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V2.05 subject to changes | Webdyn © by Flexitron Group
If several modbus RTU devices are connected, “serial” or “daisy chain” wiring is required. The cable
arrives at a modbus module and exits towards the next one.
To guarantee proper data bus operation, an RS485 bus must feature a 120 Ohm terminator at each end.
The WebdynSunPM concentrator can be at the end of the RS485 communication bus or in the middle.
As the concentrator has a 120 Ohms resistor, it made need to be enabled depending on the concentrator
position on the bus. (See wiring below)
There are 3 separate considerations for the choice of cable type:
• On installations requiring short lengths with no electric interference, plan on using a 2 pair
6/10 rigid screened cable.
• On larger installations of which the cable length is less than 500 m, plan on a 2 pair 8/10 rigid
screened cable.
• When the cable distance is more than 500 m, and even more so if there is electric interference,
plan for a shielded 2 pair 0.34 mm2 cable.
The maximum RS485 bus length is 1000 metres. (for 19200 bauds max). If the length is long,
remember to reduce the device transmission speeds if communication is difcult.
Recommendations for RS485/RS422 BUS wiring:
• The modules must be connected one after the other.
• Star connections are prohibited.
• The cables must either be screened or shielded, twisted pair per pair (see above: “cable type
for RS485 bus connection”).
• The cable screen or shielding must be connected to the concentrator box earth and not to the
0 V (only connect one end of the screen).
• Avoid any return trips in the same cable.
Concentrator side RS485 wiring:
• Strip the RS485 communication cable sheath over about 4 cm.
• Shorten the shielding down to the cable sheath.
• Strip the wires over about 6 mm.
• Connect the conductors to the terminal block marked “Serial” following the assignments in
your RS485 communication bus.
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