| Here's some further detail on 10BaseT link polarity as it relates to
connections between different types of products.
The 10BaseT standard talks about 2 types of connections or ports -- a
"Repeater" and a "Station". A repeater connection is internally
crossed; a station is internally straight-through. The thing to keep
in mind is that in an end-to-end connection, you need an ODD number
of crossovers. (Simply put, this is because you need a cross in the path
to connect receive to transmit.)
The standard 10BaseT cable is a straight-through cable. Thus, you can
use a standard cable to connect a repeater to a station; from the
above, the total number of crossovers is 1. You can't connect a repeater
to a repeater (or a station to a station) with a straight-through cable;
but you could with a crossover cable. Such cables are available. I
believe the DECconnect part # for straight-through cables (these
are marked with "=") is BN24F; crossover cables, marked with "X"
are BN26K or BN25G.
Finally, a note on how this relates to the DECbridge 900MX. Initial
protos were built with crossover 10BaseT connectors, like that of
a repeater. This meant that you needed to use a cross cable to
connect a bridge port to a repeater with a front-panel connection
(btw, when using a hub backplane connection, you don't need to worry about
any of this). Since it is normal to create a 10BaseT "LAN" by
connecting a bridge port to a repeater, we decided to change the
bridge connector in the shipping version to the straigh-through type.
As a result, you can use a straight-through cable. One might think of
this as a bridge "station" port representing a LAN's worth (10's nowadays,
sometimes 100's) of stations.
As an aside, some people who needed only 10Base ports would
use a MAU to convert the AUI port of the bridge (which is like that
of a station) to 10BaseT -- in this scenario, you again need a
straight-through cable to connect the bridge port to a repeater.
When using the DB900MX as a "private Ethernet" device -- that is, connect
a single station directly to a bridge Ethernet port so it can get a
dedicated 10 Mbps -- you need a crossover cable between the two
connectors.
Anil
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| >believe the DECconnect part # for straight-through cables (these
>are marked with "=") is BN24F; crossover cables, marked with "X"
>are BN26K or BN25G.
Change where the "X" and "=" are in the sentence and you got the
cables defined correctly for the most part. IE: The BN26K is
"=" straight-through, and BN24F, is a "X" cross-over cable. I'm not
sure what a BN25G cable is though.
I don't have a networks product book in front of me, but I do have
a BN24F and BN26K cable in front of me. :-)
Bob
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