MODBUS ERROR-RS485 Converter_ Reg

[nanotronicsnss]

Hi,

I am having data transfer problem with Modbus only when I use a specific RS485 Converter .I am using STM32H750VB as my main IC for a fuel dispenser project. I am having a problem with my MODBUS based on the RS485 converter.

i)If i am using the RS485 converter which consists of MAX 13487E, I don't have any problem with MODBUS(no error).

ii)But if I use an RS485 converter with MAX485 IC then I am having CRC error (the data received is correct but the value of CRC Changes) while receiving the response from my IC.

The error code is :0xbfff006b framing error occurred in Modbus ..1073807253 at VISA Serial Read...

Can anyone suggest how to get the correct output in Modbus in both the cases.

Note:

i)I have checked the output with three Software's : Simply mod bus master , Modbus poll and Modscan64.

ii)The baud rates are correct(9600)

iii)A and B of RS485 are connected correctly

iv)RTU Mode is used with 8 databits,1 stop bit and No parity bit

v)The address for request and response are correct.

[Pavel A.]

According to the documentation on MAX485 and 13487E, the latter does automatic direction control without a special input from the MCU.

The former needs a special input for direction control.

Some UARTs of STM32H7 can work in half duplex mode and provide the direction control output to external pins; you can use this mode with MAX485 converters.

Or just keep using MAX 13487E.

[Hoang_Tran]

before I have the same issue, my mistake is supplied 3.3V for MAX485