RS485 (max33072easa+)

[jensvanhoof]

Good afternoon,

I'm working on a Modbus RTU (RS-485) master project (115200 baud) with the ESP32, currently in the breadboard phase.
So far, it works flawlessly with one of these boards: https://arduinoinfo.mywikis.net/wiki/RS485-Modules

Since that board uses the MAX485, which is on 5V, I'm using this bi-directional level convertor for the RO/RE/DE/DI lines to get to 3.3V: https://www.sparkfun.com/products/12009

In that working setup I have RE and DE connected together to a GPIO pin, and RO-DI each a separate pin. This works great.

For the next step I would like to integrate the RS485 circuitry on my PCB design, and while I'm at it ommit the level converters and change to a RS485 transceiver on 3.3V. For this I got myself a max33072easa+ chip (https://be.farnell.com/maxim-integrated ... st=3499914).

On the breadboard, I've added 3 10k pull-up resistors for RO and DE and tied POL to ground. I've also added a pull-up resistor (20k) to A and a pull-down resistor (20k) to B. Finally, I've put a 120ohm termination resistor between A and B since my master is at the end of the chain. Additionally I have connected GND on the twisted pair cable between A and B (I had some problems before I did this - connecting the GND seems to make the communication 100% reliable).

The datasheet of the MAX33072 says that the receiver is always enabled, so there's no RE pin on this chip.

Unfortunately I cannot get it to work yet. I clearly see an attempt to communicate with my 2 oscilloscope channels on lines A and B but the slave device indicates a problem with the communication, so no responses are being send back either.
I'm certain that I didn't swap A and B, since I know that my slave device replies differently to that. Anyway, I have set POL to high for a moment just to make sure that it wasn't that.

There is of course still the option that I'm messing something up on the breadboard although I have checked and doublechecked and triplechecked. I'l quadruplecheck in a moment.

Meanwhile, if anybody would have an idea what the problem could be I'd be super happy to hear about it!

Thanks a lot,
Jens