MODBUS control for Delta VFD-M

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21 Apr 2017 01:24 - 21 Apr 2017 05:27 #91742 by screwtop
Now that I think about it, of course there isn't "a transmitting side" in this setup, as the slave is also a transmitter. :rolleyes:

I know (from scoping the signals) that there are substantial voltage spikes in the ground system while the motor is running, and the kernel messages when the operating system reconnects the USB-to-serial device actually mention EMI as a likely cause. It did also occur to me that adding the terminating resistors would increase the power demand on the RS-485 drivers. I'm not currently using libmodbus, so I think I can exclude problems with the packaging of that library. My VFD is a Delta model VFD-M, but I do have the manual, which does document the Modbus settings and registers.

On further reflection (ha), it sounds like the terminating resistors should only be needed with long cabling and/or high speeds. TI's app note says that if the bit time is greater than 4 times the cable propagation delay, the cable is not considered a transmission line and the resistors can be omitted. For a 3 m cable and 19200 bits/s, assuming a propagation speed of 70% of lightspeed, that ratio is:

s/19200 / (3 m / (0.7 c) * 4)
910.82778038194444442

( Frink syntax)

So, well above the point where the termination should be necessary.

Anyway, last night I tried connecting the RS-485 shield ground at both ends (the VFD's RJ-11 style Modbus port includes a ground pin), and lo and behold, communication is suddenly error-free while the motor is running! I might try with the termination resistors as well, but it looks like that's fixed it. :)

Thanks for the suggestions. Now on with writing this Modbus library...
Last edit: 21 Apr 2017 05:27 by screwtop. Reason: Language fix

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24 Apr 2017 10:05 #91944 by screwtop
Out of curiosity, I tried running the RS-485 link with the termination resistors installed as well as the shield being grounded at both ends, and it makes things worse (some messages from the PC get no response, though it doesn't suffer from the OS reconnecting the device). The waveform on the D+ line in particular looks, as we say, pretty munted. I can only assume that the Jaycar USB to RS-485 adapter I have isn't really properly engineered.

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02 May 2017 11:21 #92509 by bkt
Replied by bkt on topic MODBUS control for Delta VFD-M
i think you are in right ....

regards
giorgio

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