---
name: "modbus_rtu_set_rts"
section: "3"
platform: "linux"
package: "libmodbus-dev"
version: "3.1.6-2.1"
release: "Debian 12 (Bookworm)"
platforms: ["linux"]
url: "https://new.carta.tech/modbus_rtu_set_rts"
---

# modbus\_rtu\_set\_rts(3)

set the RTS mode in RTU

## NAME

modbus\_rtu\_set\_rts - set the RTS mode in RTU

## SYNOPSIS

**int modbus\_rtu\_set\_rts(modbus\_t \****ctx***, int** *mode***)**

## DESCRIPTION

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.

## ERRORS

**EINVAL**

The libmodbus backend isn’t RTU or the mode given in argument is invalid.

## EXAMPLE

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

## SEE ALSO

[**modbus\_rtu\_get\_rts**(3)](/modbus_rtu_get_rts/3.md)

## AUTHORS

The libmodbus documentation was written by Stéphane Raimbault \<stephane.raimbault@gmail.com\>