helps.



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  • MVP 2023

Your poll time is too fast for RTU and is shorter than your response time.  You're filling up your buffer.

For testing I'd set your poll time to 5 sec each until you get it functional.  Then you can speed it up and see where it starts choking.

Joe T.

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  • MVP 2023

And like is mentioned all over the forum, the best method for working with a new modbus device is to learn about it using your PC.  Any number of the free modbus utilities found on the web will suffice.  The whole point is that it is far easier to find what works and what doesn't, by using the PC.  Once you've got it all figured out, it is vastly easier to then shift all those known parameters to the PLC.  And a question I don't know the answer to...does Unistream do the 1 digit difference in register numbers like Vision does?  Perhaps this is your issue in the first place...?

cheers, Aus


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  • MVP 2023

I had a quick look at the manual and I'll leave the connection issue to Kratmel who is more used to quirky connections.  I'd need one of these on the bench to be sure of things.

Looking at some of the registers in the sensor,  I'd be very careful with your testing, so that you don't accidentally upset some crucial setting which might be done in registers, both listed and unlisted!    I again suggest doing it all on the PC first.  I use Modbus Poll and a link below..... but this is the trial that only runs for 10 minutes.  Some other freebies are fully functional.



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