Appendix C.     Termination


C.1.    Termination Diagrams

        Here are some termination diagrams about various configurations
        and their neccessary termination.

        Your host adapter may have a software-possibility to enable or
        disable termination. In this case, of course, you don't need to
        remove the resistor packs to disable termination.
        Also, with external device termination, you could also terminate
        the last external device internal, but that would limit your
        flexibility with them, so i suggest having all external devices
        not terminated and attaching an external active terminator to the
        last device's second connector.

        Only internal devices:


              ¿
        Ŀ     Ŀ     Ŀ
                          not                          ::::::: ſ
         Terminated       terminated      Termination> ... ... 
                                          Resistors  >   ...   
                          installed    
                                                                   
                                              Ŀ     ڿ           ٳ
                                                 

        Only external devices:

        Ŀ
                     ::::::: ſĿ
        Termination> ... ... Ŀ Ŀ
        Resistors  >   ...   Ĵ    Ŀ   Ŀ
                installed    Ĵ     not          Ĵ Terminated 
                                  terminated      through    
        Ŀ     ڿ           ٳ                     Ĵ <----      
                    
                                                       ^^External
                                                         Terminator

        Internal and external devices:

                       ¿
                           Ŀ
                                        ::::::: ſĿ
        Ŀ     Termination> ... ... Ŀ Ŀ
         not             Resistors  >   ...   Ĵ   not        
         terminated Ĵ            removed      Ĵ Ĵ terminated 
                                               Ĵ            
             Ŀ     ڿ           ٳ     
                                  
                                                       
        Ŀ                                  Ŀ
                                                       Terminated 
         Terminated                                 Ĵ through    
                                          External    Ĵ <----      
                              Terminator >


C.2.    How to identify a terminator

        If you get a terminator block from some device and no idea
        about what it is - passive, active, FPT or whatever special
        variant of active terminators, a few basic rules and an
        ohm-meter may be of some help.

        As stated in 4.3.4., you can try to measure the resistance
        from a SCSI signal line to any other signal line to detect the
        basic type of the terminator.

C.2.1.  Passive Terminators

        If you remember the standard passive termination circuit, it
        is clear that we get a rather complex resistor network for
        our calculation. But with only basic electronic rules, it's
        easy.

        In the terminator diagrams shown below, "#" means a resistor.

                   _______________________________
                   | | | | | |               | | |
                   # # # # # #  18 Signals   # # #     each 220R
        any signal-+ | | | | | ............. | | +-any signal
                   # # # # # #               # # #     each 330R
                   |_|_|_|_|_|_______________|_|_|

        Let's draw this a bit different and we get the following
        diagram:
                                                  220R    220R
           +--###--+--+-...-+--+--###--+   or   +--###--+--###--+
           | 220R  |  |     |  | 220R  |        |       |       |
         --+       #  # ... #  #       +--    --+       #34R4   +--
           | 330R  |  |     |  | 330R  |        |       |       |
           +--###--+--+-...-+--+--###--+        +--###--+--###--+
                  16*550R parallel                330R    330R

        The basic formula for resistors in parallel is (R1*R2)/(R1+R2),
        or R1/n for identical resistors, so 550/16 gives us 34.375 ohms

        The left part of this network (including the parallel resistors)
        can be converted by a rule called "Dreieck-Stern-Umwandlung" in
        german - translated word-by-word "Triangle-Star-conversion"
        (i don't know the real english name for it).
        Converted by this rule, we get the following:

                 +--###---###--+        The parallel resistors together
                 |  12R8  220R |        are 139.7 ohms according to the
          --###--+             +--      rule above, and together with
           124R3 |  19R2  330R |        124.3 ohms give us 264 ohms from
                 +--###---###--+        any signal to any other signal.

        So, you see, 264 ohms, if i didn't get my math totally wrong <g>

        Another method i got from Gary Fields' SCSI FAQ is measuring TP
        to Ground; a passive single ended terminator should give you a
        readout of 30.5 ohms, and a passive differential terminator
        should read 45 ohms.
        Active terminators will have much higher (or "overflow")
        readings, and if they show a value at all, should have different
        readings for both polarities.


C.2.2.  Active Terminators

        Detecting an active terminator by measuring resistance can be
        easy or impossible - depending on the circuit.
        If the recommended standard circuit is used, you will get a
        readout of 220 ohms from any signal line to any other signal
        line. But alas, this won't work with most of the newer
        single-chip-based designs - you may not get a reasonable
        resistance value as long as if the terminator is not powered.
        So, if you check a terminator and it is definitely not a passive
        one, and if you don't get 220 ohms, there are only some logical
        clues what it could be. Mainly these are only two crude
        rules-of-thumb:

        - if it has a LED, it is very likely an active terminator
        - if you measure both directions of your chosen reference
          connection and get different readings for the two possible
          directions, it is most likely an active or FPT terminator.

        Keep in mind that this may not always work. Active Terminators
        come in many flavors now, and some chip-based variants can not
        be detected this way, for the resistors are switched to the
        signal lines by driver transistors - if they are not powered on,
        they simply will have high resistance.

C.2.3.  Other termination circuits

        For FPT, i don't have any rule of thumb - i still don't know the
        exact circuit diagrams for it, and without this, it's hard to
        calculate anything....<g>


