From: Peterson, James F (FL51) (email@example.com)
Date: Fri Oct 22 1999 - 09:50:30 PDT
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,
Honeywell, Space Systems Division, M/S 934-5
13350 US 19 N., Clearwater, FL, 34624
From: firstname.lastname@example.org [mailto:email@example.com]
Sent: Friday, October 22, 1999 8:37 AM
Subject: [SI-LIST] : Zo for HCT
Can one find Zo of the 74HCT245 from: HIGH (typ): (Vcc-Voh)/Ioh =
= 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
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
the CPU on the mainbord is running 33 MHz clock.
Is there a better way to terminate tracks (74HCT245 can't drive an end
- e.g. choose an other device family and use end termination.
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