User`s manual

36
Chapter B - GDT User's Manual
SCSI Bus Width SCSI Mode Synchronous Data
transfer Rate
Number of
Participants
Maximum
Length
8 Bit, narrow Fast 10 MB/sec. 8 2.0 m
8 Bit, narrow Fast-20, Ultra 20 MB/sec. 4 1.5 m
16 Bit, narrow Fast 10 MB/sec. 8 2.0 m
16 Bit, wide Fast 20 MB/sec. 8 2.0 m
16 Bit, wide Fast-20, Ultra 40 MB/sec. 8 1.5 m
With regard to Fast-20 devices, the maximum number of participants and the maximum
cable length have to be strictly observed when a Fast-20 device (even if it is only one) is
running in Fast-20 mode. In any case , the minimum cable length is 0.5 m. In addition to
specifications mentioned above, the following should be kept in mind when selecting and
installing SCSI cables:
•
Always install SCSI cables that are as short as possible. The lengths in the table above
are absolute maximum lengths. (Total length of internal and external cables per cha n-
nel).
•
Avoid using SCSI cables with more connectors than actually needed. Never select a
SCSI mode or operate a SCSI device with a cable that is not appropriate for this mode.
•
The minimum distance between two connectors of a SCSI cable is 20 cm.
•
Avoid cable stubs. If this is not possible, keep the stub length below 10 cm.
Star cablings are not allowed.
•
Keep the number of transitions from flat to round cables and vice versa as small as po s-
sible. Best is to use flat or round cables, only.
•
Check these points when routing SCSI cables:
- Avoid kinks in the SCSI cable
- Do not roll the SCSI cable up on itself
- Avoid routing the cable next to other cables
- Avoid routing the cable in the vicinity of noise sources such as power supplies
- Avoid routing the cable over sharp edges and in areas where it could be catched
- Avoid routing/sticking the cable directly on metal surfaces
Following is a list of some manufacturers of high quality SCSI connectors and cables: 3M,
AMP, Amphenol, Fujitsu, Harting, Honda, Methode, Molex, Robinson Nugent, Y amaichi.
When making home-made SCSI cables, make sure that the insulation displacement co n-
nectors are properly aligned and firmly pressed into the flat ribbon cable. Otherwise, the
whole cable might turn out to be a big short-circuit. Furthermore, check carefully that PIN 1
of the cable connects to PIN 1of the connectors. A simple short-circuit and continuity test
before running the devices helps you to save time and money.
The same warnings as for home-made cables apply when you buy non-brand cables. If you
plan to run Fast-20 devices you should explicitly ask your dealer if these cables are appr o-
priate for the Fast-20 mode.