barnett@im4u.UUCP (Lewis Barnett @ home on the range) (08/27/87)
I have a somewhat unusual and rather urgent request to make.
I need to find a 3Com 3C300 UE (Unibus Ethernet) interface.
This was, I believe, 3Com's first Ethernet interface, and has
the unusual feature of handling the collision backoff in
software. This made it something of a CPU hog, but it is
also uniquely suited for experimenting with data link level
protocols -- which is what I'm doing. I work with a testbed
of four machines with these interfaces, but I have recently
discovered that one of our interfaces is a pre-release version
with slightly different firmware. The difference causes the
early revision board to dominate the ether at high loads when it
resides on a network with the later boards. This skews
performance results. Therefore, I need to replace this board
with one matching the hardware revision level of the other
three boards.
The details: the pre-release board is revision PCB 267-01 Rev. E 21.4;
I need a board of revision level PWA 268-01 Rev. A or later.
I understand that pre-release revision PCB 267-01 Rev. E 21.7
is identical to this revision, so I'd settle for that.
I would need to board for a maximum of six months. I am willing
to trade the early revision board in the meantime. It works fine
under normal network loads (anything under about 150% offered load)
and does significantly better than a "normal" board in higher
loads.
The firmware change: Ethernet spec says that when attempting to
transmit, an interface checks carrier sense, and if it is clear,
waits 9.6 usec. then transmits, regardless of the state of
carrier sense. The earlier board checks carrier sense *again*
after waiting and before initiating transmission. This means
that when it is stuck in an environment with boards that follow
spec., it avoids a lot of collisions that the other don't.
If you have any of these boards and would be willing to do a
temporary trade, or know of anyone who might, please contact
me by email or phone [(512) 471-9708] as quickly as possible.
Lewis Barnett,CS Dept, Taylor Hall 2.124, Univ. of Texas, Austin, TX 78712
-- barnett@im4u.UTEXAS.EDU, barnett@im4u.UUCP,
{ihnp4,harvard,seismo,gatech,ctvax}!im4u!barnett