---
name: "modbus_new_rtu"
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_new_rtu"
---

# modbus\_new\_rtu(3)

create a libmodbus context for RTU

## NAME

modbus\_new\_rtu - create a libmodbus context for RTU

## SYNOPSIS

**modbus\_t \*modbus\_new\_rtu(const char \****device***, int** *baud***, char** *parity***, int** *data\_bit***, int** *stop\_bit***);**

## DESCRIPTION

The **modbus\_new\_rtu()** function shall allocate and initialize a *modbus\_t* structure to communicate in RTU mode on a serial line.

The *device* argument specifies the name of the serial port handled by the OS, eg. "/dev/ttyS0" or "/dev/ttyUSB0". On Windows, it’s necessary to prepend COM name with "\\\\.\\" for COM number greater than 9, eg. "\\\\\\\\.\\\\COM10". See http://msdn.microsoft.com/en-us/library/aa365247(v=vs.85).aspx for details

The *baud* argument specifies the baud rate of the communication, eg. 9600, 19200, 57600, 115200, etc.

The *parity* argument can have one of the following values

• *N* for none

• *E* for even

• *O* for odd

The *data\_bits* argument specifies the number of bits of data, the allowed values are 5, 6, 7 and 8.

The *stop\_bits* argument specifies the bits of stop, the allowed values are 1 and 2.

Once the *modbus\_t* structure is initialized, you must set the slave of your device with [**modbus\_set\_slave**(3)](/modbus_set_slave/3.md) and connect to the serial bus with [**modbus\_connect**(3)](/modbus_connect/3.md).

## RETURN VALUE

The function shall return a pointer to a *modbus\_t* structure if successful. Otherwise it shall return NULL and set errno to one of the values defined below.

## ERRORS

**EINVAL**

An invalid argument was given.

**ENOMEM**

Out of memory. Possibly, the application hits its memory limit and/or whole system is running out of memory.

## EXAMPLE

```
modbus_t *ctx;
ctx = modbus_new_rtu("/dev/ttyUSB0", 115200, 'N', 8, 1);
if (ctx == NULL) {
    fprintf(stderr, "Unable to create the libmodbus context\n");
    return -1;
}
modbus_set_slave(ctx, YOUR_DEVICE_ID);
if (modbus_connect(ctx) == -1) {
    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
    modbus_free(ctx);
    return -1;
}
```

## SEE ALSO

[**modbus\_new\_tcp**(3)](/modbus_new_tcp/3.md) [**modbus\_free**(3)](/modbus_free/3.md)

## AUTHORS

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