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Single Connector Attachment (SCA, SCA-2)

The SCSI standards define four "alternative" connector types for both internal and external SCSI connections. Of those eight, seven involve the use of traditional SCSI cabling of one sort or another. Internal connection "Alternative 4", however, defines a totally different way of connecting SCSI drives to host systems. Instead of the use of discrete cables, the drives are plugged directly into the system. A single connector is used that includes all of the signaling and power inputs and outputs needed by the drive. This technique is called single connector attachment or SCA.

The reason why this attachment method was developed was to respond to the needs of higher-end systems. Better workstations and servers now employ advanced technology to allow multiple hard disks to be used together to increase performance and improve reliability. This is done through the use of Redundant Arrays of Inexpensive Disks (RAID); these high-end machines may have anywhere from two to dozens of SCSI drives in them. One important feature of RAID is hot swapping, which means that failed hard disks can be removed from the disk array and replaced with new ones without powering down the system. This allows busy machines to keep on running even if a drive fails. Conventional attachment protocols with separate power and data cables--and manual configuration--do not work well in this environment. Regular SCSI hard disk connection methods don't allow for hot swapping. In addition, all the cables involved in connecting power and data to a dozen drives are cumbersome

With the SCA system, the regular 68-pin data connector, 4-pin power connector, and several configuration jumpers on a hard disk are all replaced by a single, unified 80-pin connector. (Incidentally, SCA is a wide-only interface; there is no narrow SCA). This is a Centronics-style connector with special properties used to ensure safe hot plugging of devices into an active system. On the host side, mating connectors are mounted onto a backplane consisting of several removable drive bays (you can see a picture of a server case providing removable SCSI SCA drive bays here.)

The first SCA implementation had some issues, and the technology was revised in a new version now called SCA-2. Several attributes about the interface were changed with this new version of SCA, and SCA-2 is now the standard in the industry. In order to frame the discussion of the features of SCA, let's take a look at the single-ended signals and contact numbering scheme for the current standard 80-contact SCA connector:

Looking at this table, you will notice several differences compared to the signal chart for regular wide SCSI attachment. These really define the special characteristics of the SCA interface:

• Regular Signals: The "core" SCSI signals are the same as for wide SCSI; they are found in the middle part of the connector. The only difference is that the signals are in the left column and the returns on the right, instead of the other way around.
• Power Signals: Three voltages of power are supplied to the hard disks: 3.3 V, 5 V and 12 V. Again, these are provided so that the drive can operate without needing a separate Molex-style four-pin power connector.
• Long Contacts: The "Long Host Contact?" column has the word "YES" in it for several contacts. The connector on the host (PC) side is specially designed so that these contacts are made a bit longer than the regular length associated with most of the contacts in the connector. This is an important feature: what it does is to ensure that these signals make contact before any others when a drive is inserted, and also that they break contact last when a drive is removed. Without this feature, subtle changes in the angle of insertion or pressure applied to the drive as it is pushed into the system could cause spurious results--a voltage contact might connect before the ground contacts, for example. (Remember that compared to the speed of electricity, human hands are extremely slow; even the split-second that passes while a connector is being seated represents an eternity to electronic circuits.)
• Power Charge Signals: Each of the three voltages mentioned above has associated with it a "precharge" signal. These are on long contacts while the regular voltage signals are on short contacts. When a drive is inserted, these precharge circuits make contact a bit before the regular voltage circuits do. A drive can be designed to use these charge signals to "precharge" its internal circuits during hot plugging, to soften the sudden "surge" of current when the regular power signals connect.
• SCSI ID Signals: Instead of using jumpers, SCA drives have their device IDs set through software control. This is obviously important to enable the use of many drives and hot swapping.
• Other Control Signals: Extra signals are provided to allow control over other drive features such as remote or delayed starting of the drive motor. A signal is also provided for the drive to indicate that it is active, so the system can light the appropriate drive activity LED.

The physical SCA connectors also incorporate physical features to make hot plugging of drives easier. SCA drives slide into drive bays and mate with the system connector in the back without the user being able to see what is going on; this is called blind connector mating. To ensure that the connectors line up properly, special guides are provided on the host connector.

There is also an LVD version of the SCA interface, which is now taking over from the single-ended type of SCA, as all new hard disks use LVD. The difference between LVD SCA and SE SCA is the same as the difference between LVD wide cable signals and SE wide cable signals: there are two changes. The first is that all of the signal returns are replaced with positive complement signals; for example, contact #58 on an LVD SCA connector is "+BSY", and so on. The second is that one ground is replaced by the DIFFSENS signal, in this case contact #46.

Warning: Though physically identical, LVD drives must use the LVD version of the SCA interface to function properly.

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