MODBUS RTU decoding captures different to RS485

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Schorsch
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Joined: Wed Mar 13, 2019 6:46 pm

MODBUS RTU decoding captures different to RS485

Post by Schorsch » Wed Mar 13, 2019 6:56 pm

Hi,
I am using MODBUS RTU 8N1 decoding to monitor the transferred information of two controllers on the RS485 line.

Unfortunately, in the calculated byte stream bytes are missing if I use the MODBUS RTU decoder.

However, if I use in parallel the UART RS485 serial decoder all bytes are correctly recognized (see attached screen capture).

Could you provide a hint what leads to the dropped bytes in the MODBUS RTU serial decoding?
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Gerry
PICO STAFF
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Joined: Mon Aug 11, 2014 11:14 am

Re: MODBUS RTU decoding captures different to RS485

Post by Gerry » Tue Apr 16, 2019 10:26 am

Hi Schorsch ,

This could be due to the inter-frame delay being too small. According to the ModBus RTU Dec 2006 spec, for bit-rates higher than 19200, the minimum inter-frame delay (t3.5) should be 1.75ms. I can't tell from your posted image what the inter-frame delay is in your capture.

Regards,

Gerry
Gerry
Technical Specialist

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