Quick Q regarding Ethernet

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I'm starting to look at building an ethernet switch for a specialist
application that precludes using an off-the-shelf switch, and I'm interested
in the Realtek RTL8309SB because having pulled apart a couple of switches
and looked at the reference designs it looks like I can do what I want
by just adding some magnetics and LEDs and nothing much else, and that
means that I don't have to learn too much about Ethernet, because this is
a one-off and I have no forseeable need for an in-depth understanding
of things Ethernet in the future.

However, I might want to connect two RTL8309s together so that I can do
smart things with redundancy on a vulnerable link by switching on or
off the interconnection.  I can do that in a number of ways that minimise
my learning curve, for example I could use a relay or a FSAL200 somewhere in
the interconnection link (I have a local controller on the LAN) to either
isolate the link or not.  That will do what what I want, and I don't have to
understand a lot of stuff about Ethernet to do it.

However, if it's worth it, I am of course prepared to go into things more
deeply, but I don't want to go off on a tangent that'll turn out to be
a dead end, so I'm asking for some guidance, particularly because I don't
really know what I'm looking for at this stage.

The RTL8309SB has a 9th port, called an MII port.  Can I use that port
to somehow generate another 4-wire ethernet signal?  Is this to do with using
the port in MII PHY mode or SNI PHY mode?

Alternatively, can I connect together two 8309s with this port and some
glue logic (but *not* a controller of some sort) to extend the number of
ports on my switch?

I'd appreciate it if someone could tell me if I'm on the right track
or not!


Trevor Barton

Re: Quick Q regarding Ethernet

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No.  MII is a management interface that is used for
status/control purposes.  It does not transfer Ethernet packets.

Grant Edwards                   grante             Yow!  Oh, FISH sticks,
                                  at               CHEEZ WHIZ, GIN fizz,
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Re: Quick Q regarding Ethernet
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Oops. I mis-spoke.  MII has two sets of signal lines, one for
the serial management interface, the other for data-frames
(it's a nibble-wide path, IIRC).  I was thinking that MII
referred to the management half and there was some other TLA
for the data half, but MII is both.

Grant Edwards                   grante             Yow!  Thousands of days of
                                  at               civilians... have produced
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Re: Quick Q regarding Ethernet
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MII = Media Independent Interface (or is it Interconnect?)

I'd chase this option first.  Grant's memory may be better than mine,
but IIRC, MII interfaces are intended for using an external PHY (e.g.,
fiber), and you may find its signaling more compatible for your
interests.  There should be some more details in the datasheet.

Some things to look into - at least Realtek's 10Mb chipset uses this as
a multi-function interface, so you'll probably have to select it as MII
mode.  Also, see if activating this port comes at the expense of
disabling anything else (like one of the other ports).

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There's a possibility you could connect them together, creating a trunk
between the 2 switches.

So, what particular failover scenario are you trying to achieve?  Maybe
there's a simpler path to your goal.

Re: Quick Q regarding Ethernet
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Thanks for the info, you and Grant both.  I'll have to look into the
MII interface more, then, seeing as it's not neccessarily a non-starter!

The failover requirement is that we have a scheme that involves two
independant LANs being carried from a subsea to shipboard enviroment.
They are independant because under normal circumstances we will use one
for high data rate monitoring from one transmitter on the seabed to one
receiver on the surface, and the other for overall control of a
number of subsea bits of equipment.  We want no other data on the
high speed link in normal operation.

However both links are vulnerable to disruption for one reason or another
and if that happens we want to be able to route the two LANs together to
bypass the break.  There are several of these dual hubs between the
deck and the last node, and any interconnect can be broken, so we need
to be able to crosslink at any hub if required.

An alternative is that the local controller in each hub could act as
an intelligent router if we fit them with two NICs, and we could tell
them to route between LANs if required.  There are advanteges and
disadvantages in both approaches, though.


Trevor Barton

Re: Quick Q regarding Ethernet
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A clever approach.  Sounds reasonably resilient.  I gather you're OK
with using a microcontroller, just not to forward data between the
switch chips?

This is actually a pretty slick chip!  It'd be worth your time to study
the datasheet - it's got some powerful features for a single-chip

1) Check out the Power-down and Isolate features (section 7.1.1) - this
is one easy way to handle what you want.  Via an external MCU, just turn
the failover ports on & off as needed.  Query other status registers to
decide when.

2) Even slicker - a one-chip solution.  If you use the VLAN features of
the switch (section 8.3.1) you can keep your 2 data channels separate
while using a single switch chip.  Simpler, higher-performance, much
better failover ability.

E.g., under normal circumstances, ports 1 and 2 (downlinks) are in
separate VLANs which only talk to ports 3 and 4 (uplinks),
respectively.  If any link fails, the MCU simply reconfigures ports to
map a spare uplink or downlink cable into the failed VLAN (the two
channels only need to be co-mingled as a last resort).  This could yield
2 uplinks and 2 downlinks, plus 2 failover ports in each direction -
you'd need to lose 3 uplink or downlink cables before you'd have to
combine the channels into one VLAN.

The only exposure is using a single switch chip (and single MCU).
Lastly, use the MII port for the MCU's Etherenet so it can report status

Re: Quick Q regarding Ethernet
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Yes, the microcontroller (actually a PC104 x86) controls video multiplexing
and power distribution as well.

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Thanks, Richard, a few good ideas to look into there.  The VLAN idea sounds
neat, but I think I'm going to run into problems with the number of
ports anyway, because I actually need 9 per side although I think I cna
squeeze it into 8.  Direct connection to the controller sounds interesting,


Trevor Barton

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