Open henriterhofte opened 3 years ago
Modbus over RS-485 is specified for twisted pair cabling with characteristic impedance of 120 Ohm.
In practice it is often installed with Ethernet-grade unshielded or shielded twisted pair, which has a differential impedance of 100 Ohm.
When resistors are mounted at both far ends of the bus (between the A and B wires), each matching the differential impedance, the reflections of signals are minimized.
However, for such short cable runs, and the system speed of 9600 bps, reflections are unlikely to be a problem in your case. Also cable resistance is not likely a problem.
The SmartEVSE controller has no built-in resistors. Not sure about sensorbox-2.
Perhaps there is a strong source of interference nearby (e.g. switching power supply)? In that case twisted pair cable may help, make sure to use a single pair of twisted wires to carry A and B signals. Shielded cable, with the shield connected to protective ground may also help.
Perhaps a more likely source is a bad contact or occasional short circuit (with a single small copper strand)?
And yes, signal ground should be connected in parallel with the A and B lines in Modbus installations, although this is often omitted and the installation still works (be it by chance, not by design).
I have the same issue here. My modbus setup/devices:
If I just interconnect the devices, everything works. I can even set the SmartEVSE to SMART mode and read the values from the Eastron.
But, once I start polling the Eastron for data over the PE11, I get the same 'NO SERIAL COM' error. Code I use to poll the Eastron: https://github.com/martijndierckx/sdm630-modbus-tcp
Any ideas?
I have the SmartEVSE running fw 2.20 connected to Sensorbox 2. New this weekend: (finally) also connected to the Eastron SDM630-Modbus V2 meter.
Functionally, everything works (charching, kWh counting, smart reaction to other loads to prevent MAINS overload) but I also get this error regularly: ERROR NO SERIAL COM CHECK WIRING.
I used very short (10-15cm) non-twisted older phone cables with 4 non-stranded wire cores, hooked op 12V, GND, A and B for the Sensorbox-2 and hooked up A B and GND for the Eastrom
Given these errors (which do not seem to hinder the functionality):
1) Should I use twisted pair wiring even for these short RS-485 connectrions?
2) I not only connected not only A and B between SmartEVSE and Eastron, but also GND; is this correct?
3) Should I use 120 Ω termination resistors that seems to be recommended for RS-485 bus topology wiring?