RTU Receive issue on Raspberry PI CM3.

[Allan Irvine]

I'm having trouble receiving bytes using the libmodbus 3.1.4 package provided by Raspberry PI OS. I have also compiled the latest git sources with the same result.

As a modbus master I always and only receive bytes with value 0x00, weather any data is transmitted by a slave or not, or weather the slave is connected or not.

Example usage: (Error handling omitted)

      auto ctx = modbus_new_rtu("/dev/ttyS0", 115200, 'N', 8, 1);

       rc = modbus_rtu_set_rts(ctx, MODBUS_RTU_RTS_UP);

        rc = modbus_rtu_set_custom_rts(ctx, do_rts);

        modbus_set_slave(ctx, 5);

        modbus_set_debug(ctx, 1);

        modbus_set_response_timeout(ctx, 1, 0);

        rc = modbus_connect(ctx);        

        

        modbus_flush(ctx);

        // Write some registers

        std::array<uint16_t, 11> drs;

        drs.fill(0);

        drs[4] = 1;

        drs[5] = 1;

        drs[6] = 1;

        // auto val = dist(mt);

        std::cout << fmt::format("Sending Float Value: {}\n", v);

        modbus_set_float_abcd(v, drs.data() + 7);

        rc = modbus_write_registers(ctx, 0, 11, drs.data());

        if (rc < 0) {

            std::cout << "Modbus Write Registers Error: " << modbus_strerror(errno) << "\n";

        }        

        modbus_close(ctx);

        modbus_free(ctx); 

        

Example Debug output:

Opening /dev/ttyS0 at 115200 bauds (N, 8, 1)

Bytes flushed (0) // I call flush after connect just to be sure...

Sending Float Value: 530

[05][10][00][00][00][0B][16][00][00][00][00][00][00][00][00][00][01][00][01][00][01][04][44][00][80][00][00][00][00][6F][6F]

Sending request using RTS signal

Commanding RTS PIN to: 1

Commanding RTS PIN to: 0

Waiting for a confirmation...

<00><00><00><00><00>

ERROR CRC received 0x0 != CRC calculated 0x71C0

Modbus Write Registers Error: Invalid CRC

The data is written to the slave correctly.

On both an oscilloscope and a logic analyzer I can see the correct response being sent by the slave however the value received by libmodbus are always 0.

If I disconnect the slave I get the exact same output. In that case I would expect to get a timeout or a connection reset error.

I should also mention that a quick test using pyserial shows that the UART can and does receive bytes as sent,

I've used libmodbus in various linux projects for a number of years now and this is the first time I've ever had an issue. I have made many implementations of modbus master and slaves on raspberry PI devices, but this is my first try with the compute module.

Any advice is greatly appreciated.

Hardware: Raspberry PI Compute Module 3 on custom board.

Software: Linux raspberrypi 5.4.72-v7+ #1356, Libmodbus 3.1.4

Thanks,

Allan Irvine

[Stéphane Raimbault]

Hi Allan,

Why do you need the RTS flow control?

Stéphane

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[Allan Irvine]

Hi Stephanne, 

I should have mentioned that my board has an RS485 chip connected to the UART pins. I need to set TXE.

Thanks,

Allan Irvine.

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