Conference 7.286::fddi

    One is metal and plastic...the other is plastic and glass.  :-)
    
    Seriously though, the thinwire terminator is actually a 50 ohm (?)
    resistor that is connected to the end of a T-connector to properly
    terminate an Ethernet connection.  The FDDI loopback connector is a
    looped piece of fiber that connects the send and receive paths of a
    fiber optic cable.  The loopback connector is generally only useful for
    testing, FDDI doesn't require "terminators" for making connections. 
    
       - Larry 

    Hi,

>    Seriously though, the thinwire terminator is actually a 50 ohm (?)
>    resistor that is connected to the end of a T-connector to properly
>    terminate an Ethernet connection.  The FDDI loopback connector is a
>    looped piece of fiber that connects the send and receive paths of a
>    fiber optic cable.  The loopback connector is generally only useful for
>    testing, FDDI doesn't require "terminators" for making connections. 
>    
>       - Larry 


    are you sure? My H-8614 looks like H-8225 but behaves differently. It
    comes with the ThinWire port module to the DC500.

    Regards,
    	Torgny...

    Hmmm...sounds like the terminator in question may be used for our copper
    FDDI configurations.  Sorry, I misunderstood...perhaps Paul or someone
    else in the conference can answer your question.
    
       - Larry 

I don't know the details of that widget.  But it definitely is not a terminator
in the normal sense, since a loop test requires that the transmitted signal 
has to come back to the sender.  So it requires a mismatch to generate a
reflected signal.  Perhaps it's simply a short circuit, or it may be a resistor
different from 50 ohms.  Check with an ohmmeter.  Whatever you do, don't mix
up the two; they can't be interchanged.

	paul

    Hi,

    yes I noticed that they are different :-). I checked the with an ohm
    meter and they are both about 50 ohms.

    But why do we deliver to different parts that look so much a like that
    you can't tell them apart being a foot away? I guess the answer is
    money, and no one is looking at the over all cost.

    Regards,
    	Torgny...

    The difference seems to be that the FDDI terminator has a parallel
    capacitor across the 50 ohm resistor. The 50 ohm resistor is needed to 
    provide a DC path so that the looped back port can fake out the 
    cable detect circuit and allow the loopback test to run. This system
    detects the presence of the cable by looking for a DC current. If that
    was all there was to it you could use the same terminator as the
    Ethernet systems used. 
    
    The Thinwire FDDI is full duplex, which means that its transmit
    signal is "mixed in" with the receive signal from the port it is
    connected to. It is necessary to "subtract out" the local transmit
    signal in order to recover the signal from the other end of the cable. 
    The port does this by use of a bridge circuit. 
    
    When receiving its own loopback signal if the Ethernet terminator is
    used the bridge is so well balanced that the receiver can't find anything 
    to receive. So, the DC detection passes, but, no AC signal is received. 
    The capacitor causes the AC impedance of the terminator to be slightly 
    different than 50 ohms, so for AC signals the bridge is unbalanced, and 
    the receiver can now see its own signal, and pass the loopback test. 
    

So in short, the reason we deliver two different parts that look a lot alike
is that they perform different functions, and a single part would not be
able to do both things.  They look similar because they ARE similar, but
they are not identical and the existence of both is not a waste of money!

	paul

    RE : .-1

    you misunderstood me Paul, I was not implying that we only need one
    part. I fully understand the need for the two different parts .-2
    confirmed my guesses. The problem is that most people (I've asked
    around) will just toss the FDDI loop-back in the ThinWire terminator
    bin, and never know the difference, nor be able to find it again. Paint
    it red and everyone will be perfectly happy. (or blue if politics comes
    in :-)

    This is the same problem as with the fiber cable (do you use that
    expression in English?) It looks like any other copper cable on the
    floor. Nothing to indicate that you have to be a bit more careful with
    it.

    Regards,
    	Torgny...

Good point.  And in fact fiber cable often comes in orange jackets to help
distinguish it.  The same thing would help for the loop widget.

	paul