It varies enormously by product, and the specification must match the monitor's native mode or the whole arrangement is uncomfortable to use.

Older analogue switches handle modest resolutions well and degrade the image at higher ones - text becomes soft and the picture may be unstable.

Digital switches carry the signal without degradation up to their rated limit, which differs by product: some handle standard high definition, others 4K at 60 Hz, and the refresh rate weighs as heavily as the pixel count.

The test is the monitor's native resolution and refresh. A display forced to run below its native mode looks poor, and an operator working at it all day will notice.

Multiple monitors add another dimension - switches supporting dual or triple head keep all displays switching together, which is essential for control room use.

Also check the cables. The per-machine cable assemblies have their own bandwidth limits, and a switch capable of 4K fed by cables that are not will not deliver it.

No, and that independence is its main advantage - it works on anything with a video output and a USB port, including a machine that will not boot.

The switch operates on the physical video and USB signals. The connected computer sees a monitor and a keyboard and behaves normally, with no driver, no agent and no configuration.

That means it works with any operating system, with appliances and embedded systems, and with machines at their firmware setup screen or displaying a boot failure - which is exactly where software-based remote access cannot help.

It also means the switch cannot be compromised through the connected machine's software, which matters in secure environments.

Some switches offer optional software for switching by hotkey or mouse movement across screen edges, which is a convenience rather than a requirement.

The one thing to check is USB emulation. Good switches present a persistent keyboard and mouse to each machine so that switching away does not cause the machine to lose its input devices, which otherwise produces error messages and slow switching.

By a button on the unit, a keyboard hotkey, an on-screen menu, or mouse movement to a screen edge - and for frequent switching, the method matters more than it sounds.

Front panel buttons are simple and unambiguous, and suit a rack unit switched occasionally.

Hotkeys allow switching without reaching for the unit, which is what an operator at a desk wants. Sequences are usually configurable to avoid clashing with applications.

On-screen display menus are useful where ports are labelled, since selecting by name beats remembering which machine is port six.

Edge-of-screen switching - moving the mouse past the boundary of one display to control the next machine - is the most fluid method for multi-system desks and is what control room operators generally prefer. It usually needs software on the connected machines, which reintroduces a dependency.

For a rack switch used during maintenance, buttons and hotkeys are sufficient. For an operator position used all day, the switching method should be tested before committing to a product.

A switch built to prevent any data path between the connected machines, certified against a defined protection profile - required where systems of different classifications share one desk.

An ordinary KVM shares circuitry between ports, and in principle a signal path exists. Where the connected machines are on networks that must remain isolated, that is unacceptable.

Secure units use separate isolated circuitry per port, unidirectional data paths for keyboard and mouse, no shared memory, and physical tamper evidence. They deliberately omit features that could carry data between ports - no shared clipboard, no audio switching in some models, and restricted USB support that permits keyboard and mouse only.

They are evaluated against published protection profiles, and the certification is what the requirement usually specifies.

They are more expensive and less convenient by design. Where the requirement exists it is not negotiable, and an ordinary switch is not an acceptable substitute regardless of how it is configured.

Some can, through additional USB pass-through ports - but it is a specific feature, and secure models deliberately exclude it.

Basic switches emulate a keyboard and mouse only. Anything else plugged into them is not passed to the connected machine.

Models with USB pass-through provide extra ports that follow the selected machine, allowing a storage device, a smart card reader, a webcam or a headset to be shared. Performance varies - some pass-through implementations are limited to lower USB speeds, which matters for storage.

Switching a storage device between machines needs care, since removing it without dismounting can corrupt the file system.

Secure switches exclude this deliberately, because a shared storage device is exactly the data path they exist to prevent. Some permit an authenticated smart card reader with specific controls.

Where audio is required, check it separately - it is a distinct feature and is not universally supported.

A few metres on standard cables, considerably further with extenders - and beyond that, the answer is an IP-based solution.

Direct KVM cables are typically a few metres, and video quality degrades on long analogue runs. Digital signals either work or do not, with less graceful degradation.

KVM extenders send the signals over structured cabling or fibre, reaching tens or hundreds of metres depending on the technology. That is what allows an operator position to be some distance from the equipment room, which is the normal arrangement in control rooms and broadcast facilities - noise and heat stay with the machines.

Matrix systems extend this further, with any operator position able to reach any machine over a network of extenders.

Beyond that, an IP KVM is the correct product: it uses the ordinary network and imposes no distance limit at all, at the cost of some latency and image compression that matters for demanding visual work.

Video mode, cable cost, USB emulation behaviour, mounting and the switching method - with cable cost the one most often forgotten.

Confirm the switch carries the monitor's native resolution and refresh rate, and that the per-machine cables do too.

Price the cables. Each connected machine needs an assembly, they are model-specific, and for a sixteen-port switch they can approach the cost of the switch itself. Quotations frequently omit them.

Check USB emulation - whether each machine continues to see a keyboard and mouse when not selected. Without it, switching is slow and machines may log input device errors.

Confirm mounting: desktop, under-desk or rack, and whether rack ears are included.

And consider what happens when the switch fails. A KVM in the path to a critical machine is a dependency, so keep the means to connect a monitor and keyboard directly, and a spare switch where the arrangement matters.