Because they are not passive cables - most contain electronics that emulate input devices and hold configuration - and they are specific to the switch family.

A simple video cable is inexpensive. A KVM interface module contains a microcontroller that presents a persistent keyboard and mouse to the connected machine, converts the video for transmission over structured cabling, and stores the port identification.

That emulation is what makes switching instant. Without it, each switch selection causes the machine to detect input devices being connected, which takes seconds and can produce error messages.

They are also low-volume products specific to a switch family, so they do not benefit from commodity pricing.

The practical consequence is to include them in the budget from the start, at one per connected machine plus spares. A sixteen-port switch needs sixteen, and a quotation that lists only the switch is incomplete by a large margin.

Rarely. Modules are specific to the switch family and often to a generation within it, and there is no cross-vendor compatibility at all.

The protocol between module and switch is proprietary, so a module made for one manufacturer will not work with another's switch even where the connectors match.

Within a manufacturer, compatibility across product generations varies. Some maintain it deliberately to protect customers' investment; others change it, and modules from an older range will not work with a new switch.

That has a real consequence when replacing a KVM: the modules may need replacing too, which can double the cost of the refresh and should be established before the switch is chosen.

When ordering, always state the exact switch model. 'KVM cable' is not a specification, and modules that arrive incompatible cannot usually be adapted.

DisplayPort, HDMI, DVI and VGA, and increasingly USB-C - and the module must match what the machine actually provides.

Modern servers and workstations vary considerably. Some provide only DisplayPort, others HDMI, older equipment DVI or VGA, and some laptops and compact machines only USB-C carrying video.

Modules are made for each, and choosing the wrong one is a common ordering error - especially in a mixed estate where different machine generations coexist.

Passive video adaptors between standards work in some directions and not others, and can limit the resolution or refresh rate achievable. Where an adaptor is unavoidable, test it at the required mode rather than assuming.

Also confirm the module supports the resolution needed. A module rated below the display's native mode is the limiting element regardless of what the switch supports.

Survey the machines before ordering. Counting ports is quicker than returning modules.

Fixed KVM cables run a few metres; interface modules over structured cabling reach tens or hundreds of metres depending on the system.

Bonded KVM cables are supplied in fixed lengths of a few metres and are used where the switch is in the same rack or at the same desk.

Interface modules connect back over ordinary category cabling, which is what allows the switch or matrix to be some distance from the machines. Supported distances vary by system and by video resolution - higher resolutions reduce the distance achievable.

Matrix systems extend further again, connecting modules and operator stations across a dedicated network.

Use solid-core structured cabling rather than long patch leads for the fixed runs, and keep the number of patch points low - each connection reduces the margin.

Where the distance exceeds what the system supports, the answer is fibre-based extenders or an IP-based solution rather than pushing the copper further.

The module presents a permanent keyboard and mouse to the connected machine, so switching away does not cause the machine to see its input devices disconnect.

Without emulation, selecting another port removes the keyboard and mouse from the machine's point of view. Coming back causes it to redetect them, which takes a moment and on some operating systems produces notifications or errors.

More seriously, some firmware and some operating systems behave badly when input devices vanish - a machine part way through a firmware setup screen may lose its state.

Emulation avoids all of it: the machine always sees a keyboard and mouse present, and switching is instantaneous.

It also allows the module to hold the port's name, so the machine is identified consistently on the switch's menu regardless of which physical port it happens to occupy - which makes recabling much less confusing.

Check the specification. Cheaper modules and switches omit emulation, and the difference is immediately obvious in daily use.

At least one of each module type in use, and more where the estate is large - because a failed module isolates one machine and cannot be improvised.

Modules are the part of a KVM installation most likely to fail, because they are handled, moved and plugged in and out.

A failed module leaves one machine unreachable through the KVM. Where that machine has no other console path, it is a site visit and a wait for a delivery.

Stock one spare per module type - and note that a mixed estate may need several types, one per video interface.

Record which type each machine uses, so the correct spare is fitted rather than discovered to be wrong.

Where modules are being discontinued because the switch range is ageing, buying spares before they become unavailable is worth considering. A KVM system whose modules can no longer be bought is a system with a defined remaining life.

They draw power from the connected machine's USB port, which is usually fine but has consequences worth knowing.

Modules are powered from the machine they connect to. That means no separate supply and no additional cabling, which is the intended design.

The consequence is that a module connected to a machine that is powered off may be inactive, so the switch may show the port as absent rather than as a powered-down machine. Better systems handle this and continue to show the port; simpler ones do not.

On machines where USB ports are unpowered when the system is off, the module cannot show the power-on sequence, which removes part of the value.

Some systems offer separately powered modules for exactly this reason, and they are worth specifying where seeing the boot sequence matters.

Check also that the machine's USB ports provide sufficient current - modules on a hub or on a port group that is current-limited can behave erratically.