RE: [SI-LIST] : Zo for HCT

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From: Peterson, James F (FL51) (jfpeterson@space.honeywell.com)
Date: Fri Oct 22 1999 - 09:50:30 PDT


Johnni,

IRT : Driver source impedance (Rs) :
Your numbers look close, but I believe they're based on DC parameters and
when you're talking about Rs you need to base it on AC parameters. If you
have the IBIS model and a simulator you can derive Rs :

Rs = ((dv*Zo)/Vi)-Zo
dv = change in voltage
Zo = impedance of t-line
Vi = the initial step launched onto the t-line

Using my simulator and ibis model :
                Rs = ((5V*51ohms)/3V)-51ohms = 34 ohms

Note : the ibis model was for a TI 74HCT245 (measured not spiced!)

IRT your other question on what's the best termination scheme : as most
people on this reflector will attest - it depends on your topology...

hope this helps,
Jim

Jim Peterson
jfpeterson@space.honeywell.com
Honeywell, Space Systems Division, M/S 934-5
13350 US 19 N., Clearwater, FL, 34624
727-539-2719

-----Original Message-----
From: jfr.gaab@velux.com [mailto:jfr.gaab@velux.com]
Sent: Friday, October 22, 1999 8:37 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Zo for HCT

Hi all,

Can one find Zo of the 74HCT245 from: HIGH (typ): (Vcc-Voh)/Ioh =
(4.5-4.22)/6m
= 30 ohm,
LOW (typ): Vol/Iol = 0.16/6m = 26.67 ohm ?
Is it possible to use the Zo of the 74HCT245 as source terminator for a
trace of
30 ohms ? Is it wise ?
Should one choose an other trace impedance ?
Tracks are on a daughter card and will be both point to piont and point to
multipoint (databus),
the CPU on the mainbord is running 33 MHz clock.

Is there a better way to terminate tracks (74HCT245 can't drive an end
termision) ?
- e.g. choose an other device family and use end termination.

Regards,

Johnni

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