Inside the Ultra2 LVD SCSI Interface
A detailed look at the specs and integration issues surrounding the latest version of SCSI.
By John Lohmeyer and Bill Schmitz
With the advent of Ultra2, SCSI has been born again. Featuring low-voltage differential (LVD) technology, the new interface is expected to span many generations. Ultra2 SCSI offers VARs, systems integrators, and OEMs improved performance, reliability, and connectivity over previous SCSI technologies (see table).
The choice of single-ended or differential SCSI transceivers determines the maximum bus length and the number of devices that can be supported by the host. With single-ended transceivers, cable lengths are limited to 3 meters with four-device loads and 1.5 meters with eight-device loads. Maximum cable length increases to 12 meters for both narrow and wide Ultra2 SCSI. In addition, LVD signaling technology enables a maximum 8-device load for narrow implementations and 16 for wide.
I/O performance has been greatly improved by doubling synchronous data transfer rates. Narrow (8-bit) rates have increased from 20MBps to 40MBps for Ultra and Ultra2, respectively; wide (16-bit) rates have increased from 40MBps to 80MBps, also respectively.
These two enhancements give Ultra2 SCSI a significant advantage over its predecessors-even "differential SCSI," or high-voltage differential (HVD). While HVD can handle longer cable lengths and more loads than single-ended transceivers, its use is limited due to the need for additional signal conversion hardware and board real estate. And it is expensive. Less than 10% of SCSI devices use HVD because it requires expensive external drivers and receivers.
LVD signaling technology eliminates the need for expensive external HVD transceivers while addressing performance demands. Though it can be used for the slower transfer rates of fast and Ultra SCSI, LVD is currently targeted at next-generation Ultra2 and Ultra3 SCSI devices.
LVD in Detail
LVD technology features reduced output signal levels and increased receiver sensitivities. It provides the signal quality advantages offered by differential signaling, such as common mode noise rejection. Each LVD signal (V+ and V-) swings about 400mV around a common mode bias of 1.25V. This small amplitude change allows the signal to achieve its desired state more quickly than single-ended or HVD signaling. Faster signal swings reduce the skewing effects of cabling and device loading associated with legacy SCSI devices.
Significantly lower power consumption enables LVD transceivers to be integrated into SCSI protocol devices, giving Ultra2 SCSI a major advantage over previous versions. LVD does not depend on specific power supply voltages, enabling migration to lower power supplies such as 3.3V, 2.5V, or 2.0V while maintaining the same signal levels and performance. Constant current mode drivers are independent of frequency changes, which means driver power consumption is relatively constant over the operating frequency range.
LVD topology is compatible with the current single-ended infrastructure so that multi-mode drivers can accommodate both LVD and legacy single-ended devices. The SCSI connector pin-out for the LVD transceivers is the same as that of single-ended transceivers. When switching to single-ended mode from LVD mode, the + signal of the LVD driver becomes a virtual ground driver and the - signal becomes a single-ended SCSI driver, so the driver pair is compatible with legacy SCSI buses. In contrast, HVD employs a different pin configuration from single-ended and LVD, precluding automatic switching to this mode.
Because single-ended restrictions can be severe, integrators should consider using two SCSI buses: one that operates in single-ended mode for legacy devices and one that operates in LVD mode for new devices. Dual-channel host adapters facilitate this approach. Alternatively, consider LVD-to-single-ended expanders (also called translators). These products allow new devices to achieve full Ultra2 performance while still being able to communicate with older SCSI devices.
When a single-ended device is added to a SCSI bus that is operating in LVD mode, the DIFFSENS line on the SCSI bus informs the other devices, and all devices drop back to single-ended mode. The transceivers are limited to the maximum cable distance, transfer rates, and device load limits of Ultra SCSI or slower devices.
To run at Ultra2 SCSI rates, all devices on the bus must be capable of running LVD or the system must be configured with expanders to isolate single-ended devices. Expander chips can receive LVD or single-ended SCSI signals, recondition the signals, and transmit them to another SCSI bus segment. Some expander chips convert SCSI signals between LVD and single-ended.
