Re: [SI-LIST] : RE: [SI-LIST]: Long bus or star?

Shayle Hirschman (shayle@mho.net)
Tue, 30 Mar 1999 00:16:40 -0600

John --

What a great idea to series source each leg at the hub! Very, very nice.

Shayle

At 03:58 PM 3/30/99 +1000, you wrote:
>
>>
>>What I realize here is that you will have a reflection that hits one of the
>>devices at the end of the 3" trace. It will reflect all the way back to
>the
>>other 4 devices, which are each 6" away. Because this length is considered
>>"long" it will cause problems, no?
>
>No. If the star is being driven at the hub (this an address bus, right? not
>a bidirectional bus?)
>then the hub is a low impedance, and any returning waves will be mostly
>attenuated, so will pass through to other spokes.
>
>Seems to me that a classic series source terminator of value Zo at the hub,
>one for each spoke, is optimum. It halves the required drive, and turns the
>voltage doubling at the open circuit at the end of each spoke to advantage.
>The reflections witl be fully absorbed back at the hub in the resistors.
>
>>
>>So, is everyone basically saying that you should always *try* to daisy
>chain
>>devices, no matter how long the chain is? (Incidentally, I made a decision
>>to keep the daisy chain and terminate it).
>>
>>Thank you all for your help,
>>
>>Chris
>>
>
>
>
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