Quote:
Originally posted by crazycliffy
could explain a bit more about this ???
what is line capacitance? bit changes ?
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Bit Change: One data I/O line = 1 Bit of data (aka bit, nybble, byte, etc...). Changing A Bit means shifting the value presented on a line from 0->1 or 1->0.
Line Capacitance: The wire between the black box and the RC is a "transmission line". Think of it like a distributed resistance and capacitance, like an R/C circuit. When you change a bit, you are changing the voltage on it. It takes real time to discharge or charge that line to the new voltage value. Using a high current line driver chip on the output of your black box gives it more "omph" so any bit transitions flip quickly, and not linger in the "between" state.
Look at the StangSense schematic in the White Papers. The 74HC125 chips are the line drivers. The diodes going to the DB25 connector allow the 74HC125 chips to only SINK current (pull down), not SOURCE it (pull up).
BTW something else I forgot to mention: Without a line driver chip, the wire between the black CAN act as a "radio antenna", picking up motor brush noise as data. Hard driving it with a line driver chip helps eliminate that. You should also consider using shielded wire between the black box and the RC, connecting the shield ONLY to the RC side's ground connection.
- Keith