RE: [SI-LIST] : ATA source termination

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From: Farrokh Mottahedin (Farrokh.Mottahedin@quantum.com)
Date: Fri Apr 28 2000 - 11:14:59 PDT


I suspect you are looking at a down-level ATA spec. There are different
source termination recommendations for different signals. The device
termination impedances are not the same for all signals due to differences
in I/O cell design. For bi-directional signals such as the data lines, the
recommendation is 33 Ohms at host and device ends (it used to be 22 Ohms
prior to Ultra 33 and 66). For uni-directional control signals, the
recommendation is 33 Ohms at the sender and 82 Ohms at the receiver.

I hope this helps.

Farrokh Mottahedin

Quantum Corp.
500 McCarthy Blvd.
Milpitas, CA 95035
(408)324-7934
farrokh.mottahedin@quantum.com

-----Original Message-----
From: Rehm, Dennis [mailto:Dennis.Rehm@lsil.com]
Sent: Friday, April 28, 2000 10:00 AM
To: 'si-list@silab.eng.Sun.COM'
Subject: [SI-LIST] : ATA source termination

I am coming up to speed on the ATA spec and have found a point where the
spec seems inconsistent.

Regarding the source termination resistors at a device, I quote from D.2.2.6

"Ideal termination at each connector is when the impedance seen looking back
toward the source matches the cable impedance in the forward direction. For
devices, this means that the sum of driver output impedance and termination
resistance match the cable impedance(typically 80 to 85 ohms), minus 5 to 10
% to allow for attenuation due to the capacitive loading of other devices on
the cable."

However in Table 6 of section 4.2.2.2 the device termination impedance
specified varies from 22 to 82 ohms. I assume that 82 represents matching to
the cable impedance which would actually disagree with D.2.2.6 above as you
should match to 5 -10 % less. To be consistent with D.2.2.6 this table
should have all entries be the same at about 70 to 75 ohms. But I guess I am
missing something.

Dennis

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