Scalextric Digital Car Decoding

(((<table><tr><td>[image||{UP}eyes.gif]</td><td valign=center>For those who just want the practical stuff, go to [SSD_Decoder|Digital Car Decoder] page</td></tr></table>)))

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===Digital Car ID Signal===
Beneath the car, situated just behind the car's slot guide is an Infra-Red LED (Light Emitting Diode). This LED is constantly broadcasting the car's unique ID and lane-changer command downwards to the track. Mounted into the track at strategic locations is an Infra-Red detector which receives the car's ID signal and decode it for use. The car id and lane change status is sent over 2,200 times per second.

The decoding is for two purposes
* Lane changing command (change lane or not)
* Car number (#1..#6)

The car constantly generates the appropriate signal, based upon its programmed Car ID and the status of the Lane Change button located on the hand controller. As the car passes over the track detector, circuitry associated with the detector triggers the desired response.

===Car Signal Format===
The car generates its signal with a nominal bit time of 48&micro;s, where a 1 state is LED on.
* The time between the start bits defines the car id.
* Padding bits are placed after the start bit, meaning that Car #1 is about 190&micro;s ((1 +2 +1) * 48&micro;s)
* Nominal cycle time for Car #6 is about 530&micro;s ((1 +2 +6) * 48&micro;s)
* The LED signal is ''inverted'' when the lane change button is pressed

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====='No Lane Change' Format=====
In normal (ie, non-changing mode) the car signal is thus.
* The LED is ''mostly off'' in this mode
* First sent is a start bit, always a 0 directly after a falling edge
* Second and third bits are padding, and carry lane changer state - always 0
* Extra spacing bits appear until the car id.
* Car Id is signaled with a high bit - a 1 state.
* Car Id is therefore dependant upon the ''spacing'' between each high bit.
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[imageright|Car with ID #3 signal - without Lane Change|{UP}SSD_Decoding%2fCarId3.gif]
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=====Format when Changing Lanes=====
When the Lane Change state has been selected from the hand controller, the centre portion of the signal becomes inverted. 
* The LED is ''mostly on'' in this mode
* First sent is a start bit, always a 1, directly after a rising edge
* Second and third are padding - always 1 when changing lanes
* Extra 1 bits appear until the car id.
* Car Id bit is a zero, and effectively marks the car id
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[imageright|Car with ID #3 signalling lane change|{UP}SSD_Decoding%2fCarId3Change.gif]
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{S:CallOut|YellowGreen|GhostWhite|Note that the timing of the car id output is ''not'' perfect nor exact. There is a small amount of jitter present (approx &plusmn;500ns) on the LED, which any detector circuit must take into account.}

====List of Car LED Signals====
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[imageleft|Car IDs 1-5 when ''not'' signalling lane change|{UP}SSD_Decoding%2fCarIds.gif]
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===Digital Car Decoding===

=====Decoding Car ID=====
Details on how to decode the Car ID are provided on [SSD_Decoder|Car Decoder page].

=====Decoding Lane Change=====
The decision to change lanes or not, can be based upon deciding if the are more zero bits than one bits, or vise versa.  The changer flipper should be set to change lanes when there are ''more'' one bits than zero bits. Exact timing of the Car Id signal is not as significant when decoding the lane change state.
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Scalextric Digital Car Decoding
