ADDER CCS4-USB Specifications download pdf (Page 5)

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What is True Emulation?

TrueEmulationrepresentsasignicantbreakthroughinsharingUSBdevices

between two or more computer systems. Until this point, the problem has been

how to create a USB switch that provides all of the following:

• Quick,transparentandreliableswitching,

• AccuraterepresentationoftheconnectedUSBkeyboardandmouse,

• SwitchingcontrolviatheconnectedUSBkeyboardand/ormouse.

The difﬁculty in achieving all of the above requirements has been due to the

complexity of the USB standard. This has led to various problems that have

spawned a number of possible solutions.

Enumerated USB switching

The earliest attempts to switch USB devices applied a relatively ‘hands off’

approach.EnumeratedUSBswitchesaretheelectronicequivalentofthoseold

mechanicalKVMswitcheswithalargeknobonthefront.

EnumeratedswitchesaresocalledbecauseaconnectedUSBdevicewillbe

required to perform a full initiation (a process called Enumeration) every time it is

switched; just as if you had pulled out the plug and then reconnected it.

EnumeratedswitchessimplypassallsignalsstraightthroughbetweentheUSB

device and the computer, they do not attempt to interpret any data. For most

devices, this offers an advantage because the switch just leaves them to get on

with their jobs without any interference or any hit on performance. However, it

means that a USB keyboard or mouse cannot be used to control the switching

process - a quick and simple control method expected by most users. Reliability

of switching is also an issue that has plagued enumerated switches, especially

when used with certain USB devices and particular operating systems.

Emulated USB switching

The issues with interpreting the complex USB data streams and recreating (or

Emulating) the identity of attached USB devices were eventually solved, leading

tothecreationoftheEmulatedUSBswitch.

A neat side effect of the technique used is that each computer can be fooled

into thinking that the USB device is permanently connected to it, even when

the device is switched to another computer. This means that the enumeration

process for the USB device takes place only once, during the ﬁrst power

on. After that, a computer merely sees a dormant version of the USB device

whenever the device is actually connected to a different computer.

However, it remains a complex task to dynamically assume the identity of a USB

device, distribute it among the connected computers and maintain all of the

necessary signals, states and processes. Therefore, manufacturers have previously

relied upon a ﬁxed keyboard and mouse proﬁle that is declared to each

computer, regardless of the actual connected devices. This precluded the use of

any special keyboard or mouse features over and above the standard layouts.

True Emulation

Mindfulofthelimitationsassociatedwiththe

previous USB switching techniques, we set about

creating a more effective and elegant solution.

After a great deal of research and development,

True Emulation is the result.

TrueEmulationallowsthecompleteidentityof

the keyboard and mouse to be copied and then

presented to all of the connected computers.

This means that any keyboard offering specialist

function keys or any mouse with extra features

will be fully supported at each computer. As with

the previous emulation method, the unselected

computers will continue to see the identities

of the keyboard and mouse, which means that

no enumeration is necessary when their link

becomes active once again. This not only helps

to speed up the rate of reconnection, but also

raises the reliability of switching because USB

links are at their most vulnerable during the

enumeration process.

TrueEmulationreliesuponahighspeedcircuit,

calledanEmulationEngine,tofullyemulate

the USB device identities and also interpret

keyboard and mouse data streams. The result is

fullsupportforKVMswitchingcontrolviahotkey

pressesorthethirdbutton/scrollwheelofa

mouse.

TrueEmulationisnotnecessarilyrequiredby

other USB devices, which is why you will also

ﬁnd two enumerated circuits included (shown in

greenwithintheblockdiagram)alongsidetheTrueEmulationfeature(shown

in blue). This allows those other USB devices to operate at their highest speeds,

without any intervention. The enumerated circuits beneﬁt greatly from the

USBHubsthatarejointlyusedwiththeTrueEmulationsystem.Becausethey

interface directly and permanently with each computer, they help to stabilise the

dormant links, making errors during enumeration much less likely.

ThedualswitchingarrangementprovidesfurtherexibilitybecausetheTrue

Emulationandenumeratedsectionscanbeswitchedinunisonorindependently

of each other, as required. Thus, your various peripherals can operate with

different computers at the same time. Please see software release notes for a list

of supported products.

HOST

CONTROLLER

USB

KEYBOARD

OTHER USB

DEVICE

USB

MOUSE

USB

HUB

USB

HUB

USB

HUB

USB

HUB

EMULATION

ENGINE

The emulated section of

the switch is shown in

blue and handles only the

keyboard and mouse. The

green enumerated section

of the switch handles other

USB devices and also uses

the USB hubs to link with

the computers.

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ADDER CCS4-USB Specifications Page 5

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