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Add a external SCSI CRW

Posted on 2000-03-24
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Now I have a SCSI Epson scaner working perfectly. It has its own SCSI card with a DB25 connector. On the scanner side there are two 50-pin centronics connector.

I want to add a external SCSI CR recorder. It is a Yamaha CRW8224S. It has two 68-pin connector.

What need I to have both working with the same SCSI card?

It is enought buing a cable to connect the scanner (50-pin centronics) to the CRW (60-pin wide SCSI) or need I to buy a new SCSI card?

Thanks in advance.
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Question by:elrichal
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jhance earned 400 total points
ID: 2652680
SCSI controllers support up to 8 total devices on the bus.  The controller itself is one of those so you can use up to 7 devices + the controller.

Every device on the SCSI bus has an address from 0-7, and the controller is usually address 7.  You scanner has some address from 0-6.  You must select an address for your CDRW other than whatever one your scanner is set to.

To determine this, check in the Windows Device Manager under the SCSI controller and see that the scanner's address is.

SCSI is very backward compatible so even newer 68-pin Ultra SCSI devices will work on older SCSI 50-pin connections.  They just fall back to the slower speeds that are possible on the 50-pin SCSI chain.

Yes, you can get a SCSI 50-pin to 68-pin adapter or cable and plug it into the same SCSI card as the scanner.  Just be sure that two things are true:

1) You use a different SCSI address for the CD-RW and the scanner.

2) That the SCSI bus is properly terminated.  It's possible that the scanner is internally terminated or there may be a small terminator device plugged into the 2nd SCSI port on the scanner.  This really depends on the scanner itself.  In any case, the terminator must be the last device on the SCSI bus.
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Author Comment

by:elrichal
ID: 2652798
jhance, I think your answer is good. But I wonder if I have to connect first the scaner or the CDRW drive.

Now I have the scanner connected. It has a id selector and a termination switch (it is on).

Should I to buy a 50 to 68-pin cable to put the CDRW as second device or should I put it first?
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Expert Comment

by:mark2150
ID: 2654200
Terminator needs to be at the end. Other than that SCSI doens't really care what order devices are in. Since you have device with built in terminator place it on farend of cable. If you don't want to do that then disable onboard terminator and buy a terminator that plugs into your cable and put *it* on the end.

M
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Expert Comment

by:jhance
ID: 2654433
As mark2150 said, the teminator should be the last physical device on the SCSI chain.

If you put the scanner last, you can use it's terminator, which would be convenient.  Otherwise, you have to disable it's terminator and put another terminator on the CD-RW.


The SCSI bus is a "straight-through" bus so all devices are literally connected together by the cable.  This is why it doesn't matter which is first, middle, or last, they all are treated equally.  The terminator needs to be last, however, to help ensure that the data signals on the SCSI cable stay clean.  If you put the terminator in the middle, you then have a hanging endpoint of the line and just like a transmission line, you get reflections of the data signals from the end of the cable and it bounces back and causes interference.  
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