Parallel SCSI: Difference between revisions

Revision as of 15:12, 28 July 2022

Revision as of 21:43, 3 September 2022

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[[Single-ended signalling|Single-ended]] and [[LVDS]] devices can be attached to the same bus, but all devices will run at a slower, single-ended speed. The SPI-5 standard (which describes up to Ultra-640) deprecates single-ended devices, so some devices may not be electrically backward compatible.

Some host adapters offer compatibility using a SCSI bridge to electrically split the bus into an SE and an LVD half, so LVD devices can operate at full speed.<ref>{{cite web|url=http://download.adaptec.com/pdfs/user_guides/aha2940u2w_ur.pdf |title=Adaptec AHA-2940U2W User's Reference |date=December 1997 |access-date=2017-05-20}}</ref> Other adapters may provide multiple buses (channels).<!--[[User:Kvng/RTH]]-->

===Mixing Widewide and Narrownarrow===

Both narrow and wide SCSI devices can be attached to the same parallel bus. All the narrow SCSI devices must be placed at one end and all the wide SCSI devices at the other end. The highwider halfpart of the bus needs to be terminated in between the wide and narrow devices because the high half of the bus ends with the last wide SCSI device. ItThis can getbe done with a cable designed to connect the wide part of the bus to the narrow part which either provides a place to plug in a terminator for the high half or includes the terminator itself. {{efn|This is sometimes referred to as a cable with high-9 termination.}} Specific capability commands allow the devices to determine whether their partners are using the whole wide bus or just the lower half and drive the bus accordingly.<!--[[User:Kvng/RTH]]-->

As an example of a mixed bus, consider a SCSI wide host adapter with an HD-68 male connector connected to a SCSI narrow disk drive with an HD-50 female connector. It might make this connection with a cable that has an HD-68 female connector on one end and an HD-50 male connector on the other. Inside the cable's HD-68 connector, there is termination for the high half of the bus and the cable contains wires for only the low half. The host adapter determines that the disk drive uses only the low half of the bus, so talks to it using only the lower half. The converse example—a SCSI narrow host adapter and SCSI wide disk drive also works.