For anyone who has built or repaired a desktop PC from the late 1990s or early 2000s, the sight of a round, six-pin PS/2 connector—often color-coded purple for keyboards and green for mice—is instantly familiar. Today, nearly all keyboards connect via USB, making the PS/2 port seem like a relic. But why did older keyboards exclusively use PS/2 when USB was already available? The answer lies in a combination of technical advantages, historical timing, and design philosophy.

First, the most critical reason is the difference in how both interfaces handle data. PS/2 was designed as a dedicated, interrupt-driven interface. When you press a key on a PS/2 keyboard, the signal travels directly to the keyboard controller on the motherboard, which then triggers a hardware interrupt (IRQ 1). This means the CPU stops what it is doing immediately and processes the keystroke. In contrast, USB is a polled interface. The host controller regularly "asks" each connected device if it has data to send. For most everyday tasks, this polling interval is imperceptible, but in the early days of USB 1.1, the polling rate was a maximum of 125 Hz (once every 8 milliseconds). PS/2, on the other hand, could achieve much lower latency—a crucial advantage for professional typists, gamers, and anyone working in environments where every millisecond mattered, such as early real-time data entry.

Second, PS/2 ports offered full n-key rollover (NKRO) support without additional software. With a PS/2 keyboard, pressing multiple keys simultaneously (for example, "Ctrl+Alt+Delete") would register every single keystroke correctly, because each key press and release generates its own "make" and "break" scan code that is sent sequentially over the bidirectional clock/data lines. Early USB keyboards, constrained by the USB HID specification, typically only supported two or three simultaneous non-modifier keys. Although modern USB keyboards have overcome this limitation with custom firmware and high-polling-rate controllers (up to 1000 Hz), in the mid-1990s, PS/2 was the only reliable choice for simultaneous keypress detection—especially important for typing-heavy operations and early PC gaming.

Third, BIOS and operating system compatibility played a massive role. Before the era of UEFI and modern Plug-and-Play standards, the basic input/output system (BIOS) handled keyboard input from the very first moment the computer was powered on. The PS/2 keyboard controller was standardized and integrated into the motherboard chipset (often integrated into the Super I/O chip). It provided a universal, well-documented, and low-level interface that the BIOS could use without any additional drivers. USB, conversely, required the BIOS to implement a full USB stack—memory overhead, initialization routines, and enumeration processes—which was technically challenging and expensive in the constrained ROM space of early motherboards. Even after USB became physically present on motherboards, many BIOSes still relied on the PS/2 port for keyboard input during POST (Power-On Self-Test) and setup menus, because USB initialization often failed before the operating system loaded drivers.

Fourth, cost and simplicity mattered. PS/2 connectors are purely passive—a keyboard literally just sends electrical signals over two wires (clock and data) plus power and ground. Manufacturing a PS/2 keyboard required very few electronic components: a simple microcontroller to scan the matrix, a small crystal, and the connector. USB keyboards, by contrast, needed a USB transceiver, a more complex microcontroller capable of handling the USB protocol (including packet framing, CRC checks, and power management), and additional voltage regulation. In the price-sensitive PC market of the 1990s, shaving a few dollars off the manufacturing cost of a keyboard or a motherboard was a significant advantage.

Fifth, the timing of USB's introduction cannot be ignored. USB 1.1 was officially released in 1998, but PS/2 had been the standard since IBM's Personal System/2 line debuted in 1987. By the time USB debuted, millions of keyboards, motherboards, and KVM switches were already designed around PS/2. The adoption of USB was slow and gradual. Many early USB keyboards came bundled with a PS/2 adapter because manufacturers understood that users might need to plug into a PS/2 port to enter the BIOS setup. It was only around 2005-2007 that motherboard manufacturers began to drop the second PS/2 port (or combine it into a single combo port), and by the early 2010s, USB had largely replaced PS/2 in consumer desktop PCs.

However, there is one final nuance: PS/2 is "hot-swappable" in practice but not in specification. The standard never officially supported hot-plugging—connecting or disconnecting a keyboard while the computer was powered on could short the pins and damage the motherboard or the keyboard. USB was designed from the ground up for hot-swapping, which made it far more convenient for everyday users. Yet, this very limitation of PS/2 contributed to its perceived reliability: because you rarely unplugged a PS/2 keyboard, the connection remained clean, secure, and free from contact degradation.

In summary, the persistence of PS/2 was not a sign of backwardness but a reflection of its technical superiority in three key areas: lower latency through interrupt-driven communication, guaranteed n-key rollover, and seamless BIOS-level functionality. USB eventually won because of its hot-swap capability, universal compatibility across devices (mice, keyboards, printers, storage), and higher data bandwidth. Nonetheless, for enthusiasts and competitive typists who still seek the lowest latency and full NKRO, PS/2 keyboards remain highly sought after—even decades after their prime. The round purple and green ports have become nostalgic icons, representing a time when every millisecond of input lag was carefully avoided, and the keyboard was a direct extension of the system's core logic.