In fact, USB devices don't send anything until they RECEIVE corresponing request from host. The question reflects a common misconception that USB devices "send" something to PC on their own when a key is pressed (or mouse moves), that's why so high attention. These behaviors have been used by plenty of software that expects this behavior, and therefore keyboard controllers have continued controlling the A20 line and performing software CPU resets even when the need for a reset via the keyboard controller was obviated by the Intel 80386's ability to switch to real mode from protected mode without a CPU reset. This was a problem because the BIOS and the operating system services could only be called by programs in real mode. 1 The keyboard controller was also used to initiate a software CPU reset in order to allow the CPU to transition from protected mode to real mode 1 because the 286 did not allow the CPU to go from protected mode to real mode unless the CPU is reset. This computer also controlled access to the A20 line in order to implement a workaround for a chip bug in the Intel 80286. The IBM PC AT used an Intel 8042 chip to interface to the keyboard. In this case, the controller usually also controls the keyboard's LEDs by sending data back to keyboard through the wire. If a keyboard is a separate peripheral system unit (such as in most modern desktop computers), the keyboard controller is not directly attached to the keys, but receives scancodes from a microcontroller embedded in the keyboard via some kind of serial interface. You can't have a "read-only" USB device.Įven before USB, the PC keyboard controller would accept commands because it did a few things besides read the keyboard ( reference): That being said, there is a negotiation and enumeration process with all USB devices that require a back-and-forth conversation between host and device. So it is not an "input-only" device (meaning it only outputs data to the host). To change the keyboard LEDs, the keyboard accepts a command to do so. If using the keyboard descriptor in Appendix B, LED statesĪre set by sending a 5-bit absolute report to the keyboard via a LOCK, COMPOSE, and KANA events is maintained by the host and NOT the Synchronization between LED states and CAPS LOCK, NUM LOCK, SCROLL Devices (HID)" version 11.1" specification:
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