uart communication issue with parity bit disabled

[creatorwonny]

Hi

I already posted a question in the following link about issue in modbus communication that slave received one packet as several packets when master sends many data to the slave. The solution I found was setting the parity bit of uart to either even or odd instead of none. 

[NORMAL]

Rx:01 10 00 00 00 64 C8 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 D4 93

[ERROR]

Rx:01 10 00 00 00
Rx:64 C8 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Rx:00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 D4 93

https://community.nxp.com/t5/i-MX-RT/Modbus-serial-communication-error-in-UART3-but-not-UART1-on/m-p...

I still don't understand why the issue can be solved by setting the parity. I tested the communication on my desk where there's almost no noise. So I believe the communication should work with parity bit disabled. Is there an issue in uart communication related to parity bit? Does anyone have idea how it can solve this issue?

[Information]

BOARD: iMXRT1060 EVK

SDK: 2.9.3

TOOL: Modbus Slave from www.modbustools.com

[jeremyzhou · Accepted solution]

Hi,
Thank you for your interest in NXP Semiconductor products and for the opportunity to serve you.
The specification states that when using "no parity" that you use 2 stop bits (Taken from pages 6-7 in Modicon Modbus Protocol Reference Guide PI-MBUS-300 Rev. J).
This is essentially the same as using 1 stop bit with "mark parity".
Have a great day.
TIC

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[creatorwonny]

Hi @jeremyzhou 

Thanks a lot for the information. I tested communication with setting of non-parity and 2 stop bits, and the communication worked very well.