[SI-LIST] Re: AC coupling of SATA and SAS I/O
- From: "Ward, Richard" <richard.ward@xxxxxx>
- To: Alief <saifj_m@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>, "T.K. Jeon" <tkjeon@xxxxxxxxxxx>
- Date: Thu, 7 May 2009 16:48:54 -0500
Hi Alief,
My view is a little different from that below.
AC caps aren't used to set the Rx bias, but to allow the Rx to set it's own
bias point. This is especially useful for rack-to-rack systems where the common
mode cannot be well controlled. It also has the advantages of:
- allowing VTT referenced and GND referenced systems to work together
- allowing a single Serdes to cover multiple standards
- allowing newer (restricted supply) devices to work with legacy devices
- ... (I'm sure someone on this list has written a book or two on this...)
The hot-swap ability doesn't really demand dc blocking caps, but they give
advantages. "hot-swap" is a bit of a nebulous term too, often.
One of the risks of hot-swap is shorting the connections (signal-signal or
signal-supply). Some devices will be designed to withstand the short-term
current only (not long-term lifetime degradation). Having the caps means a
long-term effect, actually becomes a short-term one, therefore EM
considerations are less restrictive on the circuit design side.
I believe PCIE defined them at the Tx for this (shorting) reason (whereas a
majority of non-pluggable applications put them at the Rx side, why? well it
depends).
Regards,
Richard
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Alief
Sent: 07 May 2009 12:41
To: si-list@xxxxxxxxxxxxx; T.K. Jeon
Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O
Thank you for your response T.K.
Yes, a 0V DC bias would establish an automatic reference (in theory); thus it
would be akin to DC coupling.
However, I am hesitant to make my common mode 0V; because of issues with ground
loops, ground path parasitics, etc and also because the receiver would see
negative voltages.
I realize that this is to first order a biasing issue and allows for
interoperability.
However, are there other considerations ? I have heard/read that it might
permit hot swappability. Is this true. If I had a known common mode level
doesn't that make it easier for hot swapping ?
Also, are there other issues which AC coupling helps with ?
Thanks a bunch in advance,
Alief
--- On Thu, 5/7/09, T.K. Jeon <tkjeon@xxxxxxxxxxx> wrote:
From: T.K. Jeon <tkjeon@xxxxxxxxxxx>
Subject: RE: [SI-LIST] AC coupling of SATA and SAS I/O
To: "Alief" <saifj_m@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx"
<si-list@xxxxxxxxxxxxxx>
Date: Thursday, May 7, 2009, 6:55 PM
Alief,
AC coupling is needed to maintain the correct DC bias for receivers. If your
transmitter has 0V DC bias, then I guess you don't have to implement AC
coupling.
TK
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Alief
Sent: Thursday, May 07, 2009 10:46 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AC coupling of SATA and SAS I/O
Greetings everyone,
I am currently looking at SATA and SAS SerDes designs. It looks like the data
transmission is AC coupled. Is this a standards requirement ?
Could anyone enlighten me on why SATA/SAS specifies only AC coupled links [if
that is in fact the case]; i.e. what was the thinking/justification for
restricting transmission to ac coupled only.
Also, do the other standards : Gbit Ethernet, PCI Express, RapidIO, etc. have
similar restrictions/requirements ?
Thanks v much in advance,
Alief
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