Using the 'cu' Command for Serial Communication (with Examples)

The cu command is a powerful UNIX utility that allows users to connect to another system via a serial port or act as a dial-in terminal. It is particularly useful when dealing with hardware equipment that communicates over serial interfaces or for initiating file transfers without error checking. The cu command provides a simple yet robust means to interface with various devices through specified serial ports. Below are practical use cases showcasing how to use the cu command effectively.

Use Case 1: Opening a Given Serial Port

Code:

sudo cu --line /dev/ttyUSB0

Motivation:

In many scenarios, users need to connect to a serial device such as a modem, microcontroller, or console server. This connection allows users to monitor or configure these devices directly. Accessing the serial port using the cu command simplifies this process, enabling immediate interaction with the device.

Explanation:

• sudo: The command requires superuser privileges because accessing hardware ports generally needs elevated permissions.

• cu: Calls the cu command utility.

• --line /dev/ttyUSB0: Specifies the serial port to open. In this example, /dev/ttyUSB0 represents a common USB serial interface that many devices use for communication.

Example Output:

Upon execution, the terminal will display the interface provided by the connected device. For instance, connecting to a microcontroller might present its operating prompt, ready for commands.

Use Case 2: Opening a Given Serial Port with a Given Baud Rate

Code:

sudo cu --line /dev/ttyUSB0 --speed 115200

Motivation:

When interfacing with certain devices, the speed at which data is transmitted must match the device’s configuration. This speed, known as the baud rate, needs to be set correctly to ensure accurate communication without data errors or garbling.

Explanation:

• --speed 115200: Sets the communication speed to 115200 bits per second. This is a standard baud rate for fast data transmission and is often required for modern devices that handle substantial data quickly.

Example Output:

The terminal connects at the specified speed, allowing seamless interaction with the connected device, provided its baud rate matches the specified value.

Use Case 3: Opening a Serial Port with a Specific Baud Rate and Echoing Characters Locally

Code:

sudo cu --line /dev/ttyUSB0 --speed 115200 --halfduplex

Motivation:

Half-duplex communication is necessary in some systems where the device and terminal share the same communication channel alternately. This setup is useful when the user wants immediate feedback on the characters typed in without waiting for the remote system to echo them back.

Explanation:

• --halfduplex: Enables local echo of input characters. This is crucial when interacting with systems using half-duplex communication, allowing the user to see their input locally.

Example Output:

Each input character typed will appear immediately on the terminal, providing visual confirmation of the user’s typing, regardless of whether the remote system has processed them yet.

Use Case 4: Opening a Serial Port with Baud Rate, Parity Settings, and Without Flow Control

Code:

sudo cu --line /dev/ttyUSB0 --speed 115200 --parity=even --nortscts --nostop

Motivation:

Establishing communication with devices that require specific parity settings and no flow control can be critical for specialized hardware. Such configurations are often necessary in industrial systems and legacy equipment interfacing, where precise data integrity checks (like parity) are essential.

Explanation:

• --parity=even: Sets the parity to even, which means the number of 1-bits in the data should be even. This setup helps detect single-bit errors in transmitted data.

• --nortscts: Disables hardware flow control. This means the system will not use extra hardware lines for controlling the data flow, which is crucial when devices do not support hardware control lines.

• --nostop: Disables software flow control (via XON/XOFF). This allows uninterrupted data flow, ensuring that transmission and reception occur without software-induced interruptions.

Example Output:

A clean communications stream free of hardware or software flow interruptions, with the added integrity check facilitated by the even parity.

Use Case 5: Exiting a cu Session

Code:

~.

Motivation:

Exiting a session cleanly is crucial to maintain system stability and ensure that resources, like serial port connections, are released properly. This is especially important when switching between multiple devices or configurations.

Explanation:

• ~.: A command sequence that terminates the cu session. This sequence is a safeguard to ensure the connection is closed gracefully.

Example Output:

Upon entering the sequence, the command promptly exits the cu session, returning the user to the local shell, and properly closing any open serial connections.

Conclusion

The cu command provides a versatile method for interfacing with serial devices in various configurations. Whether managing connection speeds, dealing with half-duplex communication, setting parity, or ensuring a clean exit, cu effectively facilitates these tasks, supporting a broad array of serial communication needs. By understanding these use cases, users can leverage this command to its full potential, ensuring robust and reliable serial communications.