Wordlength setting when using Modbus RTU as UART

[dkim.3]

Hello, Now I'm using Modbus RTU and UART. Modbus RTU communication is being performed through UART. Modbus RTU setting is data 8 bytes, but communication is possible only when uart wordlength is set to 9 in STMcubeIDE. Why is wordlength 9 when data is 8?

[Pavel A.]

Parity bit.

According to the Modbus RTU spec, 2.5.1, support for even parity is required. Other modes (no parity, odd parity) are optional.

[waclawek.jan]

... and in the STM32 UART, confusingly different from other manufacturers' UARTS, the parity bit is counted up to the total word length.

JW