Strange issue with RS485

[ajb]

There are plenty of app notes out there you can read that show how RS485 networks should be wired, but going on about this isn't going to help the OP, because there is no indication that the RS485 link is the problem.  In fact, the OP's scope screenshots suggest that the RS485 data is coming through just fine, but there is some issue with the TTL side of things that is keeping the output of the adapter high. 

[nali]

Lots of puzzling factors and lack of photos or diagrams make it kinda hard to pin anything down exactly. What is the mystery board and "device" for example?

The continuity reading sounds like a distraction, maybe an ESD diode or voltage offset causing dodgy readings. if it was *RXD* shorting that could be a reversed pinout on J2 but not TXD.

OP needs to break things down into logical blocks. I'd start by disconnecting "the device", remove U2 and link the RO & DI pins. That should give local echo between TX & RX and test the integrity of the signal path via all the U1 gates (U1 could be counterfeit, static damaged etc). Check the enable signal on U2(2,3) works when you type. Then replace U2 and maybe use the Aliexpress adapter with another UART & see if you can type between them. Then connect the device, see if it stops working etc etc.

[S. Petrukhin]

There are plenty of app notes out there you can read that show how RS485 networks should be wired, but going on about this isn't going to help the OP, because there is no indication that the RS485 link is the problem.  In fact, the OP's scope screenshots suggest that the RS485 data is coming through just fine, but there is some issue with the TTL side of things that is keeping the output of the adapter high.

TTL can be affected by currents of parasitic loops, so the connection method is also important here.

But, as I pointed out earlier, the most suspicious thing is the automatic switching to transmission along the front of the first bit.

We are waiting for the author to check it out and discuss the essentials so as not to get bored. 

[nardev]

Well, thank you very much for all of you for your kind efforts.

Everything is pretty find with my board and another board but not everything looks to be fine with my "main controller board".

Also, why some of the boards RS485 work and not my board. Well, i have used 4069 IC for hex inverted and the working "RS485 Aliexpress" board uses 74HC04 and that's the only difference i was able to determine.

Also, i got several different RS485-TTL boards from amazon, from different manufacturers and same thing happened.

I have no idea in depth what could be the difference when CMOS Schmitt Trigger on 74HC04 is used vs this one but that's how it is.


I would love to know if it is safe, i would switch the hex on my board and use it like that.

[S. Petrukhin]

Well, thank you very much for all of you for your kind efforts.

Everything is pretty find with my board and another board but not everything looks to be fine with my "main controller board".

Also, why some of the boards RS485 work and not my board. Well, i have used 4069 IC for hex inverted and the working "RS485 Aliexpress" board uses 74HC04 and that's the only difference i was able to determine.

Also, i got several different RS485-TTL boards from amazon, from different manufacturers and same thing happened.

I have no idea in depth what could be the difference when CMOS Schmitt Trigger on 74HC04 is used vs this one but that's how it is.


I would love to know if it is safe, i would switch the hex on my board and use it like that.

Have you read the recommendations and switched to manual transfer mode control?