............................... STT20000N, STT20000N-C ............................... SCSI Minicartridge Drives ............................... (standard and NS-20 versions) ............................... ............................... Product Manual ...............................
© 1998 Seagate Technology, Inc. All rights reserved No part of this publication may be reproduced in any form without written permission from Seagate Technology, Inc. Publication Number 10005135-001, March 1998 Seagate, Seagate Technology, the Seagate logo and Sidewinder are trademarks or registered trademarks of Seagate Technology, Inc. Other product names are trademarks or registered trademarks of their owners. Seagate reserves the right to change, without notice, product offerings or specifications.
FCC Notice This equipment generates and uses radio frequency energy and, if not installed and used properly—that is, in strict accordance with the manufacturer's instructions—may cause interference to radio communications or radio and television reception.
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Table of Contents Table of Contents Introduction 1 Drive overview ........................................................................................................ 1 Features ................................................................................................................. 4 Typical system configurations ................................................................................. 5 Minicartridge technology overview.................................................................
Table of Contents Drive operation and maintenance 27 Introduction........................................................................................................... 27 Front panel LED operation .................................................................................... 27 Using Travan cartridges ........................................................................................ 28 Loading and unloading tape cartridges............................................................
Table of Contents Tape format 45 Introduction........................................................................................................... 45 Tape partitioning ................................................................................................... 45 Track positions ..................................................................................................... 45 Track numbering.................................................................................................
Table of Contents Notes Page viii STT20000A Product Manual
Introduction Chapter 1 1 Introduction Drive overview The Seagate internal STT20000 SCSI minicartridge drive extends the Seagate family of one-inch high, DC2000 drives that feature high performance, high reliability, and quiet operation. The STT20000 SCSI minicartridge drive transfers data at up to 60 megabytes per minute (Mbtyes/min) without data compression.
Chapter 1 Introduction The drive is equipped with a 512-Kbyte on-drive buffer to facilitate efficient operation. The STT20000 SCSI minicartridge drive supports the QIC-3220-MC format standard and will provide 10 gigabytes uncompressed storage capacity, 20 gigabytes compressed, with a Travan TR-5 cartridge. The drive will also read tapes conforming to the QIC-3095 standard (TR-4). A precision-burst intelligent positioning system is used to achieve high track densities.
Introduction Chapter 1 Figure 1-1 STT20000 SCSI minicartridge internal drive (3.5-inch mounting configuration) Figure 1-2 STT20000 SCSI minicartridge internal drive with mounting brackets (5.
Chapter 1 Introduction Features The STT20000 SCSI minicartridge drive embodies Seagate 's commitment to engineer reliable and durable tape drive products. In summary form, key features of the drive are as follows: Page 4 • Internal form factor for installation in a 5.25-inch half-high or 3.
Introduction Chapter 1 Typical system configurations The SCSI standard supports up to eight SCSI addresses or IDs. These IDs refer to host adapters or peripheral devices such as printers, magnetic disks, or tape drives. The eight devices or hosts are daisy chained together. Figure 1-3 shows sample configurations of SCSI systems.
Chapter 1 Introduction Flash EEPROM The STT20000 SCSI minicartridge drive incorporates leading-edge technology in the flash EEPROM, which is useful should the drive's SCSI firmware need to be upgraded at some point.
Introduction Chapter 1 For more information about a particular QIC standard, contact Quarter-Inch Cartridge Standards, Inc., c/o Freeman Associates, 311 East Carrillo Street, Santa Barbara, California 93101. About this manual The remaining chapters and the appendices in this manual are briefly described in the following table. A glossary of terms is also included.
Chapter 1 Introduction Notes Page 8 STT20000A Product Manual
Specifications Chapter 2 Specifications 2 Overview The STT20000 SCSI minicartridge drive provides exceptional reliability in storing large amounts of computer data.
Chapter 2 Specifications Figure 2-1 STT20000N (3.5-Inch mount)—general dimensions 4.00 in (101.6 mm) 6.36 in (161.5 mm) 0.157 in (4 mm) 4.00 in (101.6 mm) 1.00 in (25.4 mm) Figure 2-2 STT20000N (5.25-Inch mount)—general dimensions 0.86" (21.8mm) 5.49" (139.4mm) 5.76" (146.4mm) 0.20" (5.0mm) 5.87" (149.0mm) DRIVE ACTIVE (GREEN) Page 10 1.685" (42.
