Frame length error - Modbus RTU slave

[Mark Payne]

Hi

I have configured my project to have a Modbus RTU slave on UART1. When I poll the dev kit, using Modscan, I get frame length errors. I have checked baud rate and parity. My board connects to my PC using an rS485 line driver (I don't think this is the problem).

Any suggestions.

Regards Mark

[Mark Payne]

OK - I believe I have found the problem ...

If no parity is selected the Modbus driver sets stop bits to 2. This is incorrect, it should be left at 1.

See below (modbus_serial.c):

    switch (param.serial.parity) {
    case UART_CFG_PARITY_ODD:
    case UART_CFG_PARITY_EVEN:
        uart_cfg.parity = param.serial.parity;
        uart_cfg.stop_bits = UART_CFG_STOP_BITS_1;
        break;
    case UART_CFG_PARITY_NONE:
        /* Use of no parity requires 2 stop bits */
        uart_cfg.parity = param.serial.parity;
        uart_cfg.stop_bits = UART_CFG_STOP_BITS_2;
        break;
    default:
        return -EINVAL;
    }

I would change this to ...

        uart_cfg.parity = param.serial.parity;
        uart_cfg.stop_bits = UART_CFG_STOP_BITS_1;

Regards Mark