Meaning of flush error received from mod-bus slave

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Discussion Overview

The discussion revolves around understanding the 'FLUSH' error encountered in Modbus communication between a PC (acting as a master) and a compressor (acting as a slave). Participants explore potential causes of the error, particularly in the context of interference from burst signals and the physical setup of the communication system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the setup involving a 1000ft cable and burst signals causing errors, specifically a 'FLUSH' error.
  • Another participant speculates that the error might be related to a buffer being full and subsequently flushed.
  • Concerns are raised about the absence of 'FLUSH' in Modbus exception codes, prompting questions about the physical layer and noise coupling methods.
  • Some participants suggest that searching online may yield more information than forum discussions, referencing external documentation about Modbus.
  • There is a discussion about whether the 'FLUSH' error is specific to the libmodbus library and if the error description would differ without it.
  • One participant expresses uncertainty about how to investigate the cause of the error and seeks guidance on potential investigative steps.
  • Another participant emphasizes the need for a deeper understanding of the hardware and software involved to differentiate between research anomalies and equipment faults.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the meaning of the 'FLUSH' error or its causes. Multiple competing views and hypotheses are presented, and the discussion remains unresolved regarding the best approach to investigate and remedy the error.

Contextual Notes

Limitations include the lack of specific definitions for the 'FLUSH' error in standard Modbus documentation and the potential influence of interference on data communication errors, which may lead to buffer overflows.

Who May Find This Useful

This discussion may be useful for individuals involved in Modbus communication, particularly those dealing with error handling, physical layer configurations, and troubleshooting in industrial automation contexts.

Nikhil N
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I have a PC(as master) and a compressor(as slave) in modbus communication. I am using 1000ft cable and I want to analyze the interference effect on the data communication error. I am coupling burst signals produced by the relay switching and I could observe the level of burst to be ~50V. When I am coupling this bursts I am getting error messages from slave. The type of error is 'FLUSH'. I need to know what is mean by the flush error
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Wow. I can only guess. Could it be that a buffer got full and the contents were flushed before resuming?

I think the only one who can tell you for sure is the software's author.
 
I don't see anything about Flush in the Exception Codes at the wikipedia page about Modbus:

https://en.wikipedia.org/wiki/Modbus

What is your physical layer? Doubly-terminated RS-485? How are you coupling the noise into the cable? What cable are you using, and what value of termination resistors are you using?
 
But good old Google produced this when I searched for "modbus flush"

http://libmodbus.org/docs/v3.0.6/modbus_flush.html

Pretty often, a simple Internet search produces more than posting a question to an online forum.
 
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berkeman said:
I don't see anything about Flush in the Exception Codes at the wikipedia page about Modbus:

https://en.wikipedia.org/wiki/Modbus

What is your physical layer? Doubly-terminated RS-485? How are you coupling the noise into the cable? What cable are you using, and what value of termination resistors are you using?
I already saw this, but this function is defined if developers are using libmodbus library right? Will that error description be same even if libmodbus library being not used? I didnt see such error definition in modbus.org
 
anorlunda said:
Wow. I can only guess. Could it be that a buffer got full and the contents were flushed before resuming?

I think the only one who can tell you for sure is the software's author.
So what do you think, can I correct this error by modifying via software means? I mean with the help of developers?
 
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Nikhil N said:
So what do you think, can I correct this error by modifying via software means? I mean with the help of developers?

I think your evidence is too weak to guess at a remedy. You need to investigate more to find the cause.

Have you contacted the manufacturer?
 
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anorlunda said:
I think your evidence is too weak to guess at a remedy. You need to investigate more to find the cause.

Have you contacted the manufacturer?
I am new to this area, what kind of investigation you are suggesting to find the cause? I am not getting the idea
 
You need to find the people who wrote that "FLUSH" error message and ask them what kinds of things could cause that.

Nikhil N said:
I am using 1000ft cable and I want to analyze the interference effect on the data communication error. I am coupling burst signals produced by the relay switching and I could observe the level of burst to be ~50V.
That sounds pretty ambitious for a newcomer.

It could be "the interference effect on the data communication" is to cause FLUSH errors. :smile:

Seriously, the interference could cause data communication errors which means retries, which causes buffer overflow, which triggers FLUSH. To accomplish the research that you described implies pushing things to the margins. I would think that you need to know everything about the hardware and software, even to the extent of writing all the software yourself. Otherwise, how will you tell the difference between anomalies your research is seeking and faulty equipment?
 
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