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New to Modbus...Good examples?

Greetings,  I am new to Modbus and the 485 tool.  I need to read and write temperature to a Watlow controller and I would appreciate any good examples on how to do this but specifically what do I write to the controller for a temperature setpoint.  Would it be a string like 488.2 or is it the Modbus enumeration followed by the value?  Write now it is unclear.

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Hello teslac,

it seems that your question is beyond the topic of using SAPHIR tools to communicate in ModBus with any compliant device. Anyway, to communicate wiht your Watlow controller you have to know how the memory is organized. You can basicaly access a word table and boolean table. In order to write your setpoint you need to refer to manufacturer documentation to know which word(s) to write and how*. When you know that you use a master request 6 or 16 to write the right word(s).

* as far as modbus only handle integer, devices must interpret words to "understand" floating numbers (there's no rules for that). You can refer to our user guide http://www.saphir.fr/en/produits/gmodbus-over-serial-line-8.html to see how it's currently done.

Olivier

Message was edited by: Olivier Jourdan

Olivier Jourdan

Wovalab founder | DQMH Consortium board member | Certified LabVIEW Architect |
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    I have been using the word addresses and in the past gotten good data and have had no issues with the comminication however using the SAPHIR tool the word address was a one to one map (361 to temperature).  It appears that using the modbus tool downloaded from the NI Modbus API site (1.0.9.22) the word address would be decremented by one.   So temperature would be 360 and not 361 correct?

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Hello teslac,

I'd like to help you but I'm not aware of the NI Modbus API behavior. You could problably ask to NI developers to explain that point.

Have a great day.

Olviier

Olivier Jourdan

Wovalab founder | DQMH Consortium board member | Certified LabVIEW Architect |
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Hi teslac,

Which Watlow controller are you using.  I may be able to point you to a good starting point for communication. 

Send me a private message if you would like to discuss further. 

Best regards,

Michael

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I worked with SAPHIR on this issue and after some investigation the issue tured out to be a MAX configuration issue.  The port was set to the default which is 4 wire auto and my set up requires half duplex which is 2 wire auto.  Once I changed it all was working well.  I am not sure how it got changed since it was working fine but it will be the first thing I will check from here on.

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