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modbus_rtu_set_rts - set the RTS mode in RTU


int modbus_rtu_set_rts(modbus_t *ctx, int mode)


The modbus_rtu_set_rts() function shall set the Request To Send mode to communicate on a RS-485 serial bus. By default, the mode is set to MODBUS_RTU_RTS_NONE and no signal is issued before writing data on the wire.

To enable the RTS mode, the values MODBUS_RTU_RTS_UP or MODBUS_RTU_RTS_DOWN must be used, these modes enable the RTS mode and set the polarity at the same time. When MODBUS_RTU_RTS_UP is used, an ioctl call is made with RTS flag enabled then data is written on the bus after a delay of 1 ms, then another ioctl call is made with the RTS flag disabled and again a delay of 1 ms occurs. The MODBUS_RTU_RTS_DOWN mode applies the same procedure but with an inverted RTS flag.

This function can only be used with a context using a RTU backend.

Return value

The function shall return 0 if successful. Otherwise it shall return -1 and set errno to one of the values defined below.


  • EINVAL, the libmodbus backend isn't RTU or the mode given in argument is invalid.


Enable the RTS mode with positive polarity:

modbus_t *ctx;
uint16_t tab_reg[10];

ctx = modbus_new_rtu("/dev/ttyS0", 115200, 'N', 8, 1);
modbus_set_slave(ctx, 1);
modbus_rtu_set_serial_mode(ctx, MODBUS_RTU_RS485);
modbus_rtu_set_rts(ctx, MODBUS_RTU_RTS_UP);

if (modbus_connect(ctx) == -1) {
    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
    return -1;

rc = modbus_read_registers(ctx, 0, 7, tab_reg);
if (rc == -1) {
    fprintf(stderr, "%s\n", modbus_strerror(errno));
    return -1;


See also