SCSI Termination Tutorial
The purpose of this white paper is to help understand common field issues that you might run into when connecting a SCSI drive (or drives) to various kinds of SCSI buses, whether cable or backplane. The paper is organized into three sections: background and history of SCSI including terminology, termination basics and field issues. There are seven field scenarios discussed, each with a checklist of things to look for, and several actual case studies:
This tutorial is intended for an audience familiar with conventional IDE/SCSI terms, PC
operation and understanding of the basics of SCSI technology. While every attempt
has been made to make this tutorial as comprehensive as possible, an individual may
be confronted with a situation which is not addressed within the text.
As the SCSI interface has evolved from the original 50 pin connection to
68 and even 80 pins, so has the speed and complexity of the devices that now connect to
the bus. Connecting any SCSI device, let alone wide devices to
narrow buses and vice-versa requires a careful understanding of SCSI
terminology, termination practices, terminators and adapters, and drive specific
requirements. Lets start with an overview of SCSI and the rather confusing
terminology that is in current use (see Table 1).
The original SCSI interface used a 50 pin connector; the SCSI "bus" therefore, was 50 conductors and was typically implemented either in a flat ribbon cable or a backplane. Eight of the conductors were allotted for data lines (plus one for a parity bit), and therefore became known as an "8-bit wide" bus; the bus speed and therefore the data rate was limited to 5Mhz (5Mbytes/sec). Both Single Ended (SE) and differential versions (two different means of electrically driving the bus) were available but because they used different pinouts, could not be interchanged. Differential signaling is less sensitive to noise, and can be used over longer distances, at higher bus speeds. This first version of SCSI is based on an ANSI standard, which is now called "SCSI 1" and is also known as "narrow" SCSI. The differential version is known today as "high voltage differential", or HVD, since they had not yet invented the low voltage differential in use today.
Now it becomes slightly more confusing. As the SCSI documentation began to get unwieldy, the next ANSI standard, or "SCSI 3", was broken up into a series of smaller documents, each covering a layer of the interface. The layer that covers the electrical connectors, pin assignments, etc. is now called "SPI", or SCSI Parallel Interface (other layers cover the commands, protocols, etc.). So the main point here is that SCSI 1, SCSI 2 and SCSI 3 refer to the documentation, or specs, that further define the interfaces, not necessarily drive performance, although the latest version, SCSI 3, covers all of the latest features.
Then 8 more data bits (and another parity bit) were added to the bus so that two bytes at a time could be transferred. This became "Fast Wide SCSI" which doubled the data rate (not the bus speed) to 20 Mbytes/sec. This new "wide" SCSI required more conductors, which led to a 68 pin connector; both Single Ended and differential versions were available.
For easier connection to a backplane and to allow hot plugging, the SCA-2 (Single Connector Attachment 2) 80 pin connector was developed, which is used on wide bus versions, only. It is not designed for direct cable attachment, due to the combination of power and bus signals in the connector.
The next performance increase is "Ultra SCSI" also known as Fast-20, which uses a bus speed of 20 Mhz; for a narrow bus, this is equivalent to a data rate of 20Mbytes/sec. "Wide Ultra SCSI", for a wide bus, is equivalent to 40 Mbytes/sec.
As speed increased, maximum allowable cable length decreased, so to move up to the next performance level and keep a reasonable cable length, a new signaling method was developed called Low Voltage Differential, or LVD. This allows the bus speed to double again, and is called "Ultra2 SCSI", which is equivalent to 40 Mbytes/sec. for a narrow bus, and "Wide Ultra2 SCSI" is equivalent to 80 Mbytes/sec for a wide bus, also known as "Fast 40" define in SPI-2.
Next comes "Ultra3 SCSI",
which, according to the spec, defines as many as 63 different variations of features! A
specific set of these features has been agreed upon within the industry and named "Ultra 160" and
doubles the data rate to 160 Mbytes/sec (wide bus only), also known as Fast-80DT defined
in SPI-3. These new features are: Double Transition Clocking, CRC, Domain Validation,
Packetization and Quick Arbeitrate select (QAS). See Table 1. below for a summary of
current bus speeds and SCSI terminology, as endorsed by the SCSI Trade Association.The next
iteration, now in development, is "Ultra 320"
also known as Fast-160 paced data transfer defined in SPI-4.
Table 1. STA-Endorsed Terms & Terminology for SCSI
(1) The listed maximum bus lengths may be exceeded in point-to-point and
TERMINATION - basics
A pulse propagating along the SCSI bus will reflect from any part of the bus that is different from the rest of it. These reflections add and subtract in odd combinations and cause the original pulse to be distorted and corrupted, thereby causing data loss. To prevent or minimize reflections from the ends of the bus, terminators are added to "absorb" the energy from the pulses.
