Modbus CRC error

[sx3]

Hello,

I have been running my HVAC system with LM for while now and it works like charm.

But now I have connected my heatpump to the modbus circuit with an interface, and now it generates alot of CRC and time out errors for me.
Everything is set on 19200 baud, the HVAC runs on ID 1, and the HP on ID 8.
Since I've connected the HP interface, telegrams aren't always comming through, not just polling but also commans that I send to the HVAC system.

I'm aware that it's probably the HP interface that is poorly designed, but how can I dig deeper into this to find out the real problem?
The interface is developed by a small company, so if I just figure out whats wrong they'll probably help me with a new firmware.

[sx3]

What is the response timeout set on, for the LM? Thinking maybe the timeout is greater on the heatpump interface than the LM, causing theese types of problem?

[admin]

What is the total cable length? Are you using 2 wire connection or 3 wires with GND? For me it seems more of a physical connection problem.

[sx3]

Total length is around 25 meter, I have from LM about 10 meter 2x2x0,8 KNX BUS-cable to a junction box. From this box I have splitted the bus in 2 direction, each direction is about 7-8 meters, one goes to floor 3, and the other goes to floor 1 (LM is placed on floor 2)

Slave #1 is using 3 wires with GND (GND is connected in both LM and RTU slave)
Slave #2 is 2 wires D+ and D- only.

It is not connected as daisy-chain, because I split the Bus-cable in the junction box and send it in different directions.

I set new MB-ID to slave #1, it generated much less invalid CRC but instead more "wrong slave", see attached.
Also I have removed all mapping to Slave #2, slave to is only connected to the RS485 without having anything mapped to it, and starts making trouble for slave #1 as seen on this log.

[sx3]

additional question to above, is there any 120 ohm resistor inside LM?

[admin]

Usually, no, only LM5 has configurable terminating resistor. You can try installing one on each line end, it might help.

[sx3]

Alright, maybe I should try to daisy-chain aswell, and not star-topology like now.

[sx3]

What is the time-out set at when using modbus-mapper? Is there anyway to adjust the timeout on LM?

[admin]

There's an optional "timeout" profile field that specifies maximum ModBus operation time for the given device. Default timeout is 0.5 seconds for RTU and 3 seconds for TCP.

[sx3]

Thank you, could you give me an example how I change the timeout?
Do I adjust it in the JSON file for the profile?

[admin]

Yes, in the profile. Keep in mind that you will have to re-create device and mapping from scratch when updating the profile.

Code:

{
  "manufacturer": "Some Company",
  "description": "Some product",
  "timeout": 5,
  "mapping": [...]
}

[Angeles]

Hi:
I have the same problem. I connect with modbus devices through usb port (converter RS485). I only use two cables, without GND. Is this the problem? is it necessary GND cable? The metering devices connect with BMS only with two cables.
Thanks.

[Daniel]

Hi
Modbus allows only daisy-chain so your problem could be generated by your topology .
BR