RS-232: The Serial Port That Refuses to Die
How this 1960s standard became the cockroach of computing - surviving decades of “obsolescence” and still running critical systems today
Picture this: It’s 1962, and you need to connect a teletype machine to a mainframe computer. The engineers at Bell Labs create a standard called RS-232 (Recommended Standard 232) that defines exactly how to send serial data over copper wires.
Fast forward to 2025, and guess what? RS-232 is still everywhere. It’s running industrial control systems, networking equipment, scientific instruments, and embedded devices all over the world. Every time someone declares it “obsolete,” it just keeps on working.
RS-232 is like that reliable old pickup truck that you keep meaning to replace, but it starts every morning and gets the job done, so why fix what ain’t broken?
RS-232 signal levels and timing - this simple bit-banging protocol has been moving data reliably for over 60 years
What RS-232 Actually Does
RS-232 is fundamentally about sending data one bit at a time over a wire. It’s called “serial” because bits travel in a single file, one after another, rather than in parallel like data on a computer bus.
Voltage Levels: Backwards Logic
RS-232 uses negative voltage for a logical “1” and positive voltage for a logical “0”. This seems backwards, but it was designed for noise immunity on long cable runs.
- Logical 1 (Mark): -3V to -25V
- Logical 0 (Space): +3V to +25V
- Undefined: -3V to +3V (transition zone)
Asynchronous Communication
RS-232 doesn’t use a shared clock signal. Instead, both sides agree on a baud rate (bits per second) and use start and stop bits to frame each character:
- Idle state: Line stays at logical 1 (-12V)
- Start bit: Sender drops to logical 0 (+12V) to signal “character coming”
- Data bits: Usually 8 bits, sent least significant bit first
- Parity bit: Optional error detection
- Stop bit(s): Return to logical 1 to signal “character done”
This self-synchronizing approach means you can connect any RS-232 device to any other and they’ll communicate successfully (as long as they agree on the baud rate).
The DB-9 and DB-25 Connectors: More Than Just Power and Ground
RS-232 defines way more than just data transmission. The full standard includes control and status signals:
TXD (Transmit Data): Your data going out RXD (Receive Data): Their data coming in RTS (Request to Send): “I want to transmit” CTS (Clear to Send): “OK, go ahead and transmit” DTR (Data Terminal Ready): “I’m powered up and ready” DSR (Data Set Ready): “I’m ready too” DCD (Data Carrier Detect): “I see a valid signal” RI (Ring Indicator): “The phone is ringing” (for modems)
This handshaking allows devices to coordinate transmission and handle flow control when one side can’t keep up.
Why RS-232 Survived: The Virtues of Simplicity
While fancy networking protocols came and went, RS-232 endured because of some fundamental advantages:
Universal Implementation
Every microcontroller, every computer, every embedded system can implement RS-232 with minimal hardware. All you need is a UART (Universal Asynchronous Receiver-Transmitter) and some voltage level conversion.
Easy Debugging
You can literally see RS-232 signals on an oscilloscope or logic analyzer. When something breaks, it’s usually obvious why.
No Network Stack
RS-232 is point-to-point and stateless. No IP addresses, no routing tables, no complex protocols. Just two devices talking directly to each other.
Deterministic Timing
Unlike Ethernet or WiFi, RS-232 has predictable latency. This makes it perfect for real-time control systems where timing matters.
Long Distance Support
With the right drivers, RS-232 can work over thousands of feet of cable. Industrial systems often need this kind of reach.
Galvanic Isolation
You can easily add optical isolators to RS-232 connections, protecting sensitive equipment from electrical faults.
Modern RS-232: Still Going Strong
RS-232 never actually died - it just moved to different applications:
Industrial Automation
PLCs (Programmable Logic Controllers), motor drives, and sensors often communicate via RS-232. These systems run for decades without needing upgrades.
Networking Equipment
Most routers, switches, and firewalls have a console port that uses RS-232 for initial configuration and emergency access.
Scientific Instruments
Oscilloscopes, multimeters, spectrum analyzers, and laboratory equipment often use RS-232 for remote control and data logging.
Embedded Systems
Microcontroller projects, IoT devices, and development boards use RS-232 for debugging output and simple communication.
Legacy System Integration
Many mainframes and legacy systems from the 1980s and 1990s are still running critical business applications. RS-232 is often the only way to interface with them.
The RS-232 Family Tree
RS-232 spawned several improved variants for different applications:
RS-422: Differential signaling for better noise immunity and longer distances
RS-485: Multi-drop capability - multiple devices on one bus
USB-to-Serial: Brings RS-232 to modern computers without serial ports
Ethernet-to-Serial: Network-enable RS-232 devices remotely
These variants maintain the same basic protocol while solving specific limitations of the original standard.
The Hardware Evolution
Modern RS-232 implementations are tiny compared to the original:
1960s: Discrete transistors and transformers, room-sized equipment
1980s: Dedicated chips like the MAX232 for voltage level conversion
2000s: USB-to-serial adapters brought RS-232 to laptops
2020s: Single-chip microcontrollers with built-in USB-serial bridges
You can now get complete RS-232 functionality in a chip smaller than your fingernail.
The Software Side: Terminal Emulators Forever
Every operating system still includes terminal emulator software that speaks RS-232:
Windows: HyperTerminal (classic), PuTTY, Tera Term macOS: Terminal with screen/cu commands Linux: minicom, screen, picocom Embedded: Built into development environments
The fact that every platform still supports RS-232 terminal access shows how fundamentally useful this simple protocol remains.
A Personal Note
I’ve been using RS-232 since the 1980s, connecting modems, printers, and terminals to computers. What strikes me most is how consistent the experience has been across decades.
The same PuTTY settings (9600 baud, 8-N-1) that worked on a 1990s router console port still work on a 2025 network switch. The same terminal commands that controlled a 1980s modem still work on modern embedded systems.
There’s something deeply satisfying about a technology that just works and keeps working. In our world of constantly changing APIs and protocols, RS-232 represents stability and predictability.
Every time I connect to a router console port or debug an embedded system, I’m reminded that sometimes the old ways are the best ways. RS-232 doesn’t need firmware updates, security patches, or compatibility shims. It just moves bytes from point A to point B, reliably, simply, and forever.
That’s the mark of truly great engineering.
Want to explore more vintage protocols that shaped computing? Check out our posts on Telnet (early remote access) and Ethernet (networking that conquered the world). Or browse our complete protocol collection.
