The Apple Color OneScanner 600/27 has two SCSI connectors. The connector you use depends on where the scanner is placed in the SCSI chain. Appendix C of the Color OneScanner 600/27 User's Guide contains information on the various locations in the SCSI chain, and what the SCSI termination should be set to.
Below is a chart providing the basic information provided in Appendix C of the Color OneScanner 600/27 User's Guide.
Scanner is the ONLY device in the SCSI Chain
============================================
Use the Macintosh SCSI System Cable. The Macintosh SCSI System Cable has a 50-pin connector on one end, and a 25-pin connector on the other. The 25-pin connector goes to your Macintosh computer, the 50-pin connectors goes to the Color OneScanner 600/27.
+-----------+ +-------------------------+
| | (1) (2) | |
| Macintosh |=================| Color OneScanner 600/27 |
| | (3) | |
+-----------+ +-------------------------+
(1) 25-pin Connector
(2) 50-pin Connector
(3) SCSI system cable
Scanner is at the Beginning of the SCSI Chain
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When you have multiple SCSI devices an the Color OneScanner 600/27 is placed at the beginning of the SCSI chain, you need two (2) Macintosh SCSI System Cables, and a SCSI peripheral cable for each additional SCSI device.
+-----------+ +-----------+ +--------+ +--------+
| | (1) (2) | | (1) (2) | SCSI 1 | (2) (2) | SCSI 2 |
| Macintosh |==========| 600/27 |==========| Device |==========| Device |
| | (3) | Scanner | (3) +--------+ (4) +--------+
+-----------+ +-----------+
(1) 25-pin connector
(2) 50-pin connector
(3) SCSI system cable
(4) SCSI peripheral cable
Scanner is in the Middle of the SCSI Chain
==========================================
If you have the Apple Color OneScanner 600/27 in the middle of a SCSI chain, you need two SCSI Systems cables and a SCSI peripheral cable.
+-----------+
| |(1) (2)+--------+(2) (2)+---------+ +--------+
| Macintosh |==========| SCSI 1 |========| 600/27 |(1) (2)| SCSI 2 |
| | (3) | Device | (4) | Scanner |========| Device |
+-----------+ +--------+ +---------+ (3) +--------+
(1) 25-pin connector
(2) 50-pin connector
(3) SCSI system cable
(4) SCSI peripheral cable
SCSI termination information is explained in the Tech Info Library article titled "Apple Color OneScanner 600/27: Terminator is not required".