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

[Einar Thorsrud]

Hi Mark,

There is not much to go on here, to be honest. Also, Nordic does not provide Modbus support directly (though I see there are Modbus libraries in Zephyr). Assuming you use that I suggest you describe the issue you are seeing and what you have find by debugging in more detail, and hopefully you will get feedback from the Zephyr community.

Einar

[Mark Payne]

Hi

Yes I am using the Zephyr Modbus module.

I have put a line analyser on the input pin for Modbus UART and verified that a good frame is being presented to the development kit. I am still getting frame length errors. The UART is configured for 19200 bps operation. My console UARTs seem to work OK, suggesting that the core clock configuration is OK.

Regards Mark

[Mark Payne]

I have configured the project to implement a simple UART and not the Modbus module. I have verified that the frame is coming in ....

but it seems to come in a character at a time i.e. each time I receive a character I get an interrupt and the character is handled individually. It is as though the driver doesn't tolerate contiguous characters. So I don't get a complete message in the FIFO.

Is there something I am missing in the UART configuration ?

Regards Mark

[Einar Thorsrud]

Hi Mark,

Mark Payne said:

Is there something I am missing in the UART configuration ?

That could be. How have you configured / implemented UART communication on the nRF side? It is certainly possible to configure single byte transactions, but the UARTE peripheral does have DMA and allows long transactions without CPU involvements.

[Mark Payne]

Hi Einar

I configured the UART in the boards dts file:

&uart1 {
    status = "okay";
    compatible = "nordic,nrf-uarte";
    current-speed = <19200>;
    tx-pin = <40>;
    rx-pin = <38>;
};

However, I have now managed to get the Modbus driver to work by hard coding the UART configuration in modbus_serial.c.

/*TODO remove following hard configuration */
    uart_cfg.baudrate    = 19200;
    uart_cfg.parity        = UART_CFG_PARITY_NONE;
    uart_cfg.stop_bits    = UART_CFG_STOP_BITS_1;
    uart_cfg.data_bits    = UART_CFG_DATA_BITS_8;
    uart_cfg.flow_ctrl    = UART_CFG_FLOW_CTRL_NONE;
    
    if (uart_configure(cfg->dev, &uart_cfg) != 0) {

The only thing I can think of is the original code is setting the Modbus comms to something I don't want. I will look into this and if I find something I will post you.

For now, thanks for your help.

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