RS-485: Imported from Wikipedia (overwrite)

2026-05-02T18:03:31Z

Imported from Wikipedia (overwrite)

New page

{{Short description|Computer bus used on VAX computers}}
The '''Digital Storage Systems Interconnect''' ('''DSSI''') is a [[computer bus]] developed by [[Digital Equipment Corporation]] for connecting storage devices and clustering [[VAX]] systems. It was designed as a smaller and lower-cost replacement for the earlier [[VMScluster|DEC Computer Interconnect]] that would be more suitable for use in office environments. DSSI was superseded by [[Parallel SCSI]].<ref>{{cite web |url=https://gordonbell.azurewebsites.net/Digital/timeline/1988-5.htm |publisher=Gordon Bell |title=Digital Storage Systems Interconnect}}</ref>

It was introduced in 1988 and has a bandwidth of 32 Mbit/s (4 MB/sec) and was typically limited to a length of {{convert|25|m}}. DSSI support was later extended to [[MIPS architecture|MIPS]]-based [[DECsystem]]s and [[DEC Alpha]]-based [[AlphaServer]] systems.

DSSI peripherals could be distinguished from narrow SCSI devices by their characteristic five-conductor power connector featuring an extra lead intended for Battery/UPS connection to power critical components in the event of a power failure.

== Characteristics ==

The major characteristics of the DSSI bus are:
* Eight-bit data path
* Up to eight nodes on the bus including variable numbers of [[SCSI initiator and target|initiators]] and [[SCSI initiator and target|targets]]
* Distributed arbitration
* Peak bandwidth of 4 megabytes per second
* All systems connected to the same DSSI bus must have a common power/ground.
* "The [[Q-Bus]] 5-Enclosure Rule": For Q-bus VMScluster configurations, a maximum of five enclosures, ''excluding'' SF200/210s, can be configured on a single DSSI bus. Note that only ''one'' (1) SF100 is allowed on any DSSI bus.
* "The [[VAX 6000]] Cabinet Rule": For a VAX 6000 DSSI VMScluster system, a maximum of two or three VAX 6000 systems and one storage cabinet, typically an SF200 or SF210, can be configured on a single DSSI bus.
* Maximum DSSI bus length cannot exceed 27 meters (89 feet) and ground offset voltage cannot exceed 30mv (dc) or 10.5mv (rms).

Bus Length Allowable Offset
Meters/Feet (DC) (AC)
----------- ----------------
up to 20/65 200mv 70mv (rms)
20-25/65-82 40mv 14mv (rms)
VAX6000 QUAD-Host--> 27/89 30mv 10.5mv (rms)
VAX7000
VAX10000

== Operating system support in VAX/OpenVMS ==

DSSI SUPPORT IN [[OpenVMS]]:
OpenVMS VAX V5.0-2:
o Single- and Dual-Host support with 2 EDA640s ([[MicroVAX]] 3300/3400
series).
o Systems Communications Services (SCS) for the VMScluster is run
over the Network Interconnect (NI) for MicroVAX 3300/3400.
OpenVMS VAX V5.1:
o Standalone support for the KFQSA.
o Up to 6 ISEs on a single DSSI bus regardless of whether 1 or 2
hosts systems are attached to the bus.
OpenVMS VAX V5.1-1:
o Dual host support for KFQSAs (1 KFQSA per system).
OpenVMS VAX V5.2:
o For 3300/3400 series, SCS communication over the DSSI.
o Multiple KFQSA's per system, but *not* multiple dual-hosted
DSSI's per system (here, multiple means 2, since only 2 hosts
are allowed on the DSSI bus).
o DSSI adapter must connect to another like adapter, e.g. EDA640
to EDA640 or KFQSA to KFQSA.
o Up to 7 ISEs on a single DSSI bus when only 1 host system is
attached to the bus.
OpenVMS VAX V5.3:
o Mixed DSSI adapters, i.e. EDA640 to KFQSA.
o Mixed DSSI adapters, i.e. EDA640 to KFQSA.
o Double dual-host configurations, i.e. 2 DSSI buses between two
CPUs. Each bus is terminated at its own pair of adapters.
OpenVMS VAX V5.3-2:
o Support for VAX 4000. (The VAX 4000 has 2 SHACs and can support
two KFQSAs on the bus, so it can have up to 4 DSSI buses).
OpenVMS VAX V5.4-2:
o Support for the KFMSA, an XMI to DSSI adapter. The KFMSA may
only be used to single-host a VAX 6000 model or dual-host 2 VAX
6000 models together.
OpenVMS VAX V5.4-3:
o Tri-host configurations supported.
OpenVMS VAX V5.5-2:
o Quad-host configurations with VAX 6000, [[VAX 7000/10000]]
series systems now supported.
NOTE: KFQSAs do not support SCS communication over the DSSI bus,
they MUST have a NI connection in order to run the cluster
software.
OpenVMS Alpha V1.5, OpenVMS VAX V6.0:
o Quad-host DSSI configurations with [[DEC 4000 AXP]] systems supported.
DEC 4000 systems can be configured only as end nodes on a DSSI
bus, therefore, quad-host configurations must include at least
two VAX systems.
o KFQSA Q-bus to DSSI configurations supported.
VAX processors that use the KFQSA adapter can be placed on the
same DSSI bus as DEC 4000 processors.

== Third Party Peripherals ==

Other companies, such as [[Silicon Image Inc.|CMD]] and [[Symbios Logic]] made DSSI compatible chipsets or peripherals, for example,
CMD manufactured various models of the CDI-4000 which would allow [[SCSI]] peripherals to be used on a DSSI bus.

==See also==

* DSSI VAXcluster Installation and Troubleshooting manual, EK-410AB-MG
*DEC Technical article: [ [[Ultrix]] ] How To Configure a kfqsa on a DECstation (sic) 5500 (DS5500), Last Technical Review: 6-SEP-1991 Size: 110 lines
*DEC Technical article: [OpenVMS] Configuring RF Drives Connected To DSSI Adapter In A Cluster, Last Technical Review: 8-OCT-1996 Size: 332 lines
*DEC Technical article: [OpenVMS] Guide To DSSI VAXcluster Configurations
*DEC Technical article: How To Initialize An RF72 For High Speed Mode
*DEC Technical article: [OpenVMS] Chart Overview Of Digital Disk Drive Storage Capacities
*DEC Technical article: [PERFORMANCE] Disk Drive Performance Specifications
*"OpenVMS Release Notes Addendum for [[VMScluster]] Systems", October 1993, (AA-Q1X7A-TE).
*"Dual-Host MicroVAX Systems", VAXcluster Systems Quorum, February 1990, (EC-P0623-57), pp. 3–31.
*"[[VAX 4000]] Systems Info Sheet", (EC-F1424-41).
*"VAX 6000 Systems Info Sheet", (EC-F1417-46).
*"KFESA DSSI Adapter Installation and User's Guide", May 1994, EK-KFESA-OP.A01.
*CDI-4000 DSSI to SCSI Adapter User's Manual, MAN-004000-000

==References==
{{reflist}}

{{Digital Equipment Corporation}}
{{Computer-bus}}

[[Category:DEC hardware]]
[[Category:Computer storage buses]]
[[Category:Computer-related introductions in 1988]]

Digital Storage Systems Interconnect - Revision history

RS-485: Imported from Wikipedia (overwrite)