Desired Serial Mode Not Supported -9600 5 N 1- Hot! Jun 2026

Decoding the Error: "Desired Serial Mode Not Supported -9600 5 N 1" – Causes and Fixes In the world of embedded systems, industrial automation, and legacy hardware communication, few errors are as cryptic—and as frustrating—as the message: "desired serial mode not supported -9600 5 n 1-" If you have landed on this article, you have likely encountered this error while trying to establish a serial connection via a terminal emulator (like PuTTY, Tera Term, Minicom, or screen) or within a custom software application. You specified the parameters 9600 baud , 5 data bits , No parity , and 1 stop bit —and the system rejected it. Why? And more importantly, how do you fix it? This comprehensive guide will break down every component of that error string, explain why your operating system or hardware is rejecting it, and provide step-by-step solutions. Understanding the Error String Before troubleshooting, let’s deconstruct the exact message: desired serial mode not supported -9600 5 n 1-

"desired serial mode not supported" : The OS or driver cannot apply the requested communication parameters. "-9600" : Baud rate (9600 bits per second). This is almost always supported. "5" : Data bits (5 bits per character). This is often the culprit. "n" : Parity (None). "1-" : Stop bits (1 stop bit).

The core issue almost always revolves around the 5 data bits setting. Most modern UARTs (Universal Asynchronous Receiver-Transmitters) and virtual COM port drivers default to 8 data bits. Why Does This Error Occur? The error arises from a fundamental mismatch between what your software requests and what your serial hardware or driver supports . Let’s break down the layers. 1. Legacy vs. Modern Hardware The 5 in the string refers to 5-bit character encoding. This harks back to the days of teletypes (TTY) and Baudot code (ITA2), where characters were transmitted in 5 bits. Modern UART chips (16550, FTDI, CP210x, CH340) fully support 5-bit mode in their hardware specifications— but not all operating system drivers enable it by default, and not all USB-to-serial adapters implement it correctly. 2. The Operating System Constraint

Windows (via legacy COM ports) : The Windows serial driver (Serial.sys) nominally supports 5 data bits, but only for specific parity and stop bit combinations. For 5 data bits, Windows often expects 1.5 or 2 stop bits, not 1. Linux (TTY layer) : Linux can handle 5 data bits well, but requires proper termios flags ( CS5 ). However, some USB-serial converters (especially cheaper ones) have faulty drivers that reject CS5 . macOS / FreeBSD : The POSIX termios implementation supports CS5, but certain USB serial adapter drivers (e.g., Prolific PL2303) have been known to return EINVAL (invalid argument) for 5-bit modes. desired serial mode not supported -9600 5 n 1-

3. The Driver/Firmware Limitation Many modern USB-to-serial chips are optimized for 8-N-1 (8 data bits, no parity, 1 stop bit). While the chip may advertise 5-bit support, the actual firmware on the device may:

Silently ignore the 5-bit request. Round up to 7 or 8 bits. Reject the setting and return "not supported."

This is especially common with:

Prolific PL2303 (older revisions) Silicon Labs CP210x (some firmware versions) Generic CH340 chips

Is the Error Always Literal? Sometimes, "not supported" is a misinterpretation. The OS may actually be saying: "The combination of 9600 baud, 5 data bits, No parity, and 1 stop bit is invalid." Specifically, with 5 data bits , many protocols require 1.5 or 2 stop bits to allow the receiver time to resynchronize. Requesting 1 stop bit with 5 data bits is unusual and may be rejected outright. Check the original device documentation. You may find that the true expected mode is 9600 5 n 1.5 or 9600 5 n 2 . Step-by-Step Troubleshooting and Fixes Step 1: Verify the Expected Parameters Before changing anything, confirm that 9600 5 N 1 is indeed the correct specification for the target device. If you are trying to communicate with a legacy teletype, an old industrial PLC, or a custom embedded device, consult its manual. You may discover the device actually expects:

9600 5 N 1.5 (1.5 stop bits) 9600 5 N 2 (2 stop bits) 9600 7 E 1 (7 data bits, even parity) – sometimes misreported. Decoding the Error: "Desired Serial Mode Not Supported

Step 2: Change the Stop Bits (Easiest Fix) In your terminal software, change the stop bits from 1 to 1.5 or 2 while keeping 9600 baud , 5 data bits , and No parity . Test each combination.

In PuTTY : Go to Connection > Serial > Stop bits, select 1.5 or 2 . In Tera Term : Setup > Serial Port > Stop Bit: 2 bit. In Minicom : sudo minicom -s → Serial port setup → Stopbits → 2.