Modbus errors

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Thread Starter

Piet

I'm trying to connect two climate devices that's using modbus and jbus communication, with a KNX system using the gateway: IBOX-KNX-MBRTU-A of manufacture Intesis software.

The climate devices are:
- ventilation-unit with a modbus interface and,
- chiller with a jbus interface.

For monitoring the modbus messages, I use the software tool: modbus poll.
The monitoring of the jbus messages doesn't show any errors. Only during monitoring the modbus messages the following errors are shown:

- Bytes missing error
- Read error: Framing error
- CRC error

What mean these errors and how can i solve these problems?

Hopefully someone can help me.
 
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Lynn August Linse

Framing errors is a bad/unusual thing, something you should never see in a correctly designed system - look it up on the Internet. What it usually means:

#1) a baud rate mismatch - I don't think that's your problem since the Modbus device won't answer if the master sends 'garbage' which is what incorrect baud rate data looks like.

#2) improper RS-485 grounding, which means bits/bytes are being misinterpreted because of no ground wire and a ground potential approaching the over-voltage levels.

#3) lack of line bias on RS-485, which means when the master stops sending & before the slave starts responding, the RS-485 lines float which means you likely might see some garbage 0xFF or 0xFE bytes at the start of the response.

#4) just plain too much line-noise. You might need to double check your shielding.

#5) a slave which starts replying BEFORE the master is ready to receive, therefore part or all of the first byte is lost ... leading to a possible framing error during the first received byte and also missing data, which leads to CRC error.

- Lynn
 
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Patrick Lansdorf

Not much more to add that Lynn did not cover. The termination resistors are usually about 120 Ohms and normally I use 400-500 Ohms on the Bias resistors.

Just my 2 cents

//Patrick L
 
@Lynn

In the modbus device, I see the ground wire is connected to a contact. But on the gateway, where the modbus cable is connected, isn't a contact for the ground wire.

I'm not sure if the problem will be the ground wire, because the JBus device communicates with no errors while this earth wire also isn't connected to the gateway.

How can I solve this problem?

(Thanks for the fast reply)
 
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Intesis Support

> How can I solve this problem?

Your issue can be due to multiple causes (e.g. not matching parity between master and slave). we recommend you monitoring the Modbus communication by using the tools embedded in the own LinkBoxEIB software instead of using external tools. this way you can debug and log the communication activity in a file and send it to us along with your project configuration. this will allow us to provide you full support on this issue. please send it to [email protected].

Intesis Software
 
Hello

You can't use Modbus Poll to monitor a serial line. Modbus Poll is the master and you must disconnect the existing master before using it.

If the situation is that you actually use Modbus Poll as master the problem could be that your serial converter or slave device echoes back the request. Then the response consist of a request followed by the real response. In modbus poll you can select "remove echoes" in the advanced settings. (Located in the connection dialog)

Regards
Brian
www.modbustools.com
 
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