The core challenge in designing a split keyboard lies in determining how the two halves communicate with each other and which half reports data to the computer. Current mainstream solutions fall into three categories: wired physical connections, independent wireless connections, and hybrid bridging modes—each involving different trade-offs regarding latency, convenience, and structural complexity.
I. Wired Physical Connection: The Most Stable Signal Path
This is the most traditional and reliable method, utilizing a physical cable to link the left and right halves.
Dedicated Cables (USB-C / TRRS): The most common approach today involves using a USB-C to C cable. For instance, the ROG Falcata requires the use of its included, proprietary cable to ensure stable and secure signal transmission; using third-party cables may result in malfunctions. In the custom keyboard community, TRRS cables or 4-pin JST connectors are also frequently used to transmit matrix data and provide power.
Firmware-Defined Communication Protocols: In a wired setup, the firmware (such as QMK) typically establishes a master-slave relationship. The half connected to the computer acts as the “Master,” while the other serves as the “Slave.” The Master scans its own key matrix, sends commands to the Slave, receives key-press data from it, and then consolidates all information to transmit to the computer. It is important to note that hot-plugging a TRRS cable while the device is powered on carries a risk of short-circuiting, which could damage the controller.
II. Independent Wireless Connection: The Ultimate in Wireless Flexibility
This represents a “truly wireless” split configuration, where each half is equipped with its own independent wireless module and battery, eliminating the need for any physical cabling.
Independent Pairing (Dual-mode/Tri-mode): In this setup, the left and right halves communicate with the computer as separate, independent wireless devices. For example, with the NocFree keyboard, the left half, right half, and numeric keypad can each connect wirelessly—whether via Bluetooth or 2.4GHz—requiring the user to perform the pairing process only once. Master-Slave Wireless Architecture (ZMK/QMK Framework): In the custom keyboard space, ZMK firmware supports a “Central and Peripheral” architecture. The left half (the “Central” unit) communicates with the computer via Bluetooth, while the right half (the “Peripheral” unit) connects to the left half via Bluetooth. Keypress data is transmitted from the peripheral to the central unit, which then aggregates and reports all inputs to the computer. The trade-off is that the “Central” unit consumes significantly more power, and the wireless communication between the two halves introduces additional latency (according to official ZMK data, this adds an average of 3.75ms, with a worst-case scenario of 7.5ms).
III. Hybrid Bridging Mode: A Compromise Between Stability and Freedom
This is an ingenious approach—”wired for one half, wireless for the other”—often found in certain tri-mode split keyboards.
Wired Left Half, Wireless Right Half: Taking the “wired mode” of NocFree keyboards as an example, the user connects the left half to the computer via USB-C, while the right half (and any attached numpad) connects wirelessly to the left half. This ensures a stable connection between the primary controller and the computer, while freeing the right side from the constraints of a cable.
Mandatory Interconnection in Wireless Mode: Some products require the two halves to be physically connected by a cable even when operating in wireless mode. For instance, the Mistel Barocco MD600 Alpha explicitly requires that the left and right units be linked via a USB cable when in wireless (Bluetooth) mode; they cannot connect to the computer independently via wireless, and the left unit functions as the primary controller.
IV. “Master-Slave” Logic at the Firmware Level
Regardless of the physical connection method, the firmware must define “master” and “slave” roles to determine which unit is in charge.
Determining the Master Controller: In QMK, the half into which the USB cable is plugged is designated as the master (typically the left half). It is responsible for processing all logic and communicating with the computer. If the master is incorrectly identified—for example, if the right half is mistakenly treated as the left—the key mapping will be mirrored. This can be corrected by manually specifying the roles using settings such as `EE_HANDS`. Data Synchronization: While aggregating key data, the master unit must also synchronize layer states, LED statuses, and even OLED display content to the slave unit, ensuring visual and functional consistency across both halves.
