T300 Modbus Master operation

[Windsor]

I have a T300 RTU running firmware version 2.8.4 which is talking to a PM8240 meter using Modbus over an RS485 link. The RTU has been configured using Easergy Builder V1.7.9 with the following Modbus master parameters for the meter device:

Offline period : 10000 ms

Retries after timeout : 5

Reties after error : 3

What I am finding (by examining the Modbus channel trace display in the web server) is that occasionally one of the Modbus replies from the meter has the top bit of the function code byte set to indicate an error and contains an exception code of 2.

The trace then shows no further communications to the device until approx 10 seconds have passed (the offline period presumably). 

However the trace does not seem to show the retries that should have been performed prior to the backoff period. Is this expected behaviour or am I misunderstanding something?

[TomBerry]

In this case the exception code 2 means invalid address.

The firmware assumes that this is due to a wrong configuration, so it makes no sense to retry.

These are the list of exceptions codes that behave like this.

• ILLEGAL_FUNCTION                0x01  
• ILLEGAL_DATA_ADDRESS        0x02
• ILLEGAL_DATA_VALUE              0x03
• NEGATIVE_ACKNOWLEDGE      0x07

[Windsor]

Hi @TomBerry,

After looking at the exported Modbus channel trace (file attached) again it seems to me that at least 2 reties are taking place before the response containing the exception code is received but there is nothing in the trace to suggest why the retries are occurring.

If you have the time to take a quick look it would be much appreciated (the lines of interest in the file have been annotated starting with //)