Re: [SI-LIST] : Termination for bi-directional lines

Dennis Tomlinson (det@tellabs.com)
Wed, 17 Jun 1998 09:49:02 -0500

Poulet P. wrote:

> Dear Members,
>
> I have read and found a lot about the different techniques in use to
> terminate unidirectional line ( series, parallel, ac, diode, etc.). I
> would like to find more specific information about terminating
> bi-directional lines.
> We need to interconnect two PCB and the transceiver characteristics on
> each board are different.
>
> Any reply will be greatly appreciated.
>
> Thanks in advance,
>
> --
> Philippe Poulet
> Hardware Engineer
> RICOH Corp
>

Hi Philippe,

Let me throw my $0.02 in here, and perhaps stimulate other responses.

If you're fortunate enough to have these bi-directional lines be
point-to-point, you can series terminate both ends. This allows
you to use different terminator values at either end to match your
differing transceivers to the line. You'll suffer a slight increase in
apparent flight time due to an R-C time constant at each
receiver. Parallel termination techniques can also be used, but at an
increase in power dissipation.

If your nets are multi-point driven, then the choice of termination technique

depends on whether or not pulse monotonicity and first incidence switching
are required. If so, then you will need shunt termination at both extreme
ends of the line, and the appropriate driver strengths. Included among these
are shunt (simple R to either power rail, or to a termination voltage),
Thevenin (an R to both rails), or AC (series R-C to ground).

If monotonicity and first incidence switching are not required, then I'd use
series termination. There are some 8 wide and 16 wide drivers/tranceivers
with series terminators (roughly 22 Ohms) built in for this sort of
application.
This can result in some ugly signals, but sometimes ugly is good enough.
(And,
good enough is perfect).

Diode termination, though helpful in some circumstances, is a rather rude
termination technique. There was a thread about this technique on the SI-LIST

a few weeks back.

Other inputs?

Cheers,

Dennis

> ------------------------------------------------------------------------
>
> philippe Poulet <poulet@fax.ussj.ricoh.com>
> Hardware Engineer
> Ricoh Corporation
>
> philippe Poulet
> Hardware Engineer <poulet@fax.ussj.ricoh.com>
> Ricoh Corporation
> 3001 Orchard Pkwy Work: (408) 944-3347
> San Jose Fax: (408) 434-5390
> CA Netscape Conference Address
> 95134 Netscape Conference DLS Server
> USA
> Additional Information:
> Last Name Poulet
> First Name philippe

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