RS-485 Comms Cap on BeagleBone – TX works, but RX only after pull-up/down added

General Discussion
1

Hi everyone,

I’m working with a RS-485 Comms Cape on a BeagleBone Black. According to the documentation, it should work out of the box by simply plugging it into the board.

Initially, I was able to transmit data just fine, but I couldn’t receive anything.

After some troubleshooting, I added an external pull-up on the B line and a pull-down on the A line, and then everything started working correctly (TX and RX both functional). This seems like the classic RS-485 fail-safe biasing setup.

My question is:
• Shouldn’t the cape already handle biasing internally?
• Is there a way to make it work without the external pull-up/down resistors?
• Has anyone experienced the same issue or has any idea why RX wouldn’t work without biasing?

Any insight is appreciated!

Thanks!

2

It happened for me but for esp32. External pullups solved the issue.

I think internal pullups are not able to handle the burst current of rs485 (just a guess).

3

yes it could be but i don’t know why pepole working with it wihout adding anything, i tried with two caps and get the same results i feel like im missing something, while reading the documentation of the tranceiver used in to it they are saying that (Resistance biasing are required) and in electrical schematic of the comms cap they are not used so i think they should be used, and as i understand they could not make it on the comms cap because the resistance vary depending on the circuit you work with