E-Mu ESI-32 Operation Manual page 210

Tip: Always use 50-pin Centronics-type
cables with the ESI-32 These cables have
much better electrical characteristics and
have fewer problems. Additionally, make
sure that all 50 pins of the cables are
actually wired. Some SCSI cables have only
half of the pins wired.
!
Caution: Turn all devices OFF whenever
you change the SCSI cable. Serious damage
to the ESI-32 may result from connecting
and disconnecting SCSI cables with the
power on
In addition, devices left on when the cable is
unplugged may not respond properly until
powered off and on.
202
ESI-32 Operation Manual
The following is a list of the default SCSI ID numbers:
ID - 7 (fixed) Macintosh
ID - 6 Unassigned
ID - 5 ESI-32 (this number can be changed)
ID - 4 Unassigned
ID - 3 Unassigned
ID - 2 Internal Syquest 270MB Drive (if installed)
ID - 1 Unassigned
ID - 0 Internal Macintosh HD (if installed)
Types of SCSI Cables
There are two basic types of SCSI cables in use: DB25 and 50-pin
Centronics. The two types of cable can be identified by their connectors,
which are quite different. When buying SCSI extension cables, it is
important to choose ones with the right type of connectors, since the
two types will not interconnect without a special adapter. The type of
SCSI connectors on the Macintosh are DB25 connectors. These are the
type commonly found on the back of personal computers. The SCSI
connector on the optional SCSI port is the 50-pin Centronics type. Use
only high quality shielded cables that have all the pins connected. Many
cables only have some of the pins wired.
Terminating SCSI Cables
The total length of the SCSI chain should not exceed 15 feet. A general
rule for SCSI cables is: The Shorter the Better!
Terminating resistors or terminators are special resistor packs that are
installed on the first and last device in the SCSI chain. Terminators
are used to reduce line echoes or standing waves on the SCSI bus. An
analogy in an audio system would be impedance matching.
Terminators can either be hidden inside a SCSI device or they can take
the form of termination blocks which can simply be plugged into the
SCSI port.
Terminators apply power to an array of resistors which ensure a full 5
volt swing between high and low levels on the SCSI line. The host
system is required to provide Termination Power. Termination power
has its own wire in the SCSI cable and supplies power to the termination
block. On external hard disk drives, SCSI terminator power is almost
always turned Off and the drives are left un-terminated.
The ESI-32 is shipped with termination power On with the termination
resistors left in place. This assumes that the ESI-32 will be placed at one
end of the SCSI chain. The ESI-32 has an automatic system which
ONLY supplies terminator power when it is NOT being supplied by
another device. Therefore, you don't need to worry about terminator
power when connecting the ESI-32.
The more devices you have connected to SCSI and the longer your
cables are, the more it becomes important to have the cables properly
terminated.