Modbus RTU slave not working

[d-solis-tovar]

Hi there,

I am using an esp32 with Micropython "ESP32_GENERIC-20250415-v1.25.0.bin" and a MAX485 converter to communicate via Modbus using this library: https://github.com/brainelectronics/micropython-modbus

When the esp32 acts as master, it works flawless (tested using an USB-to-RS485 converter and my computer acting as slave).

However, when the esp32 acts as slave, the master (tested both on computer and on other esp32) raises "No data received from slave" error.

I tried exactly the same examples as in https://github.com/brainelectronics/micropython-modbus/blob/develop/examples/rtu_client_example.py and https://github.com/brainelectronics/micropython-modbus/blob/develop/examples/rtu_host_example.py, but it didn't work.

I also tried to add some resistors between A and B, and pullup and pulldown resistors as well. Furthermore, I add a delay before initializing the UART interface in the slave esp32.

Is this problem familiar to anyone? I don’t know what more to try.

Thanks in advance!

[Lobo-T]

I think you forgot to actually link to the library.

[d-solis-tovar]

Edited. Sorry for the inconvenience. Thank you!

[Lobo-T]

What does your hardware look like?
I notice that those examples do not use the DE/RE pins.

[Lobo-T]

Also which line is called A and B is often confusing.
What most RS485 driver ICs call A and B is opposite of what most protocols call A and B.
Try swapping the wires.

[mathiasflick]

I recently built a Modbus to MQTT converter using the same library and a Pi Pico W.
From my own struggling with the basic setup (though the library works like a charm) I would guess your problems are about the wiring ...
As a first hint: You should start with ESP to ESP communication - leave out the RS485 drivers; simply connect TX to RX, RX to TX and GND to GND for the two ESPs, one configured as the master, the other as a client (this actually worked out-of-the-box with above examples for two Pi Picos).
If this works insert the RS485 drivers: For each ESP TX to TX(RS485), RX to RX(RS484) and GND to GND(RS485); A to A and B to B for the both RS485 converters.

Good luck and greetings from Cologne, Mathias

Edit: Corrected the 484 to 485 (Once wrong all copies will be wrong ... ;-))

[d-solis-tovar]

Thank you @Lobo-T and @mathiasflick for your help.

Following @mathiasflick advice, I first tried ESP to ESP communication without RS485 drivers, and it works like a charm.

However, when I connect RS485 drivers, communication stops working and I got an OSError: no data received from slave.

My wiring is the following:

ESP32_1 TX -> MAX485_1 DI
ESP32_1 RX -> MAX485_1 RO
ESP32_1 3.3V -> MAX485_1 VCC
ESP32_1 GND -> MAX485_1 GND

MAX485_1 A -> MAX485_2 A
MAX485_1 B -> MAX485_2 B

ESP32_2 TX -> MAX485_2 DI
ESP32_2 RX -> MAX485_2 RO
ESP32_2 3.3V -> MAX485_2 VCC
ESP32_2 GND -> MAX485_2 GND

ESP32_1 GND -> ESP32_2 GND (I also tried without this connection)

I also tried three RS485 drivers, in case one was faulty.

[Lobo-T]

You need to use the RE and DE pins too.
The MAX485 will only transmit when the Driver Enable is high.
Receive enable is active low, so the normal way is to connect both to the same microcontroller pin. And drive it low when listening and high when transmitting.

[mathiasflick]

Now that we know which type of RS485 converter you are using @Lobo-T is absolutely right (I myself use very straightforward modules without the control pin, i.e. RX, TX, GND only ...).
But you are making good progress: just tie the two pins mentioned by @Lobo-T together and connect them to a free output pin.
Don't forget to enable this control pin in the client as well as in the master example code! It is part of the parameter list where the basic constructor is established (it is named ctrl_pin) ...

Excerpt from client example:

client = ModbusRTU(
addr=slave_addr, # address on bus
pins=rtu_pins, # given as tuple (TX, RX)
baudrate=baudrate, # optional, default 9600
# data_bits=8, # optional, default 8
# stop_bits=1, # optional, default 1
# parity=None, # optional, default None
ctrl_pin="Your pin number here", # optional, control DE/RE
uart_id=uart_id # optional, default 1, see port specific docs
)

[d-solis-tovar]

It finally worked! I have already tested this configuration but I think it was an issue with using GPIO12 as control pin, which was causing problems at boot. Thank you so much @Lobo-T and @mathiasflick