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Attaching devices to a scsi bus (transmission lines related)?



This is a question about attaching devices to a scsi bus which is transmission 
lines related.

As we all know a SCSI bus has to be terminated on its 2 end points. I guess 
that a classical situation is the following:


     (one end termination)
               |
         (1st device)
               |
               .
               .
               .
               |
         (n-th device)
               |
     (other end termination)


Further more there is a narrow (50 wires) SCSI bus and a wide (68 wires) SCSI 
bus. (in the above picture, the bus is represented by the vertical bars and 
dots that are connecting the verbal devices in parenthesis.)

Now suppose my bus is narrow (50 wires), and my n-th device is wide (68 
wires). Basically the SCSI standards allow me to have this wide device 
connected to the bus. However, must I or must I not have the high byte of the 
wide device terminated on both sides? Pictorially, should the situation be 
like this:


Situation A:

               |
         (nth device, WIDE)-(a termination only for the high byte)
               |
     (other end termination, now for all the lines of the wide bus)


 or like this:


Situation B:

               |
         (nth device, WIDE) (simply ignoring the other lines)
               |
     (other end termination, now for all the lines of the wide bus)


For those who are not familiar to SCSI, a rough explanation for this high byte 
are all the lines that are found in the wide bus (68 lines) but not on the 
narrow bus (50 lines), supposedly 18 = 68 - 50 lines.

On one hand, situation A fully adheres to the SCSI standard. Each portion of 
the SCSI bus gets it 2 ends terminated. But on the other hand, isn't the n-th 
device a source of radiation which for all practical usage get loaded by the 
other end termination and does not get loaded by the narrow bus at all?

Please bear in mind that although I wrote `for all practical usage', the 
system should be a fully reliable one.
-- 

	Shaul Karl <shaulka@bezeqint.net>



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