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

Jon Keeble ([email protected])
Tue, 30 Mar 1999 15:58:23 +1000

>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
>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
>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,

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