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Downward Compatibility Overview
Definition of Downward-Compatible
The general definition of a "downward" or "backward"-compatible product
is one that is compatible with both current and earlier versions of the product and
technology. Specifically, if a downward-compatible product is operating in a system
containing components designed for use with an earlier technology, the specifications that
applied to the earlier product and technology would again apply.
An example of this situation occurs in applications which integrate Low Voltage Differential (LVD)
products. These LVD products are designed to operate in an LVD mode OR are capable of automatically
switching to the older, Single-Ended (S.E.) technology. When an LVD-capable product is used in a
S.E. application, it is considered to be operating in a downward-compatible capacity.
SCSI Types and Specifications
|
SCSI Type |
Bus Speed
max. MB/sec |
Bus
Width |
Max Bus Length (meters) |
Max. Devices
Supported |
|
S.E. |
LVD |
HVD |
| Narrow |
5 |
8 |
6 |
- |
25 |
8 |
| Fast |
10 |
8 |
3 |
- |
25 |
8 |
| Fast
Wide |
20 |
16 |
3 |
- |
25 |
16 |
| Ultra |
20 |
8 |
1.5 |
- |
25 |
8 |
| Ultra |
20 |
8 |
3 |
- |
- |
4 |
| Wide
Ultra |
40 |
16 |
- |
- |
25 |
16 |
| Wide
Ultra |
40 |
16 |
1.5 |
- |
- |
8 |
| Wide
Ultra |
40 |
16 |
3 |
- |
- |
4 |
| Ultra2 |
40 |
8 |
- |
12 |
25 |
8 |
| Wide
Ultra2 |
80 |
16 |
- |
12 |
25 |
16 |
| Ultra160 |
160 |
16 |
- |
12 |
- |
16 |
| Ultra320 |
320 |
16 |
- |
12 |
- |
16 |
Note:
It is recommended that the SCSI devices be uniformly spaced between connectors with the end
devices located as close to the terminators as possible.
If ALL devices on the bus support LVD, 12-meter operation is possible at typical LVD speeds.
However, if ANY device on the bus is capable of only S.E. operation, the entire bus will switch
to the S.E. mode and the cable distances provided in the S.E. column will apply.
Sometimes the word "Narrow" precedes 8-bit SCSI, Ultra SCSI,
or Ultra2 SCSI.
StorCase SCSI Ultra160 Products
Following the specifications
defined above for SCSI S.E. and LVD cable length limitations
and termination methods, StorCase enclosures that are identified
as supporting SCSI Ultra320 LVD operation are completely downward-compatible
with SCSI Ultra, Ultra2 and/or Ultra160 devices implemented
into S.E. configurations. In those S.E. applications, products
such as the StorCase removable Data Express®
Ultra320 drive enclosure should not be expected to accomodate
the longer cable lengths or bus speeds associated with the complete
Ultra320 LVD system.
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