Flow Control: XON/XOFF and RTS/CTS

Why Do We Need Flow Control?

Imagine you are sending a huge amount of data to a device. If the receiving device is slow and can't process the data as fast as it's coming in, its input buffer will overflow. When the buffer is full, any new incoming data is simply lost. Flow control is a mechanism to prevent this by allowing the receiver to tell the sender "Hold on! I'm busy, stop sending data for a moment."

Software Flow Control (XON/XOFF)

Software flow control uses special characters sent over the regular data lines (TX/RX) to control the data flow. It's often called "in-band" flow control because the control signals are sent within the same data stream as the actual data.

• XOFF (Transmit Off): When the receiver's buffer is getting full, it sends a special control character (ASCII 19, or Ctrl-S) to the sender. The sender, upon receiving XOFF, must pause transmission.
• XON (Transmit On): Once the receiver has processed some of the data in its buffer and is ready for more, it sends an XON character (ASCII 17, or Ctrl-Q) to the sender, which can then resume transmission.

Pros: Doesn't require extra wires, making it suitable for 3-wire serial connections (TX, RX, GND). It's a simple solution for basic text-based communication. Cons: The control characters (XON/XOFF) cannot appear in the actual data stream, which can be a problem for binary data transfer. If binary data happens to contain the XON/XOFF character, it will be misinterpreted as a flow control signal, leading to data corruption or stalled communication. It also introduces a slight delay as characters are sent and processed.

Hardware Flow Control (RTS/CTS)

Hardware flow control uses dedicated signal wires in the serial cable to manage the flow of data. It is generally more reliable and faster than software flow control because the control signals are "out-of-band"—they use separate physical lines.

• RTS (Request to Send): The sending device asserts this line (sets it to a specific voltage, typically low) to indicate it has data to send.
• CTS (Clear to Send): The receiving device asserts this line (sets it to a specific voltage, typically low) to signal that it is ready to receive data. If the receiver is not ready, it will de-assert the CTS line, and the sender will pause until CTS is asserted again.

Pros: More reliable and doesn't interfere with the data stream, making it ideal for binary data transfer. It's faster because the control signals are immediate electrical changes rather than characters that need to be parsed. It provides a robust mechanism for preventing buffer overflows. Cons: Requires additional wires in your cable (RTS and CTS), typically a 5-wire or full RS-232 cable. Many simple 3-wire serial connections (TX, RX, GND) do not support it, meaning you need compatible hardware on both ends.

When to Use It?

For simple debugging with Serial.println() from a microcontroller, where data rates are relatively low and occasional data loss is acceptable, you rarely need flow control. However, for critical applications, flow control becomes essential:

• Uploading Firmware: When transferring large binary files (like new firmware) to a device, data integrity is paramount. Hardware flow control is highly recommended to prevent data loss during the transfer.
• High-Speed Data Logging: If a sensor or device is streaming data at a very high rate, the receiving computer or microcontroller might not be able to process it fast enough. Flow control ensures no data packets are dropped.
• Interacting with Peripherals: Some industrial equipment, modems, or specialized sensors require flow control to be enabled for reliable communication, especially if their internal buffers are small.
• Long Cable Runs: Over longer serial cables, signal integrity can degrade. Flow control can help manage retransmissions or pauses to ensure data arrives correctly.

Configuration: Most serial terminal applications and microcontroller libraries allow you to select the type of flow control (None, XON/XOFF, or RTS/CTS). Always ensure that the flow control setting in your terminal matches the setting on your connected device.

By understanding and correctly implementing flow control, you can build more robust and reliable serial communication systems, preventing frustrating data loss and ensuring your devices speak to each other clearly.