How to solve Bad File Descriptor for Linux and Modbus

Question

I am trying to setup a Half Duplex RS-485 communication using libmodbus on a Raspberry Pi running Raspian Buster, with a FTDI USB to Serial adapter. My FTDI adapter shows as ttyUSB0 when I run ls /dev/.

I tried the following sample code:

#include <modbus.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

int main(void) {
   modbus_t *ctx  = modbus_new_rtu("/dev/ttyUSB0", 19200, 'N', 8, 1);
    if (ctx == NULL) {
        fprintf(stderr, "Unable to create the libmodbus context\n");
        return 0;
    }

    if (modbus_rtu_set_serial_mode(ctx, MODBUS_RTU_RS485) == -1) {
        fprintf(stderr, "Error setting the serial port as RS-485\n");
        fprintf(stderr, "errno: %d\n (EBADF == 9)", errno);
        modbus_free(ctx);
        return 0;
    }
}

Compiled with gcc test1.c -I/usr/include/modbus -lmodbus. And I get errno as 9, or EBADF, even if I run this code with sudo.

Answer 1

There is a very easy solution to your problem: just don't set MODBUS_RTU_RS485, quite likely you don't need it.

This mode is actually a workaround for devices without automatic (hardware) direction control. As you know, Modbus RTU works over a half-duplex RS485 link (only one device is allowed to talk while all others must be listening only), and hence requires an additional (to RX and TX) signal to control what device is writing to the bus at all times (direction control).

So you would only need to set MODBUS_RTU_RS485 if your device lacks this feature, which is nowadays quite unlikely or if you are building your own transceiver. Most devices based on the FTDI chip, in particular, should have this feature since the chip itself has a TXDEN (transmit enable) pin. See here for more details and a trick to expose the TXDEN signal to a non-default pin.

It is when you don't have this feature (one frequent scenario is when you want to use the embedded UART on your Rpi for Modbus over RS485, implementing your own transceiver) that you need a software (or hardware) workaround. And that's where MODBUS_RTU_RS485 should come handy, repurposing the RTS flow control signal. Unfortunately, most serial drivers (including ftdi_sio, the one you are probably using) don't support this mode (refer again to the link above).

Luckily, there are workarounds to the workaround: see here for a complete discussion. You can also take a look at this answer where I explained how to set up libmodbus with support for toggling the direction on the bus using a GPIO pin on a Rpi (also applicable to most SBCs, I've used this method successfully with a Pocket Chip computer, for instance).

You can find more background on this issue elsewhere: here and here.