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EXPANDING & CONVERTING THE SCSI BUS

SCSI Expander

This parallel SCSI bus has become the most widely utilized mass storage interface in the computer industry.  Its strong position in the market place permeates from the high end server applications through workstations down to the low end single user computer.

The following needs have arisen due in large part to SCSI's inherent protocol flexibility, large support infrastructure, continued speed increases and the acceptance of SCSI Expanders in applications:

  • Longer SCSI Cable Lengths
    In Stand alone applications, where all of the devices are internal, shorter cable lengths may not be a problem but in applications where external device packages are to be used in a system, the short single ended cabling becomes a serious issue. 

    High Voltage Differential (HVD) applications allow for longer cable lengths, but add an additional expense per device.  There are also limits to the variety of HVD devices available, as not all SCSI device manufacturers make HVD versions of their devices.

    Low Voltage Differential (LVD) devices have the low cost of single ended devices and the longer cable lengths and higher signal stability of HVD devices.  They are also able to operate at the faster Ultra 2 SCSI transmission rates. Even with LVD there are cable length issues. The larger SCSI based systems of today have varying requirements that exceed the maximum cable lengths of all of the current parallel transmission mediums. The answer to cable length issues resides in the use of expanders as SCSI bus extenders.
Longer SCSI Cable Lengths Using Expanders
Length in Meters.  Point to Point = No devices between expanders.
  ASYNC/
SLOW
FAST ULTRA ULTRA2
Single Ended SCSI        

No Expanders
1 Expander
2 Expanders
2 (point to point)

6
12
18
24

3
6
9
12

1.5
3
4.5
6

NA
NA
NA
NA

High Voltage Differential (HVD) SCSI        

No Expanders
1 Expander
2 Expanders
2 (point to point)

25
50
75
100
25
50
75
100

25
50
75
100

NA
NA
NA
NA

Low Voltage Differential (LVD) SCSI        

No Expanders
1 Expander
2 Expanders
2 (point to point)

*
*
*
*

*
*
*
*

*
*
*
*

12
24
36
48

* = LVD can go longer distances at slower transmission rates

Note: Cable segments of different types can be used in the same SCSI domain with converting expanders. Longer cable lengths are possible with serial expanders.

Parallel SCSI Expanders used as extenders separate a SCSI bus (SCSI domain) into more than one physical segment, each of which can have the full SCSI cable length for that type. Two expanders can increase the extension of the bus, creating 3 segments.  If no other devices are connected to the center segment, the point to point   communications between two expanders will allow for a longer than normal cable length in that segment. Serial Expanders are similar in function as two Parallel expander implementations, with the point to point serial transmissions in place of the middle SCSI segment.

All of the above SCSI Expanders have certain characteristics. The expanders are transparent to the software and firmware on the bus. They don't take up a device ID.  They are able to provide termination if located at the end of a bus segment. They can have termination disabled if they are in the middle of a bus segment. They can provide termination power, and they provide a complete regeneration of the SCSI bus signals as each SCSI bus segment must maintain the complete physical bus characteristics of a SCSI bus in and of itself.

SCSI Domain consisting of a single segment:


SCSI Domain with two segments using an expander:


SCSI Domain with three segments using two expanders:


SCSI Domain with middle segment used as a point to point extension of
the bus:


SCSI Domain with expanders creating branches:

      T = Terminator

NOTE: SCSI Domains contain a maximun of the total number of addressable SCSI devices as specified in the SCSI standards (normally 8 or 16 devices). The total cable length from any device to any other device within a SCSI Domain must not exceed SCSI arbitration timeout limitations as specified in the EPI technical report for SCSI.   Any segment within a domain can independently use Single Ended, LVD or HVD transmission mediums when connected with the proper type of expander.

  • The above are just a few examples of how expanders can affect segments within a
    SCSI domain.

    SCSI Expanders have certain characteristics:
    - The expanders are transparent to the software and firmware on the bus.
    - They don't take up a device ID.
    - They are able to provide termination if located at the end of a bus segment.
    - They can have termination disabled if they are in the middle of a bus segment.
    - They can provide termination power,
    - and they provide a complete regeneration of the SCSI bus signals as each SCSI bus segment must maintain the complete physical bus characteristics of a SCSI bus in and of itself. 

  • Compatibility between Various SCSI Transmission Mediums
    Devices that transmit Single Ended SCSI signals cannot normally communicate with HVD SCSI based devices. The LVD specifications have a definition for Multimode LVD that allows a multimode LVD based bus segment to switch down to Single Ended transmissions if a Single Ended device is connected to it. This allows for compatibility at the cost of degrading the speed and cable lengths of the total bus segment when single ended device is physically connected to the segment.

    LVD and HVD devices are also incompatible with each other. Add the problems of connecting narrow SCSI (8 bit) devices with wide SCSI (16 bit) devices and it starts to get complex. This is where SCSI expanders are used as converters to become the glue that brings these divergent transmission types within a single SCSI domain.

    Expanders can be used to maintain the faster (Ultra 2) communication rates of LVD devices within an LVD segment, while retaining the long cable lengths of devices within a HVD segment, and supporting low cost devices within a single ended segment.  This could all be done within a single SCSI domain, with each segment having its full cable length available to it. Expanders that convert between HVD and Single Ended SCSI are used in HVD systems to add low cost single ended drive packages onto the longer HVD cabling.

    The newer expanders that implement Multimode LVD on both sides will automatically convert between Single Ended and LVD allowing LVD devices to keep their transmission rates high when communicating within their segment.  They do this while still allowing the use of single ended devices. Wide SCSI Expanders also address all of the questions of connecting a narrow SCSI segment with a wide SCSI segment. All signal lines for the wide segment are terminated at the expander eliminating the potential hazards of improper terminations for the wide bus and simplifying the cabling for the narrow segment.
  • Increasing the Number of Addressable Devices
    The majority of current SCSI applications see the number of devices addressable by a single SCSI port as completely adequate.  But in some larger SCSI configurations, where large banks of devices must be accessed, SCSI's total target/initiator devices in the narrow SCSI bus (8 devices) and wide SCSI bus (16 devices) are viewed as a limitation. Logical Unit Number Expanders (LUN Expanders) can increase the number of devices possible through a single SCSI port. They do this by controlling the accessing of target/initiator ID based devices such that the LUN addressing specified in the SCSI standards can be used to address them.
  • Expanders Taking SCSI into the Future
    SCSI Expanders have been in existence in various forms since the mid 1980s. The simpler expanders which are used to transparently convert between single ended and differential transmission mediums have evolved from providing just a bus conversion to being able to provide full SCSI cable lengths on each side of the conversion, and allowing for back to back bus extension for even longer cable lengths. Developing from the converters, bus repeaters/regenerators have become popular in providing that extra cable length, without having to go through a conversion.
    (This document is provided by STA - SCSI Trade Association.)

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