VME320? (fwd)

[Elton Smith <elton@jlab.org>]

---------- Forwarded message ----------
Date: Tue, 4 Sep 2001 14:50:01 -0400
From: Fernando J. Barbosa <barbosa@jlab.org>
To: Elton Smith <elton@jlab.org>
Subject: VME320?

Hi , Elton. At our last meeting, we briefly discussed the possibility of
using dedicated pins for distribution of the reference clock, trigger,
syncreset and start signals in differential mode. The TDCs will also have
these input pins present on the front panel for use in systems where the
backplane implementation is not required nor feasible.

    Standard VME backplanes have 64 pins of User Defined pins on P2/J2.
Some of these could be used for signal distribution by means of wired
piggy-back connectors, as opposed to designing a special backplane.

    VME64x adds 46 User Defined pins to P2/J2 and has standard provisions
for an additional connector (P0/J0) between the P1/J1 and P2/J2
connectors. This connector adds 95 User Defined pins. Boards using the
P0/J0 connectors cannot be installed on standard VME backplanes as there
is a bar for support and rigidity of the backplane in this location. The
piggy-back connections mentioned above can also be implemented on this
backplane.

    The most recent VME standard is called VME320 for 320 Mbytes/sec
transfers. VME320 builds on VME64x with two distinct enhancements:
Synchronous Source Termination (SST) and "lumped" backplane layout. Two
edge transfers are also implemented as on VME64x. The traces in VME320
(a.k.a. 2eSST VME) are routed from slot 1 (master) to slot 11 and then
from slot 11 to all other slots (2 through 21). This technique doubles the
speed on the backplane as compared to earlier VME standards as those
backplanes had traces routed from slot 1 to slot 2, from slot 2 to slot 3
and so on. Because of the reflective nature of those earlier backplanes, a
signal from slot 1 to slot 2 could only be qualified after the signal
propagated to slot 21 and back to slot 2. The SST doubles the speed once
more to 320 Mbytes/Sec because transmission is synchronous and does not
require the handshake overhead required for master/slave acknowledgements.
VME320 backplanes are already available.

    I suggest adopting VME320 as a standard for Hall D, as well as, a
standard for future upgrades at the lab. We should also discuss signal
distribution schemes for backplanes and systems in general at our next
Hall D TDC meeting.

    Best regards,
        Fernando
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