TABLE OF CONTENTS SCSI, the Small Computer Systems Interface, has been in use on Macs since 1986 to connect hard drives, CD-ROM drives, scanners, printers and other peripherals to personal computers. SCSI is a parallel rather than a serial interface, so data transfer rates are quoted in megabytes/second (MB/s) rather than megabits/second (Mb/s).
| Note | It is easy to confuse these. Just remember that Mb means Megabits while MB means MegaBytes. The lower case b is for bits, and the upper case B is for bytes which, of course, are bigger than bits because there are 8 bits in a byte. |
There are actually three types of electrical interface used for SCSI connections: Single-ended, High Voltage Differential (HVD) and Low Voltage Differential (LVD). It is not normally possible to mix these types and you can even cause electrical damage if you connect an LVD device to an HVD device. The situation is slightly less serious if you connect a Single-ended device to an HVD device in this case the HVD device will simply disable its drivers. Single-ended and LVD devices can be mixed using the more recent multi-mode SCSI, although in this case the data throughput will drop back to the slower speed of the Single-ended devices.
The SCSI connector on the back of older Macs uses a DB-25 25-pin connector. This connects to the SCSI Controller on the computer s main logic board, which uses the original narrow SCSI standard. Some models such as...
