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2204A Serial Decoding Modbus RTU
2204A Serial Decoding Modbus RTU
Trying to serial decode Modbus RTU frames with 19.2kbd, 8 data, no parity, 1 stop bit using a 2204A with A-B differential probing. Setup as +/-5V and DC coupling for Ch A and B, collection time is 10ms/div, 10MS. Slow sampling collection time is 100ms/div. Currently the setup is all on a desk in a noise free environment. If all works as expected, I need to take it out to industrial system to figure out frames being sent/received with respect to a black box system.
Presently sending master request every 500ms to read 1 coil address and getting correct slave response in my test program. The 2204A is able to identify slave addresses in the beginning of the decode but appears incorrect from function code through to the end. Capture shows valid start and end. The rest of the decode CRC, gap and overall data validity are incorrect.
Any advice how this modbus RTU decode could be set up to show the correct data.
Presently sending master request every 500ms to read 1 coil address and getting correct slave response in my test program. The 2204A is able to identify slave addresses in the beginning of the decode but appears incorrect from function code through to the end. Capture shows valid start and end. The rest of the decode CRC, gap and overall data validity are incorrect.
Any advice how this modbus RTU decode could be set up to show the correct data.
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- Modbus RTU decodes 20210320-0001.psdata
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Re: 2204A Serial Decoding Modbus RTU
your sample rate is far to low
for 19200 baud you need at least 5 times that as sample rate so 80kS or 100kS /sec should do it.
Benno
for 19200 baud you need at least 5 times that as sample rate so 80kS or 100kS /sec should do it.
Benno
Re: 2204A Serial Decoding Modbus RTU
Thank you for your post
I believe the decoding problem with the modbus decoder is lack of configurable number of stop bits. A lot of modbus RTU slaves I use seem to have 1 stop bit with no parity. The modbus standard appears to require 2 stop bits when no parity is used. Any chance picoscope modbus RTU decoder can support configurable 1 or 2 stop bits?
I have tested modbus polls with even parity and 1 stop bit and the decoder appears to work okay with this
I believe the decoding problem with the modbus decoder is lack of configurable number of stop bits. A lot of modbus RTU slaves I use seem to have 1 stop bit with no parity. The modbus standard appears to require 2 stop bits when no parity is used. Any chance picoscope modbus RTU decoder can support configurable 1 or 2 stop bits?
I have tested modbus polls with even parity and 1 stop bit and the decoder appears to work okay with this