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MITTENTITEL |
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Zeiten und Spannungen, nominal und real. Die angegebenen Werte wurden alle gemessen. D.h. nicht, dass die Werte bei allen Bahnen gleich sein müssen. Es kann kleine Abweichungen geben. Ein Abfrageintervall von 102ms ist möglicherweise nominal (sozusagen sollte eigentlich) ein 100 ms Signal sein.
LSB und MSB Wir sind es gewohnt, dass die höchste Stelle bei einer Zahl links sthet. Das entspricht der MSB Anordnung. Bei der LSB Anordnung sind die Bits genau anderherum angeordnet, also zuerst die niederwertigsten Bits. Das heisst MSB 101110 entspricht LSB 011101. |
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Physical level - terminals The SCX System is a two wire system. just as all other current slot car systems. The tracks have an additional wire, but that ist power supply only and meant to power additional devices without influencing the cars. With an analog system the voltage betwee both wires is varied and the voltage change results in a rpm change of the motor and that in a speed change. Digital systems use a constant voltage between the wires, SCXl uses 18V. As the voltage is not available as a control method for the speed, the cars mut be informed in a different way. And that different way are data packets that are sent over the same two wires, which are used to supply the energy for the cars. To transmit the data packets the 18V line ist switched on and off in a certain rhythm comparable to a morse code. The image below shows what it looks like.
You can see 9 groups up pulses. Each group composes a data word. A data packet consists of 9 data words. The distance between data words varies slightly, the total duration of a data word is about 1.57 ms. data words a serial word using NRZ (Non return to zero), that's just a fancy name for what you know from PC's serial interface. The baudrate is 115200 baud so a bit changes every 8,68µs.
On the PC the serial sigal is a logic 1 when idle. Each datword begins with a startbit. That's just a logical 0 with the length of a bit. But SCX is different the startbit consists of 1.5 logical 0 followed by 0.5 bist logical 1, so it's 2 startbist so to say. That's not standard but good news, because you can always detect a startbit, because it has always a 1.5 bit length logic 0 portion. Whe you do your own projects keep in mind that the 1.5 long startbit does not fit into the ordinary bit grid and might fool serial interface circuitry. The startbit is ollowed by 8 data bits. They are supposed to have the same length, but because of driver asymmetries ther 0 phases are a bit longer than the 1 phases. But that's no problem. In the following text I will use hexadecimal numbers. I mark them with a leading $. If you do not know what that is please use google or wikipedia to look it up. The first data bit is the LSB (least significant bit). You will have to switch the order of the bits to get the logic value. The sample aboove will be binary 10101010 oder hexadecimal $AA oder decimal 170 or in the next image:
1110000 or $F0 After the las data bit there is a stop bit which is always a 1 so it can be distinguished from the next start bit. |
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Physical level - cars When crossing the finish line or enteringthe pit stop the car will send its id. The car will do so by sending a data packet made of 4 bytes.
It's a NRZ, LSB word with 57600 Baud. The startbit has 1.5 bit length 0 phases and 1 bit length 1 phase.
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