UART MAX485 Module

[bnjroos]

Good evening,

I am currently trying to communicate with a modbus slave in RS485. For that I am using a MAX485 UART TTL to RS485 module for Arduino (http://yourduino.com/sunshop//index.php?l=product_detail&p=323).

I have wired as followed:
WiPy -> MAX485 Module
P3 -> DI
P4 -> RO
P7 -> DE/RE jumped
5V -> VCC
GND -> GND

I use a library adapted from minimalmodbus for modbus, however when I try to communicate with my slave, the response is always a chain of b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\ ...

Any idea where that would be coming from ?

from machine import UART
import utime
import ModBus
uart = UART(1, baudrate=9600)
uart.init(9600, bits=8, parity=None, stop=1,pins=('P3','P4','P7',None))
instrument = ModBus.Instrument(uart,0x01,mode=ModBus.MODE_RTU)
print(instrument)
v = instrument.read_registers(2,2)
print(v)

I have tested the modbus library with a uart mockup with success, so i think it might be a hardware issue.

Thank you in advance.

[Jurassic Pork]

hello,
be carefull :
The GPIO pins of the pycom modules are NOT 5V tolerant, connecting them to voltages higher than 3.3V might cause irreparable damage to the board.

your rs485 module uses a MAX485 which is a 5V chip.
for the RO pin the specification voltage is :
Receiver Output Voltage :
RO -0.5 to (VCC+0.5) V
may be you must use a module with a 3.3V chip as the MAX3485 and powers it with 3.3V .
example : SparkFun Transceiver Breakout - RS-485

Fully equipped with SP3485 RS-485 transceiver and supporting components
Operates from a single +3.3V supply
May be i am wrong. If someone can confirm what i have said.

[danielm]

I successfully used MAX3485 with LoPy with my code acting as ModBus slave with range of input registers.

[bnjroos]

@Jurassic-Pork said in UART MAX485 Module:

hello,
be carefull :
The GPIO pins of the pycom modules are NOT 5V tolerant, connecting them to voltages higher than 3.3V might cause irreparable damage to the board.

your rs485 module uses a MAX485 which is a 5V chip.
for the RO pin the specification voltage is :
Receiver Output Voltage :
RO -0.5 to (VCC+0.5) V
may be you must use a module with a 3.3V chip as the MAX3485 and powers it with 3.3V .
example : SparkFun Transceiver Breakout - RS-485

Fully equipped with SP3485 RS-485 transceiver and supporting components
Operates from a single +3.3V supply
May be i am wrong. If someone can confirm what i have said.

I just had a look at the datasheet and I think you're right, I am gonna order a SP3485 before going further.

[Jurassic Pork]

hello,
@bnjroos
are you using an expansion board with your WiPy module ?
The expansion board connects P0=RX0, P1=TX0, P4=SD-CMD, P8=SD-DAT0, P9=LED, P10=button, P16=Vbat, P19=CTS, P20=RTS, P23=SD-CLK.
If you use an expansion board, maybe you must use another pin than the P4 pin for RO.
@danielm how have you connected your Lopy to your MAX3485 ?

[bnjroos]

@danielm said in UART MAX485 Module:

I successfully used MAX3485 with LoPy with my code acting as ModBus slave with range of input registers.
Thank you or the info.
Good to know, me I am trying to make it act as a master. I will continue my tries once I receive my SP3485.

Yes I am using the expansion board and the wipy version 2.0. Actually it comes from the documentation quickref: # this uses the UART_1 default pins for TXD and RXD (P3 and P4)
from pycom

@danielm I am also interested on how you wired your setup, thank you in advance :p

[urbanzz]

@danielm Hello! Could you share your modbus library?Can it acts as Modbus Master? (in oder to get data from another slaves)

[danielm]

@urbanzz
There is no library, I only wrote simple ModBus client routine which waits for a specific type of command and sends back data from internal register including CRC. If you need ModBus master functionality for commercial purposes, I can help you with that.

[ledbelly2142]

@danielm
I am also interested in this.

[danielm]

@ledbelly2142
Please send me a PM with details.

[vicu]

Hello,
any update on this?
I have some old modbus plc lying around and i'm trying to use my lopy to communicate as a modbus master with a SP3485 breakout board...but i cannot find a decent modbus library for micropython

[bnjroos]

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[bnjroos]

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[bnjroos]

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