One way to use this technology is to connect LVD devices to the main bus segment and attach an LVD-to-single-ended expander to a second bus segment for single-ended devices. This type of configuration allows data transfer at Ultra2 SCSI rates to LVD devices on the primary bus segment, and data transfer at Ultra SCSI rates to the single-ended devices on the secondary bus segment.
Several board layout issues need to be considered for Ultra2 SCSI technology. Differential pair lines must be equal in length and should be routed together to avoid introducing skew. Minimal stub length (less than four inches) and impedance matching are required to keep signal reflections to a minimum.
Trace impedance should match the impedance of the media and the termination. Ultra2 SCSI configurations are required to meet trace and cable impedance characteristics of 110 ohms to 135 ohms differential and 90 ohms single-ended.
Ultra2 SCSI also has special terminator and connector considerations. Some terminators can handle either LVD or single-ended signaling interfaces and potential switching between these two modes. Designs may require auto-termination circuitry to handle configurations in which the host adapter is placed in the middle of the bus and the onboard terminators need to be disconnected.
VHDCI (very high density cabled interconnect) connectors are available as well as commonly used HD68 (High Density 68-pin) connectors. VHDCI connectors are smaller, allowing more SCSI channels on an adapter board (up to four channels on one PCI card).
As SCSI data rates continue to increase, integrators will need to use high-quality cables and follow device spacing rules. Still, with LVD, it is possible to configure systems otherwise inconceivable with single-ended technology (16 devices on 12 meters of cable vs. 8 devices on 1.5 meters of cable).
Backplane designers, too, will face challenges. While it is possible to design backplanes that work in single-ended and LVD modes, it requires careful attention to the layout to get the trace impedance right for both modes.
Also, since LVD signals are weaker than single-ended signals and timing is more critical, it is important to minimize differences in signal capacitance.
Minimal changes to software drivers are required, allowing the use of existing software. To upgrade to Ultra2 SCSI, you`ll need to negotiate for Ultra2 SCSI rates and to set the clocking. And additional code is required to handle potential bus mode changes between LVD and single-ended.
When the mode changes, an interrupt alerts the driver of a change. At that time the driver can re-negotiate the transfer rate that is appropriate for the type of signal used by the devices on the bus.
Ultra2 SCSI with LVD offers the best of single-ended and high-power differential transceivers, improving system I/O performance and resolving many of the cabling and signal issues of current Ultra SCSI devices.
With today`s computer systems demanding higher and higher transfer rates, LVD technology delivers the needed performance without the sacrifices of previous SCSI technologies.
Here Comes Ultra3 SCSI
The ink is not quite dry on SPI-2 and the T10 Technical Committee is already working on SPI-3, a.k.a. Ultra3 SCSI. SPI-3 will double data transfer rates again to Fast-80 (160MBps on a wide SCSI bus) using double-transition clocking. Both edges of the REQ and ACK signals will be used, which doubles the transfer rates without doubling the frequencies. SPI-3 will also add CRC error checking, which should ensure that hot plugging works without undetected errors.
Domain validation is the technical term for verifying that the cables, terminators, and expanders (if any) are adequate for the negotiated data rate and width. This is similar to modem protocols that automatically negotiate a slower transmission rate if the quality of the phone line is insufficient. For example, connecting two wide SCSI devices with a narrow cable results in consistent parity errors today. In Ultra3 SCSI with domain validation, the wide SCSI devices will just re-negotiate for narrow SCSI.
Finally, SPI-3 will include options for overhead reduction, called packetized protocol. These options are particularly valuable in environments that use a lot of small block-size I/Os and will become even more important as SCSI data rates are pushed still higher. Expect SPI-3 products late next year or early in 2000.
Example of LVD signaling shows the SCSI ACK plus and minus signals at the first drive of 15 Ultra2 SCSI drives on a 12-meter cable--the maximum device loading and cable length allowed by SPI-2 (SCSI Parallel Interconnect - 2). The resulting differential combination of the plus and minus signals is shown in the lower half of the window.
Bill Schmitz (firstname.lastname@example.org) is a systems engineer, and John Lohmeyer (email@example.com) is a senior consulting engineer, at LSI Logic Corp., in Colorado Springs, CO.