The terminators on the SCSI bus hold the bus in a negated state, any one of up to 16 drivers could be driving the bus or none. The bus is held in the negated state as required by the SCSI protocol. The original drivers on a SCSI bus only asserted (Open collector) and the terminators were used to negate the bus. Drivers that assert and negate the bus are known as active negation drivers. The negation is less current than the assertion, the drivers are designed for 2 terminators negating the bus.
Where do you terminate the bus?
[Terminate: from the latin terminus meaning "the end".]
Termination must be present at two and only two positions on the SCSI bus, at the beginning of the bus, and at the end, and must occur within 4 inches of the physical ends of the SCSI bus. Often, the host will be installed at one end of the bus, and will provide one of the two terminations required.
When do you terminate the bus?
Always. Period. Whenever you want the SCSI bus to operate reliably!
How are terminators powered?
Terminators are powered from the "term power" line on the SCSI bus. Term power can come from any device on the bus, and is provided by either the host, a drive on the bus, the backplane, or any combination thereof. Term power is provide through a diode and fuse - historically - the drop accross the diode and cable allows for a term power range of 4.0 to 5.25 volts.
What kinds of terminators are available?
What is the difference between passive and active termination?
Passive termination is a resistor network, 330 ohms from ground to the signal line and 220 ohms from the signal line to termpower, which is an equivalent impedance of around 132 ohms. Since typical characteristic impedance of most cable is between 85 and 110 ohms, this is not a great match. The negation current varies with the term power voltage which can be from 4.0 to 5.25 volts, the pull up current would range from 16.5 to 24 mA. This would often required multiple steps to insure the voltage reached the 2.0 volt at the receiver. An active termination incorporates a small voltage regulator which generates a stable termination voltage from the raw Term Power (which may fluctuate) and allows the termination resistance to be lowered to 110 ohms, more closely matching the cable impedance. Active termination is recommended for all Single Ended buses at any speed, and is required for Single Ended Ultra SCSI speeds and above. New systems with active terminates may be powered from a 3.3 volt source, termpower can be from 2.7 to 5.25 volts.
Other important terminology related to termination, for the discussion that follows:
Active Negation. Asserting a signal on a Single Ended SCSI bus requires driving the signal low; de-asserting (negation) is the opposite, allowing the signal to be pulled high by the terminator. For fastest bus speeds, the signal may be driven high by a driver in either the host or SCSI device, thus active negation. Active terminators must not prevent active negation. Active negation is required for Ultra SCSI (Fast-20).
High Byte. The upper data byte (bits 8-15) on a wide bus. This includes eight data and one parity bit, for a total of nine bits.
Low Byte. The lower data byte (bits 0-7) on the SCSI bus. This includes eight data and one parity bit, for a total of nine bits.
Multidrop. A SCSI bus with more than two devices on it.
Multimode LVD/MSE. LVD devices respond to the voltage on the DIFFSENS line on the bus, and can switch modes from LVD to Single Ended. Active terminators must drive the diffsense line with 1.3 volts with low current, and therefore be able to respond to the DIFFSENS line, such that if the voltage is < 0.7v, the terminator will be in the Single Ended mode; > 0.9 and < 1.9v is the LVD mode. Should the DIFFSENS line see > 2.4v, the terminator should tri-state off the bus as it is seeing HVD (High Voltage Differential).
Point-to-point. A SCSI bus with only two devices on it; one at each end.
Stub length. The distance from the drivers/receivers on the SCSI
device to the bus where it is plugged in. This may include the traces on the drive pcb,
inline terminator, etc.
FIELD ISSUES RELATED TO CONNECTING SCSI DRIVES
The following seven scenarios represent common field interconnect issues encountered when connecting SCSI devices to various buses. Typically the drive either wont be recognized by the host system, may not spin up, or the performance may be degraded.
1. Connecting NARROW drives (50 pin) to a NARROW bus (50 pin).
A narrow bus by definition is 50 conductors, but may be either Single Ended (SE) or differential (HVD). They cannot be mixed on the same bus. There are 3 50 pin connectors defined in the SCSI standards; 50 pin internal (0.1 inch spacing), 50 pin low density (Centronics) and 50 pin high density (0.05 inch spacing). 25 pin connectors are often used, but are not defined in the SCSI standards, there are a limited number of ground connections which often cause problems if used on fast or Ultra SCSI speeds.
2. Connecting WIDE drives (68 pin) to a WIDE bus (68 pin).
A wide bus can run in Single Ended (SE), High Voltage Differential (HVD) or Low Voltage Differential (LVD) mode. Single Ended and LVD drives can be mixed, however the bus will revert to the slower Single Ended speeds (Ultra SCSI) and mode of operation when mixed, which is determined by the DIFFSENS level on the bus. Although rare, there are some wide bus applications still running in the HVD mode, which are not compatible with LVD or SE.
