speed of data bus

HI,
I am revising for an avionics exam and I have a question that is probably quite simple.
In my notes, I have two diagrams of data stream formats: bi-polar RTZ
format and Manchester bi-phase.
The two graphs showing voltage with time have different periods, the RTZ fromat has a bit-time of 10us (on an ARINC 429 databus) and the Manchester format has a bit time of 1us (on a MIL-STD-1553B databus). Is the bit-time difference related to the format or the type of databus?
Also, can anyone explain why a manchester bi-phase formatted signal needs a higher bandwidth than an asynchronous communictaion?
When I think of the bandwidth, I am guessing that one oscilation is represented from the voltage going from one value to another and back to that value. I think the frequency of the RTZ format is fixed because the voltage returns to zero at the end of each bit, but the Manchester format has two frequencies, imagining two logic 'ones' having a frequency, and a 'one' and a 'zero' in succesion having half of that frequency.
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Adam Chapman wrote:

The type of databus -- bit timing and bit format are more or less independent.

What sort of asynchronous communication? Bit for bit, Manchester encoded data would need a higher 'bandwidth' than NRZ data; since nearly all formats that are called 'asynchronous' are asynchronous NRZ, the statement is generally true, if not true in general.
Try looking at 8 bits worth of Manchester encoded data, and the same data encoded using asynchronous serial (don't forget the start and stop bits!). See which has fewer transitions.
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Tim Wescott
Wescott Design Services
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