From: David Instone (firstname.lastname@example.org)
Date: Fri Jun 01 2001 - 01:53:51 PDT
The Fibre Channel specifcations require that the differential
impedance is 150 ohms. This is an interoperability requirement.
Thus, if your system you are designing is for your own use only you can
design for any impedance you wish.
However, if you intend to sell the product on the open market then the
customer connection points must be 150 ohm. Inside your product you are
free to do what you will. Note however that if any of the FC items
items in the product are replacable by the customer and available from
manufacturers other than yourself you may wish to make those interfaces
150 ohm, or specify that replacements should only be obtained from you.
A good example here is a disk drive, currently many manufacturers who
make diskdrive enclosures (JBODs) are designing their backplanes to be
100 differential, this has advantages as most multiway connectors tend
to be 100 not 150 ohm. However, the diskdrives are all 150 ohm, so
these manufacturers are qualifying drive manufacturers and model numbers
for use in their products.
Note that when the signal reaches the 150 ohm traces and terminator in
the drive the amplitude rises to very near the level it would have been
if the entire transmission path was 150. You might want to make sure
you have series/source termination in place on short runs to kill the
re-reflections on short interconnects ie backplanes. On 'your'
receiving end you can of course use 100 termination provided the
receiver is sensitive enough to take the 2/3 voltage, but as this will
only be an internal interconnect amplitude should not be a problem, and
you gain something from the lower loss of the 100 ohm interconnect. ( I
can see a whole new thread developing on that statement<g>)
There was a move sometime ago to make 100 ohms an 'official'
alternative. This was overwelming rejected as all installed cable plant
would be affected, addtionally the drive manufacturers did not want to
have to produce two variations of each drive. On the other hand chip
vendors who provide on chip termination are tending to provide
switchable termination impedances ( both source and far end), so
internally go ahead and design 100 ohms. BTW the new small format
pluggable modules are 100 ohm source and sink on the 'inside' interface.
Nekrylova, Julia wrote:
> Hi All,
> It was probably discussed before but here is my question:
> I noticed that Fibre Channel transceivers sometimes work with
> 100 Ohm differential pairs, other times with 150 Ohm pairs.
> If an application note talks about using 150 Ohm,
> does it mean that I have to stick with 150 Ohm, or I can go with 100 Ohm
> (using, of course,
> 100 Ohm matching terminations)? It seems to me, that p2p voltage amplitude
> will be smaller,
> but if diff pair is designed carefully, and the signal is above the
> threshold, it should not matter.
> Am I right?
> Thanks a lot,
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Dave Instone. Compliance Engineer Storage Systems Development, MP24/22 Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK. Tel: +44 (0)23-92-496862 (direct line) Fax: +44 (0)23-92-496014 http://www.xyratex.com Tel: +44 (0)23-92-496000
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