[SI-LIST] Re: Why we need to use "Series resistor" at Transmitter?
- From: "Sandor Daranyi" <sandord@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 29 Oct 2002 11:10:30 +1100
> 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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