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Can't read rs485 ASCII data

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Can't read rs485 ASCII data

Postby sky30766417 » Thu May 17, 2018 4:52 am

I am using the 2000 serial to read the RS485 data. I have follow the instruction to use the serial decoding, but I can't see the data which I send and read. I can only see “?” in the window.

Is there anything which I set is wrong?

Thanks.
Attachments
20180516235121.png
20180516234934.jpg
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Re: Can't read rs485 ASCII data

Postby Martyn » Thu May 17, 2018 5:34 am

The clue is in the message "Sampling rate may be too low"

Try dropping the timebase from 100ms/div to 20ms/div, and increase the number of samples requested from 100kS to 1MS, and see if that helps. You may need to add a trigger so that you catch the packet, try using a Repeat trigger with a rising edge at 4V on Channel A and at 20%

If not then post the psdata file here and we can play with the settings and data.
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Re: Can't read rs485 ASCII data

Postby sky30766417 » Fri May 18, 2018 3:05 pm

Thanks for the help. I have tried your suggestion and it looks better. However, I still can't get the ASCII data.You can check the attachment for the psdata, and also the picture below.

Really appreciate your help.

Thanks
Attachments
RS485.psdata
(142.2 KiB) Downloaded 47 times
20180518100425.png
20180518100414.png
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Re: Can't read rs485 ASCII data

Postby bennog » Sat May 19, 2018 8:19 am

Can you tell us what you are expecting to see in the data

The most logical i can make of it start data 0x8D 0x03 0x7A

pico1.png


output

pico2.png
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Re: Can't read rs485 ASCII data

Postby sky30766417 » Sat May 19, 2018 4:34 pm

Thanks for your reply.

I am sending data from PCB board to Windows side with RS485. I am using the PicoScope to test if there is any noise or the data has been damaged during the transportation, in this way, it will help a lot to our equipments' testing.

The date should included many informations, like the version number of my board, such as "402". I hope that I could see the data in ASCII, just like the documents did with the serial decoding tools. However, I can only see "?" in ASCII, not real number or character. Hex may be a little difficult to read. So I just afraid if there is anything I set is wrong or there is something that I misunderstood. Sorry for my trouble. This is my first time to use it.
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Re: Can't read rs485 ASCII data

Postby bennog » Sun May 20, 2018 8:39 am

RS485 is balanced 2 wire protocol, so if you subtract the signals then you will lose the common mode noise you see in the scope capture.

Som tips to increase the size of the decoding data.

settings.png


1. Disable decoding during capture (start the decoding after you stopped capturing)
2. Make longer snapshots of the data, to achieve this you need to adjust settings in the above picture to the following values
- value 2 needs to be set al 100Ms
- value 1 needs to be as high as is possible for your type of scope, with the limitation that 3 may not fall below 250 kS/s

So if you can do 2 s/div while sample rate is above the 250kS/s then you are fine.

the .psdata file will probably be to high to post over here so you have to do something like dropbox or google drive link.

And pleas give a complete string you are sending over the line, then we know what we are looking for

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