Ultra320 SCSI In Detail
For the first time in the long history of SCSI, several things have undergone fundamental changes. Introducing packetized SCSI will help to reduce the so-called overhead. Where until now command and status information have been transmitted asynchronously (and slowly to boot) with the Ultra320, they are part of the data packets that are on board, with their full bandwidth.
Another innovation of packetized SCSI is the introduction of a streaming procedure that is supposed to simplify and speed up the transmission of data packets. This is a simplified version of the request/ acknowledge process (the core of handshaking), in which acknowledging is not done until the end of the data packet, instead of occurring after every block of data.
While Serial ATA cable allows lengths of up to one meter, all of 25 meters with only one device are possible with the Ultra320 bus. Unlike Serial ATA, however, with Ultra320, all devices can operate on one channel, distributing the total bandwidth among all devices. Serial ATA allows only one device per port, so that the full 150 MB/s are available for it. For this reason, no more than five to seven drives can operate on one SCSI channel in performance-critical settings. The bandwidth will be used when individual drives are functioning at high capacity.
The other details are already familiar from the previous SCSI standards: termination of the bus at each end, allocation of SCSI IDs on all available devices including the host adapter, automatic detection of all devices by the BIOS of the SCSI host adapter, connection of external devices via SCSI.
For those interested in details, we recommend reading the SCSI specifications, which can be obtained on many websites or from one of the well-known manufacturers.
Ultra 320 Host Adapters: Adaptec And LSI Logic
Adaptec and LSI Logic can be indisputably characterized as driving forces in the SCSI sector. We received Adaptec's current 39320D-R model, which provides one SCSI channel internally and yet another externally. The "R" in the type labeling represents the basic RAID capabilities. Modes 0 and 1 are supported, although the former with no more than four hard drives. Adaptec also offers three other Ultra320 host adapters that are technically founded on the same basis.
We received two models from LSI Logic, the 20320-R and the 21320. The former offers integrated mirroring (RAID 1); the latter has two, rather than only one, internal channel.
For this comparative test we used the LSI 20320-R; because of the rudimentary RAID capabilities, it is better suited to the Adaptec 39320D-R.
