Idea

raymiller suggested Jul 9, '20

ET340 Carlo Gavazzi Energy Meter Victron Online Manual omits correct Modbus termination.

In the information on Victron Energy ET430 online manual fails to mention that the ModBus should be correctly terminated by connecting the EM340 pin 7 to Pin 9. When this is done you should be able to measure 120 ohms resistance between the ModBus pair (without the cable connected). The Master end (RS485 USB adaptor) should also be terminated in 120 ohms, again it's easy to check with a multimeter. By making sure each end of the RS485 cable is correctly terminated the Modbus transmission line now will reduce any errors which may be caused by pulse reflections and also reduces the impedance of the line which lowers the susceptibility to any interference.

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Izak (Victron Energy Staff) commented Jul 13, '20

Hi @RayMiller,

You are right about the ET340 containing this nifty built-in termination resistor. Termination may however not be required on all networks, so to say that it should always be terminated is not true.

If the propagation delay is much less than the width of one data bit, termination is not needed and is often skipped for ease of installation or to avoid adding a load to the bus.

Since the meters communicate at a relatively low baud rate (9600bps) and the USB cables are quite short, termination isn't a requirement in this application. It becomes essential only if you extend the cable significantly.

It would however be nice to mention this feature of the ET340 in our documentation, so I will amend that.

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Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

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ET340 Carlo Gavazzi Energy Meter Victron Online Manual omits correct Modbus termination.

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