Modbus RS-485 RTU gives Checksum error while trying to communicate. If I select Odd Parity, the error changes to insufficient bytes received. How can I remove these errors?

 

how is the modbus interface of the master and slave(s) configured? They must be the same.
Sometimes, because of a bad interface, bits get damages in the communication, don't forget to terminate the RS-485 lines with 120ohm between the A and B line. And use proper shielded and impedance corrected cabling, like the 120ohm impedance corrected cable, specifically designed for RS-485 communication. The Lapp Unitronic BUS LD. Can be ordered per meter at Conrad https://www.conrad.nl/nl/p/lapp-217...8jnR4C0g9l6SJ33RoCjugQAvD_BwE#productTechData

 

A Modbus RTU checksum error is not caused by parity. The Modbus checksum is at a higher level. The parity bit in each character is a sort of checksum, but on the scope of each single character in the Modbus frame. The Modbus checksum is performed after successful reception (i.e. no parity, framing, overrun, etc. errors for any characters) of all characters in the frame, and is performed on the entire frame.

Checksum errors are typically caused by some sort of additional modification to the signal. It may be from noise, improper grounding (i.e. 0V reference), another device transmitting at the same time, or even a timing issue where the master is still driving the bus when the slave starts responding.

 

Hi Sir,

I'm trying to read data from a meter connected to a transformer. I have read data from different devices before, but on this device I constantly receive 'checksum errors' and 'insufficient bytes received' errors. Can you be more descriptive?

 

Unfortunately we cannot be more descriptive with our responses until you are more descriptive in explaining the problem. We need to know all equipment involved (vendor and models), what Modbus software is being used (if any), what USB to RS-485 adapters are being used (if any), the exact steps you are performing, what troubleshooting steps you've already tried, etc.

In general, with any Modbus RTU communication issues, the first thing to check is your wiring (try using 3 wires for +, -, and 0V ground and try swapping your + and - wires at one end since vendors are inconsistent in "A" and "B" labeling for RS-485 devices). Next confirm your communication settings (baud rate, parity, stop bits, address).

Please provide additional details if you would like additional assistance.

 

I am trying to communicate with EMH brand LZQJ-XC model meter RS-485 via Modbus RTU. I'm using a US-101-485 model converter to convert from RS-485 to USB.

 

Thank you for the additional details. Can you please describe or provide a diagram of how you have your EMH meter wired to your RS-485 to USB converter? What happens if you swap your + and - wires at one end?

Can you tell us what Modbus software you're using on your PC. Please also include a description or screenshots of the connection settings and the registers you're trying to access.

 

Another follow-up question. What module are you using with the EMH meter to provide RS-485 Modbus RTU? Are you using the VARIOMOD-XC from EMH or another module, such as the LKM614 from REDZ?

 

I use VARIOMOD-XC from EMH. But I think Modbus is not directly compatible with communication.

 

I think that might be your issue. I can't find any documentation or indication online that the LZQJ-XC or VARIOMOD-XC support the Modbus protocol.

If you need Modbus with the LZQJ-XC, it seems you may need to use the LKM614 from REDZ instead of the VARIOMOD-XC. However, I would recommend contacting EMH first to confirm.