RS485 automatic RX/TX rpi pico

[roshV]

Hi,

I want to build a PCB with RPI pico that can read devices over RS485.
I found the official schematic of RS485-TTL converter of M5.

Here in the official schematic (https://docs.m5stack.com/en/unit/rs485), the TX from microcontroller is connected to Enable pins, DE and RE. The DI of the chip is connected to ground. How can this achieve bi-directional communication?

 

[barry]

This looks pretty goofy to me. I'm guessing the way it's supposed to work is like this:

When TX is high, the enables are low, enabling the receiver which is a high impedance input, so A is pulled high and B is pulled low, if not driven by the remote transmitter.
When TX is low, the transmitter is enabled, and since DI is pulled low, A is low and B is high.

This just looks a little too clever for my taste. I guess it saves an enable line.

 

[roshV]

Thanks for explanation.

I tested M5, it does work well.

I was confused how come I am able to send serial data out from microcontroller to an RS485 device with DI pin not connected to Tx of microcontroller.