Modbus Circuit, Laptop with Modscan and one device

Discussion in 'The Projects Forum' started by skylight, Jul 16, 2014.

  1. skylight

    Thread Starter New Member

    Jul 16, 2014
    1
    0
    Hi everyone,

    Sorry if my inquiry seems basic. I do have some modbus instruments, one of them is a CAMERON MC-III, which is really a turbine meter flow computer. Now, here's the thing: HOW CAN I CONNECT TO IT TO READ/WRITE REGISTERS USING MODSCAN/MODSIM? Remember, I do not want to connect to a full modbus network with several slaves, I just want to connect to this single device with my laptop. So far, I have tried to connect using a MICROFLEX USB to RS485 converter, directly to the modbus pins of the instrument (+ and -), but that does not work at all. I have also tried to connect to it placing a 120hms resistance in parallel but that does not work either. This device talks modicon modbus RTU, which I guess is the standard version, but even so I have tried to set it to Enron, Daniels, etc, and it's not working either. I have also made sure that both the MICROFLEX converter and the modbus device have the same baud rate, bits, etc, etc, etc...

    I am not too sure of what the right circuit should be. Am I missing something? do I need a power supply in series or something like that? I really do not know, as I do have more slaves to connect to make a fuller network. In addition, what voltage (DC) could I expect to find in both modbus pins to make sure the unit is trying to communicate?

    I have also tried to connect the unit to a NET485 device (made by GRIDCONNECT, s Modbus RTU to Modbust TCP converter) but although MODSCAN detects the NET485 itself, it is not receiving any information from the modbus unit, I guess the same circuit configuration problem... So I can't read it directly using the USB to RS485 converter or through a MODBUS RTU to MODBUS TCP Converter...

    Thanks to whoever might help and sorry for the long explanation...
     
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