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RS485 troubleshooting

Mr. Petkov

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Hello Dear Support Engineers,

We have troubles with our RS485 network at a client site.

This network was never made stable (yet).

We have 3 Jazz R31, 1 V1040 and 3 MCX06D (Danfoss) in a RS485 network and we run Modbus RTU over it.

We have tried 9600 and 19200 bps, 8N1, with similar success.

Signal grounding is made as described in Unitronics document "Unitronics devices and RS485.pdf".

The network cable we used is twisted pair, LiYCY, 2x0.75mm + shielding.

Its characteristic impedance is expected to be 74 Ohms.

We encounter a data loss at our single master device, the V1040 controller (Node 1).

Our controller (Node 1) registers a loss of connection mostly to Node 3 and Node 7. This varies with different termination resistors we installed.

We tried various termination resistor values but none is good enough to solve the data loss.

We have tried to use:

A) no termination at all,

B) termination only at Node 1 and 7 (its the 150 Ohms of Unitronics devices),

C) term. at Node 1 and 7 + a pair of additional external 150 Ohms to match the cable impedance of 75 Ohm.

None works nice enough.

What makes connection more stable is switching termination at Node 1,3,6 and 7 to ON.

We hope, that with all this information you may advice us, what have we did wrong, how to workaround.

Kind regards,


post-2575-011594600 1308435042_thumb.jpg

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Just wish to complete the thread.

I finally paid attention, and read carefully the Unitronics document "Unitronics devices and RS485.pdf".

I noticed the recommended cable for RS485 as directed in the document.

Found several serious disadvantages of the braided LiYCY twisted pair compared to the recommended cable.

1) Braided LiYCY is a shield, but covering 80% the data wires, and not 100%.

2) The PVC isolation of LiYCY data wires means ~3.2 times higher capacitance at data wires, than the optimum 40pF/m. The proper is polyethylene, not PVC.

3) Amplitude/frequency chart is rather worse.

Some people i talked to, speak for successfully operated RS485 network in such a noisy electrical environment as our client's site.

These people explained, that the cable used was a S/STP or a S/FTP. I am yet not ready to comment that.

Its for sure, cables like S/STP and S/FTP have a DC resistance of about 20 Ohm per 1000meters, and recommended cable by Unitronics has about 8 Ohms/1000 meters.

So please, do not hesitate to comment, opinions welcome.

I just do not know what are the limits of RS485 hardware (neither at MJ20-RS, not the new machine V1040).

Kind regards,


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