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Re: Addressing a SCSI film-scanner - SUCCESS!



Tim wrote:
> On Wed, 2006-03-08 at 13:49 -0600, Bruce Bales wrote:
>> I kinda hate to jump in here where I haven't been in the loop before, but 
>> unless scsi has changed in the last few years, as I remember, most scsi 
>> devices have two connectors.  Either can be input and the other then is 
>> output.  The scsi bus is serial - up to seven devices in a row.
> 
> It's a parallel bus, not serial.
> 
>> The last device has a terminator plug on it which electrically terminates
>> the data lines with resistors.
> 
> Or some other method (doesn't have to be a plug), and it doesn't have to
> be the last device, and it doesn't have to be resistors as such (active
> termination isn't just resistors).
> 
It does not have to be the last device if you use a terminator after
the last device, but both ends of the cable need to be terminated.
You should not put a terminator in the middle.

Active termination it better then passive (resisters). It also works
better if you do not mix termination type. Last but not least, if
you are using separate terminators, and not the last device in the
line, you have to make sure that one device is providing termination
power to the SCSI cable. (It is not a good idea to have more then
one device doing this.)

> It's the transmission line (the cable) that needs terminating (at both
> ends), not the devices.  You can plug anything in anywhere in between,
> so long as the ends are properly terminated.
> 
> e.g. term ------ plug ------ plug ----- plug -------- plug ----- term
>                  HDD         host       burner
> 
> 
>> The other end of the bus is terminated on the scsi card.
> 
> It's important to check that it does.  Some cards do not terminate the
> line, you have to take care of that yourself.
> 
Also, some cards will terminate automatically, while others need a
jumper. This can be a problem if you have internal devices, and you
sometimes plug in external device. With both internal and external
devices, the controller card is in the middle of the bus, and should
not be terminated.

It gets even better when you have a controller that supports both
narrow and wide SCSI connections. You may not be able to use both
narrow and wide internal devices and external devices at the same
time. (Or you may only be able to terminate two out of the three
cables.) Plus the narrow cable does not terminate all of the wide
bus, so the controller may have to provide termination for the rest
of the bus. Then there is differential SCSI...

Mikkel
-- 

  Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!


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