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Unistream PLCs & SDI-12 sensor networks?


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Hi all,

A very general enquiry: I'm interested in using a Unistream PLC (I have a US5-B10-T24 available) as a backup to log data from a SDI-12 sensor network (in this case soil moisture sensors buried somewhere deep in the Amazon ). I guess the most direct solution should be to use a RS232/USB  to SDI-12 converter and then handle the requests and responses with message composer.  If anyone has any experience or thoughts to share as to whether this will work or recommendations for compatible converters, they would be most welcome and appreciated!

Thanks & all the best,


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  • MVP 2023

I took a quick look at the SDI-12 protocol and it's very well documented.  So, yes, it will work.  I'd stick with RS232 on the Unitronics side as it's more predictable.

It looks like there are quite a few RS232 to SDI-12 converters out there.  My rule of thumb on a converter for a customer application is get one that's  from a company you've heard of before that has a real tech support staff and a phone number.  

Joe T.

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  • 1 year later...
  • MVP 2014

I am working through an SDI-12 interfacing project, to read sensors using a Unistream PLC.

I originally started down the track of using a SDI-12 to Modbus gateway. However for various reasons I am now using a RS485 to SDI-12 gateway and will be creating and parsing the messages as ASCII. I'd appreciate hearing of others' experience with decoding the data that is returned from the aD0! command and similar. I'm about to do my own testing, but any tips and points to note would be appreciated. Happy to share my own experience also.

I am using 3 sensors. The most complex is an Atmos 14 Gen 2 environmental sensor. I have set the sensor to SDI address 2, an example data exchange is as follows:

Send: 2M!
Receive: 20014<0D><0A><00>
Send: 2D0!
Receive: 2+1.437+24.9+0.457+100.38<0D><0A><00>

values are:
* vapour pressure
* temperature
* Relative Humidity
* Atmospheric pressure

One of my hopes is that the message length is always the same, and the number of decimal places always stays the same for each value. I don't know SDI-12 well enough yet to be sure of that.

FYI, I also purchased a USB to SDI-12 adaptor to assist with setup and testing, which was a wise decision. 

I look forward to any words of wisdom I receive.

Kind Regards,




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