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Serial decoding MODBUS RTU Frame start/end time

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Serial decoding MODBUS RTU Frame start/end time

Postby Vdhproducts67 » Wed Feb 14, 2018 9:45 am

When using the serial MODBUS RTU decode function in Picoscope 6.13.2.3439 Beta I encounter some problems.

1. Why do I have to invert the A-B RS485 signal for proper decoding?

2. How is the start and end frame time calculated. I get an "invalid start" and "invalid end" error. If I calculate the 3.5 chars time -> 1/115200*1*3.5=334.2us. Picoscope only gives a valid when gap is bigger than 2.66ms.

Could you help me out?

I have added a ps datafile.

Kind regards,
Peter van Riessen
Attachments
20180214-0001.psdata
MODBUS RTU Decoding data
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Re: Serial decoding MODBUS RTU Frame start/end time

Postby Vdhproducts67 » Thu Feb 15, 2018 10:19 am

Addition to second question:
The inter-frame delay (t3.5) for bitrates higher than 19200 is 1.750ms(modbus protocol dec 2006). Picoscope's serial decoding is correct.

Would it be possible to make this inter-frame delay adjustable?

Kind regards,
Peter van Riessen
VDH Products
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Re: Serial decoding MODBUS RTU Frame start/end time

Postby Gerry » Tue Feb 20, 2018 5:32 pm

Hi Vdhproducts67,

If by your 1st question you mean "why do I have to reverse my signals going to channels A and B in order to get valid decoded data", then the answer is that you just have to make sure that Channel A is +ve going and Channel B is -ve going because, as you can see in the serial decoding drop down list for the Master (channel selection for the Master on the bus) the listed option is A-B, and a for our decoding of RS-485, what will generate a data value of 1 is a +ve differential of A relative to B.

Regarding your second question/comment, are you saying that for the lower bit rates (down to our limit of 4.8k baud) the inter frame delay is less than 3.5 characters? If not, then I'm not sure that I understand what your asking. If the minimum inter-frame delay is correct according to the specification, then as long as the correct increased inter-frame delay from the bit-rates lower than 19200 baud still provide a valid decode, why would you want to adjust the inter-frame delay for bit-rates higher than 19200 baud (which would invalidate the specification)?

Regards,

Gerry
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Gerry
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