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Porter

[LVTN] Plasmionique Modbus Master

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Hello, Porter.  I'm new to Modbus and I'm trying to use the Plasmionique Master library to communicate with a 2-wire RS485 device over a USB port.  Do I need to have a 2-wire USB-to-485 converter, or is there a way in the Plasmionique library to configure the Modbus communication as 2-wire? 

Background: I'm trying to communicate with a Mellen furnace Love controller.  I can communicate with it using the manufacturer's software, but whenever I try to use the Modbus Comm Tester in the Plasmionique library I keep getting timeout error (-1073807339).  I'm pretty sure I've got the baud rate, parity, stop bits, and flow control configured correctly.  The Open Port function on the Comm Tester seems to work, but whenever I try to read coils or holding registers I get the timeout.  Mellen tech support suggests it is because the controller is a 2-wire RS-485 device.  I'm using a Tripp-Lite USB-to-485 adaptor cable, their tech support says the cable should support either 2-wire or 4-wire communication.  So now I'm wondering if 2-wire/4-wire needs to be configured somehow in the Plasmionique library.  Thanks.

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When you say Love Controller, do you mean a Dwyer "Love" Temperature Controller? They definitely work with this library.

For configuring the USB-RS485 adapter, I have never used labview to specify 2-wire or 4-wire mode. That is usually done via hardware (dip-switches or jumpers) or the driver (device manager). I have never used the Tripp-Lite adapter though. I typically use USB-Serial converters from Moxa or Sealevel Systems. They have proven to work very will with labview.

If you can provide the model number or user manual of the Love controller, maybe I can give you some example commands to try.

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Yes, it is a Dwyer Love Temperature Controller, the manual can be found here: http://www.dwyer-inst.com/pdf_files/e_90_bpc.pdf

I can communicate with the device using the Dwyer software, which also tells me which parity, stop bits, etc. to use.  But when I try the Plasmionique Serial Comm Tester I get the timeout error.  I can open a port, but cannot read the coils or holding registers.

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Be sure to double check the ASCII/RTU mode setting.

Make sure that you are specifying the starting address in hex format. The PV value is supposed to be at holding register 0x1000.

You could also try increasing the timeout value to 1000 or 2000ms (instead of the default 300ms).

A screenshot of the dwyer communication setup and the Plasmionique Modbus Comm Tester setup would be useful.

 

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