In this the second article on Storage Basics, we continue our look at the Small Computer Systems Interface (SCSI), focusing on implementation considerations such as signaling, termination and connector types. We'll start by looking at SCSI signaling.
The manner in which data is transmitted across a SCSI bus is defined by the method of signaling used. There are three types of SCSI signaling that can be used: High-voltage differential (HVD), Low-voltage differential (LVD) and Single Ended (SE).
HVD has been around since the earliest SCSI specifications (SCSI standards were discussed in SCSI Basics Part 1) and found popularity primarily due to the fact that HVD offers significant signal integrity, which allows for the utilizaton of longer cables without data loss or corruption. To give an example, Ultra2 SCSI using HVD can use cables up to 25 meters in length before the signal begins to corrupt, whereas Ultra2 SCSI using LVD can only use 12-meter cables.
However, for a couple of reasons, the ability to accommodate longer cables was not enough to make HVD the signaling method of choice. First, HVD uses two wires for each signal to increase signal integrity, making HVD expensive to implement and at the same time increasing its power requirements. The second reason is that in a practical situation, running a SCSI cable 25 meters is rarely necessary.
On the other side of the signaling equation is SE signaling. Like HVD, SE has been around since the early SCSI standards. SE sends data on a single wire and, as a result, is far more susceptible to electronic interference, limiting it to far shorter cable lengths than its HVD counterpart. The SCSI-1 standard, for example, supports cable lengths of 6 meters when using SE signaling (compared to 25 meters when using HVD). Despite the shorter cable lengths, SE was cheaper and adequate for most SCSI implementations, making it, in its day at least, far and away the most common type of signaling used.
If SCSI technology was stagnant, SE signaling would still be in use today; however, when the SCSI bus was reaching speeds of 40 MHz and higher, SE couldn't cope. Each time SCSI speeds doubled, the length of cable used with SE singling halved. Fast SCSI using SE, for example, had a cable length of 3 meters, and Ultra SCSI dropped the cable length to 1.5 meters. SE is not defined for SCSI standards beyond Ultra SCSI, which is a good thing as attaching an external tape drive with a 2-inch cable would be rather difficult!
For the faster SCSI standards, LVD signaling is used. In many ways, LVD is a compromise between its two predecessors, providing a cheaper alternative to HVD and at the same time offering longer cable distances than SE can accommodate. LVD defines cable lengths of 12 meters up to 25 meters (if only two SCSI devices are attached to the bus).
Like HVD, LVD uses two wires for each signal; however, the lower voltage requirements allows for a reduced cost and power requirements to be kept under control. As a bit of a bonus, this also allows SE devices and LVD devices to coexist on the same SCSI bus. HVD, on the other hand, is completely incompatible with the other two signaling methods. There is a significant caveat when talking about SE and LVD compatibility, though. If not all devices on the SCSI bus are LVD capable, the bus will operate within the confines of SE signaling. Not only will the speeds be limited, but the cable lengths must also conform to the SE specifications.
Today's modern implementations will almost certainly use LVD signaling. In fact, SE signaling is not defined for SCSI speeds beyond Ultra2 SCSI, and HVD is not defined for SCSI standards beyond Ultra2 SCSI.
Termination is an important topic for SCSI, as without the correct termination the entire SCSI bus will most probably be inoperative. In some cases the SCSI devices on the bus may still operate, but you will experience seemingly random problems.
The basic purpose of termination is to prevent the data signals that reach the end of the bus from reflecting back down the bus and then affecting other signals. There are two basic types of termination, passive and active. Passive termination, the method used by the lower performance SCSI standards, uses a group of resistors to "soak up" the signals on the SCSI bus. A more advanced system, active termination, uses a group of voltage regulators to deal with the signals. This results in a more defined level of control, which is preferred and in some cases required on some of the higher performance SCSI standards.
In most cases, SCSI termination is set on the host adapter. If you are only using internal SCSI devices, this is correct, but if you also have external devices then the termination must be removed from the SCSI host adapter and placed on the last device in the chain. A SCSI bus can only have two termination points -- one at each end of the physical bus.
SCSI supports both internal and external devices with an array of connector options for each. Over time, many of these connectors and their associated cable types have fallen into obscurity, but there are plenty of legacy SCSI devices out there using the various connector types.
As far as internal connectors are concerned, there are only a few variants. The original regular density 50-pin SCSI connectors were designed to be used with SCSI-1 devices. These regular density cables resembled the 40-pin connectors used with IDE devices. The SCSI-2 standard brought with it a new internal connector, the high-density connector, which comes in a 50-pin and a 68-pin version. Today's SCSI implementations are likely to use the 68-pin internal connectors.
When it comes to external SCSI connectors, there are a range of possibilities which can be grouped into 2 distinct categories, D-shell connectors and Centronic connectors.
A D-shell connector was first used with the original SCSI-1 standard. This original 50-pin connector was replaced with the high-density shielded connectors in SCSI-2. The newer high-density D-shell connectors are available in both 50-pin and 68-pin versions. The latest and greatest external SCSI D-shell connector is referred to as the Very High Density Cable Interconnect (VHDCI) connector. The VHDCI connector is only available in a 68-pin version and is the one you are most likely going to encounter when working with modern SCSI implementations.
An alternative to the D-shell SCSI connectors are the Centronic connectors. While Centronic connectors are now most often associated with PC printer cables, they were once common for connecting external SCSI devices. Today, these 50-pin SCSI Centronic connectors may still be found for connecting scanners or printers, but they have fallen out of favor for the more versatile VHDCI connectors.
As technologies go, SCSI has had a good run in an industry where the only constant is change. SCSI's ability to adapt and improve help ensure that it is not destined to fade into the background. The world of SCSI has always been one riddled with complexity and confusion, and the bad news is that the future for the veteran technology does not look any less complicated. In addition to new standards such as Ultra320 SCSI coming out, the advent of iSCSI will continue to make the world of SCSI one mired in compatibility issues and questions about capabilities and speeds.
In terms of application, SCSI is likely to remain a popular choice for connecting devices in small to medium sized storage applications. Even though other connectivity technologies like Fibre Channel and iSCSI will continue to chip away at SCSI in terms of market share, "traditional" SCSI is sure to be with us for some time yet.