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Solo Temp Controller Modbus RTU

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Hello everyone, 

     This is my first time ever messing around with a unitronics plc.  I have a V130 I started playing with.  I'm trying to learn modbus rtu, so I purchased a Solo 4824 temp controller from automation direct in order to practice.  I followed the examples in the modbus help files.  I want to display the controllers temperature on the HMI display.  I have port 1 configured as RS485.  The cable goes to a terminal block and from there I use pins (+)1 and (-)6 to go to the temp controllers data A and data B terminals.  I've tried for a week to connect to the Solo controller but no luck.  I used the loopback test function block but get no response.  I configured modbus and a read holding register function block in the plc logic but have not gotten anything.  I am reading all I can on modbus rtu since this is my first time ever using it and want to eventually implement it at work for some other units that use modbus rtu.  Please help.  Any information would be appreciated. 

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Hi Eyal,

     Yeah I changed the jumpers to rs485 and also left the termination on.  I am not really sure what the termination is used for.  Should I have a termination at the temp controller also?  


The terminations is crucial for the correct function of an RS485 network. In such network, the two end nodes must have either the termination jumper set to on, or alternatively a resistor (typically 120 Ohm) between the + and - signal line. The termination has two main purposes - to prevent data corruption and to minimize electric noise.

For more information please see the attached guide for RS485 (taken from Visilogic help).


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  • 2 weeks later...

Post your code so we can see if you have the serial port configured correctly.  I have actually written a program to access the Solo control and it works; the hardest thing in serial communications is making first contact. I feel your pain.


Joe T.

Hello Joe, 

     Was the code I wrote correct or am I missing something else?  I also went ahead and put a 120 ohm resistor on the temp controller but I still get no where.  Thank you. 

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