Modbus over RS485 (RJ45 to DB9) cable lenghth and pinout

Hi All

I am about to take delivery of my SunSynk inverter which does not seem to have any binding support (using the new SunSynk logger).

So it appears that I will have to revert to MODBUS over RS485, so I have a few questions which I have not been able to answer. Google seems to produce an endless collection of possible answers - so hoping for so hoping someone here has run into and solved this?

  • What is the maximum cable length I can use. The standard for RS485 says 1.2 km (4000ft). But not sure if that is valid in this case. My cable length between inverter and openHAB will be about 30m (all indoors)
  • Would CAT5/CAT6 cable work for this?
  • What is the pinout of the required cable from RJ45 (inverter) to DB9-female (openHAB serial port)
  • Any other suggestions/recommendations to avoid issues

Any help would be appreciated.

Pinout depends on the hardware, but maybe this is correct:

CAT5/CAT6 cable should be sufficient for the job, keep in mind, that RS485 is a bus (RS232 is point to point), so maybe you’ll have to terminate the ends (but it’s possible that resistors are integrated).
Max length of cable is really that long, it’s all about frequency and maximum load, not critical at all.

1 Like

So on further research I found the following:

So will probably go with:

And hopefully the wiring in the first link works.

I use the ethernet version of this device: Industrial serial server, RS232 to RJ45 Ethernet, TCP/IP to serial, rail-mount support, with POE function(optional) | RS232 TO ETH (B) | RS232 TO POE ETH (B)

I simply used an audio cable, only needed 2 wires.