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Unread June 13th, 2018, 10:23 PM   #1
GSNITZ
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Modicon M258 "RX Message Error"

We are running into some issues with serial communications dropping out after a few days. When viewing the PLC hosted web page, we found the following "RX Message Error" count was increasing. Does anyone have information on this error?

Specifically, I'm trying to find out if these errors are indicative of general network issues (noise or frame impacts with messages from other devices on the network) or are specific to network issues with the PLC itself (noise or frame impacts for messages sent to/from the PLC itself).

We have good communications with the PLC for several days at a time, then suddenly the PLC stops responding to requests from the serial master device. If the PLC is power cycled, or the diagnostic counters are reset, comms return to normal (for a few more days).

Thanks for your help!
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Unread March 6th, 2019, 12:33 PM   #2
GSNITZ
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SERIAL_R is an array of SERIAL_R_STRUCT types

Variable Name: i_udiRX_MessagesError
Type: UDINT
Comment: Number of frames received with detected errors (checksum, parity).
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Unread March 6th, 2019, 12:35 PM   #3
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RX Message Error is an evaluation of ALL frames received; includes frames from the master as well as all slave devices on the serial network.
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Unread October 17th, 2019, 05:00 PM   #4
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To close the loop on this, we discovered another device on the network was not adhering to the Modbus standard for a minimum time between frames requirement (also known as silent duration). We were able to log the network and determine that the delay between when the master requested data and this other slave responded was very short (between 1-5ms typically). The M258 was configured for the minimum acceptable interframe delay for the given baud rate (3ms). Any frames which were received before the silent duration timer expired were flagged as RX Message Errors. After a certain number of these errors were logged, the MBS port would stop responding. We were not able to determine why exactly the port would "lock-up" and stop responding; it may have been related to a hardware level bug in the serial port, however this is pure speculation. We resolved the issue by installing a gateway between the serial network and the M258; the gateway still flagged these fast messages as RX Errors, however the gateway itself has never stopped responding. The equipment has been operating with the fix in place for several months at this point in time without any further losses of communications.

Last edited by GSNITZ; October 17th, 2019 at 05:02 PM.
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