Classes of USB KVM Switches
With the popularity of USB; USB keyboards, mice and I/O devices are still the most common devices connected to a KVM switch. The classes of KVM switches reviewed are based on different types of core technologies in terms of the KVM switch handling USB I/O devices--including keyboard, mouse, touch screen display, etc. (USB HID class devices- Human Interface Devices ).
1. USB HUB based KVM
- Also called Enumerated USB KVM switch. A connected/shared USB device is required for the full initiation process (USB enumeration) EVERY TIME the KVM is switched to another target system/port. The switching to different ports is just as if you were to physically plug and unplug a USB device into your targeted system.
2. Emulated USB KVM
- Dedicated USB console port(s) are assigned to emulate special sets of USB keyboard or mouse switching control information to each connected/targeted system. Emulated USB provides an instantaneous and reliable switching action that makes keyboard hotkey and mouse switching possible. However, this class of KVM switches uses only generic emulations and consequently have only been able to support the most basic keyboard and mouse features.
3. Semi-DDM USB KVM
- Dedicated USB DDM console port(s) work with all USB HID devices (including keyboard and mouse), but do not maintain the connected devices' presence to all the targeted systems simultaneously. This class of KVM switch takes advantage of DDM (Dynamic Device Mapping/ True Emulation ) technology to quickly enumerate the first connected system and keep the enumeration to the console side of the KVM switch for the next switching action to the other connected systems. It is only DDM to the console side but not to every connected system (of the PC side connections).
4. DDM USB KVM or True Emulation KVM
- Dedicated USB DDM console port(s) work with all USB HID devices (including keyboard and mouse) and maintain the connected devices’ special functions and characteristic to each connected/targeted systems. This class of KVM switches overcomes the frustrating limitations of an Emulated USB Class KVM by emulating the true characters of the connected devices to all the computers simultaneously. This means that you can now use the extra function keys, wheels, buttons, and controls that are commonly found on modern keyboards and mice.
Comparison - Classes of USB KVM Switches
| Comparisons | Hub Base Class | Emulated Class | Semi-DDM Class | DDM Class |
|---|---|---|---|---|
| USB RE-Enumeration Required | Required on Every Switching | NO, only for keyboard/mouse | NO, for all USB HID Devices | NO, for all USB HID Devices |
| Latency in sharing connected USB devices | Longest, depending on connected systems’ OS. (about 10~15+ seconds) | Short | Short | No Latency |
| Hot-key Command supported | NO | Yes, only on dedicated keyboard port | Yes, all the console Semi-DDM ports | Yes, all the console DDM ports |
| Special keyboard mouse functions supports | Limited * | NO, only act as standard keyboard and mouse | Yes | Yes |
| Windows 7 devices stages showing correct devices connected | Limited * | NO, show as the standard keyboard and mouse, no matter what keyboard and mouse connected to the KVM | Yes | Yes |
| Windows 7, 8 built-in touch monitor driver support or touch mouse/ touch pad | Limited * | NO | Yes* | Yes |
| Wireless combo keyboard and mouse support | Limited * | NO | Yes* | Yes |
| USB HID devices (other than keyboard and mouse ) support | Limited * | NO | Yes* | Yes |
| USB Touch Screen sharing support | Limited * | NO | Yes* | Yes |
| Drawing Tablet support | Limited * | NO | Yes* | Yes |
| Logitech USB Wireless Unifying Receiver | Limited * | NO | Yes* | Yes |
| Pros | Pass all signals between USB devices and target system/computer | – USB Keyboard/mouse switching control
– Shorter switching time – Hot-key commands |
– Full USB Keyboard/mouse switching control
– DDM ports can work with all USB HID class devices – Short switching time ( latancy: within 1 sec.) – Hot-key commands (apply to all USB Semi-DDM ports) – Lower Cost than Full DDM class switches |
– Full USB Keyboard/mouse switching control
– DDM ports can work with all USB HID class devices – Shortest switching time (no latency ) – Hot-key commands (apply to all USB DDM ports) |
| Cons | - Longest latency, delay in device availability
- Can’t use USB keyboard/mouse to control KVM switching process - No Hot-key command - Generate HPD error when switching under particular OSs |
- Support only limited/fixed general keyboard mouse profiles
- Special keyboard buttons or mouse features will not work - Can only share “standard” USB keyboard and mouse - Can not share other USB HID devices, such as touch screen monitor, drawing tablet …etc. - Generated HPD Error while using other USB HID devices |
- Still have latency in time while switching | Higher Cost |
Limited * = supported but need to do USB re-enumeration – which not only cause long latency in switching but also sometimes caused HPD (Hot-plug Device) errors to the OS systems.
Yes * = Latency time within 1 sec while switching among channels/ ports.
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