[SI-LIST] Re: Why we need to use "Series resistor" at Transmitter?

> Jim,
>=20
> In regard to your comment: "Series termination
> shoulkd NEVER be used wher the path is going to more
> than one input i.e bus configuration.".  The ATA/IDE
> interface used in PCs is series terminated at the
> source and is bussed to two loads on data and
> strobe lines.  The number of working systems in the
> field is in the millions (every PC that has 2
> drives).  The value of the series termination is a
> great consideration (in ATA it is 22-50ohms).
>=20
> So series termination in busses can work.  Though, I
> recommend a thorough understanding of the system
> before employing this technique.
>=20
>=20
> Charles Hill, consultant

I concur.  In theory, I agree that it's not "nice", but I've seen series te=
rminations (with no parallel termination at the far end) used successfull=
y in designs where there were multiple inputs on a trace.  "Never" sounds=
 neat but as with most good rules, there are exceptions.

I also agree with the caveats.  In the examples I remember, the system was =
running at a low clock frequency and all the inputs (not more than 3 in t=
hat case) were close to each other at the end of a long trace.  It was of=
 a daisy-chain topology, i.e. there were no branches on the trace, to min=
imize the impedance discontinuities and reflections.  The inputs had high=
 impedance.  Of course, one gotta think twice, even thrice, before doing =
this on a clock signal.

Sandor

---
Sandor Daranyi
Snr Design Engineer
Aristocrat Technologies Australia



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