3. Connecting NARROW drives (50 pin) to a WIDE bus (68 pin).
A wide bus can run in Single Ended (SE) or Low Voltage Differential (LVD) modes, however certain older narrow drives could be differential, or HVD, which is not compatible with LVD or SE (be careful not to mix LVD and HVD). If a narrow drive, running in the SE mode, is mixed with LVD drives, the entire bus will run in Single Ended mode.
4. Connecting WIDE drives (68 pin) to a NARROW bus (50 pin).
A narrow bus by definition is 50 conductors, but may be either Single Ended (SE) or differential (HVD). The two have identical looking connectors and cannot be mixed on the same bus. A wide drive can only be added to a narrow bus if the bus is Single Ended (SE).
Case study: a Customer connected WD Vantage drive to narrow bus, using
Customer supplied 68 to 50 pin adapter. Drive would not spin up. Bus was properly
terminated, externally. Problem: upper data byte was not terminated with +5v. Solution: WD
provided special firmware (available from WD by special request) that internally
terminates the upper data byte. When downloaded, the drive worked OK. An alternative
solution would be to use a different 68 to 50 pin adapter that provides the necessary
upper byte termination.
5. Connecting WIDE SCA-2 (80 pin) drives to a WIDE bus (68 pin).
A wide bus can run in Single Ended (SE), High Voltage Differential (HVD) or Low Voltage Differential (LVD) mode. Single Ended and LVD drives can be mixed, however the bus will revert to the slower Single Ended speeds and mode of operation when mixed, which is determined by the DIFFSENS level on the bus. Although rare, there are some wide bus applications still running in the HVD mode, which are not compatible with LVD or SE. There is no term power line on the 80 pin connector, termination must be provided in the adapter or backplane. The 80 pin connector is designed for hot plugging, providing sequenced power. Power must be provide by the adapter or backplane. The adapter has the ID switches, there are no ID settings on the SCA-2 drives, the backplane position determines the device ID.
The SCA-2 connector is designed for hot plug systems for minimum bus loading. The used of an adapter for each drive doubles the bus loading, it is recommended that 2 adapters maximum be used on a system. It is better to use a backplane or 68 pin drives.
Case study: a Customer attempted to attach a WD Vantage 80 pin LVD drive
to a 68 pin bus, and the drive was not recognized by the host. The system was a PC with
host adapter, 68 pin cable properly terminated at both ends of the cable. The Customer
used an unknown 68 pin to 80 pin adapter purchased at a local electronics store, which
worked on some drives, but not all. Term power was present on the bus without the drive
present, so the issue was not termination. The problem was determined to be that the
adapter cards signal ground lines were all tied together, and grounded, thus forcing
the drive to operate only in the SE mode. However, the DIFFSENS line (pin 46 on the 80 pin
side) was left floating, not connected to anything. This caused some drives to try to
enter the LVD mode, however the adapter card only supported SE mode, so the drive was
unable to communicate with the host. A few drives using this adapter worked because a
floating DIFFSENS signal (an illegal state) caused them to enter the SE mode. Solution:
Change adapters to one with a grounded DIFFSENS line for SE operation.
6. Connecting WIDE SCA-2 (80 pin) drives to a NARROW bus (50 pin).
A narrow bus by definition is 50 conductors, but may be either Single Ended (SE) Low Voltage Differential (LVD) or differential (HVD). HVD devices cannot be mixed SE or LVD devices on the same bus. A wide drive can only be added to a narrow bus if the bus is Single Ended (SE).
7. Connecting WIDE SCA-2 (80 pin) drives to a WIDE bus (80 pin or proprietary).
A wide bus can run in Single Ended (SE), High Voltage Differential (HVD) or Low Voltage Differential (LVD) mode. Single Ended and LVD drives can be mixed, however the bus will revert to the slower Single Ended speeds and mode of operation when mixed, which is determined by the DIFFSENS level on the bus. Although rare, there are some wide bus applications still running in the HVD mode, which are not compatible with LVD or SE.
Case study: a Customer uses a backplane with proprietary DIN
connectors; a small paddle card is connected between the WD Vantage 80 pin drive and the
DIN connector. A standard 68 pin connector is provided on backplane for plug-in
terminator. Terminator has LED showing term power present. System is running in SE mode.
Problem: WD Vantage 80 pin drive wont spin up. The problem was determined to be that
pin 46 (DIFFSENS) on the SCA-2 side of the paddle card was open, not connected. This
allows the drive to go into the LVD mode, not the SE mode. Solution: Customer grounded pin
46 on the paddle card.
Updated - corrections Paul Aloisi, 05/30/00