modbus_rtu_set_rts - Man Page

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 RS485 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.



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, "Connexion 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



The libmodbus documentation was written by Stéphane Raimbault <>

Referenced By

libmodbus(7), modbus_rtu_get_rts(3).

01/25/2024 libmodbus v3.1.7 libmodbus Manual