Specifications Chapter 2 Power specifications The following table lists the power specifications for the STT20000 SCSI minicartridge drive. (Power specifications are measured at the tape drive power connector and are nominal values.) Specification Measurement DC Voltage +12 VDC +5 VDC Voltage Tolerance ± 10% ± 5% Operational Current 2.0 amps 1.0 amp Standby Current 0.2 amp 0.6 amp Peak 2.
Chapter 2 Specifications Drive performance specifications The following table lists the performance specifications for the STT20000 SCSI minicartridge drive. Feature Specification Capacity 10.0 gigabytes—1650 Oe 740' Travan cartridge (uncompressed) 20.
Specifications Chapter 2 Environmental requirements The following table lists the environmental specifications for the STT20000 SCSI minicartridge drive. The drive may be mounted either vertically or horizontally.
Chapter 2 Specifications Reliability The STT20000 SCSI minicartridge drive is designed for maximum reliability and data integrity. The following table summarizes the reliability specifications.
Specifications Chapter 2 Recommended tapes The STT20000 SCSI minicartridge drive uses pre-written minicartridges. Description Seagate Travan (740 ft) (QIC-3220) 10.0 Gbyte capacity (20.0 Gbyte capacity with data compression) 20000TT Note. The STT20000 SCSI minicartridge drive reads and writes the QIC-3220-MC format. The STT20000 SCSI minicartridge drive reads but does not write the QIC-3095-MC format.
Chapter 2 Specifications Notes Page 16 STT20000A Product Manual
Installation Chapter 3 3 Installation Introduction This chapter tells you how to install the STT20000 SCSI minicartridge drive. The following paragraphs briefly outline the organization of this chapter. • The following section, Before You Begin contains general information that you should read before you begin the installation.
Chapter 3 Installation Three simple steps make up the installation procedure: 1. Configure jumpers. 2. Mount the drive unit. 3. Complete the power and interface connections. The installation procedure is the same for both models except physically mounting the unit in the computer. After you configure the jumpers and decide whether or not you need to add terminators, follow the steps provided for the size of enclosure into which you are mounting the drive.
Installation Chapter 3 Figure 3-2 Jumper configurations 15 13 11 9 7 5 3 1 16 14 12 10 8 6 4 2 JUMPER ON JUMPER OFF SCSI ID PARITY ENABLED JUMPER 7-8 ON RESERVED RESERVED ACTIVE TERMINATION JUMPER 13-14 ON TERMINATOR POWER ENABLED JUMPER 15-16 ON PIN PIN PIN 5 6 3 4 1 2 OFF OFF OFF OFF ON ON ON ON OFF OFF ON ON OFF OFF ON ON OFF ON OFF ON OFF ON OFF ON SCSI ID 0 1 2 3 4 5 6 7 Notes: In Figure 3-2, ON means that the jumper shunt is placed over the two vertical pins.
Chapter 3 Installation Figure 3-3 Daisy-chain configurations 1 2 SCSI Host Adapter (terminated) SCSI Tape drive (not terminated) other SCSI device (terminated) other SCSI device (not terminated) SCSI Tape drive (terminated) SCSI Host Adapter (terminated) The STT20000 SCSI minicartridge drive supports active termination and does not require the use of terminating resistor packs. Terminator power Terminator power (+5-volt) is enabled by default.
Installation Chapter 3 Installing the drive The following steps guide you through installing the internal Travan drive. Caution: Note: Turn off your computer before you begin the installation. Failure to do so might result in damage to your equipment or electrical shock to you. Because computer models vary between manufacturers, refer to your computer manual for specific installation instructions. 1.
Chapter 3 Installation Pin 1 on the SCSI connector is to your right as you look at the back of the drive. (See Figure 3-1.) Your SCSI cable should have Pin 1 highlighted by a color strip. Be sure to mate Pin 1 on the cable to Pin 1 on the drive. Failure to do so could make the drive inoperative. Note: 10. Replace the computer cover and reconnect any peripherals that you disconnected during the installation.
Installation Chapter 3 Figure 3-5 Mounting hole locations for drive with mounting brackets 0.4 in (10.2mm) 0.62 in (15.7mm) 0.86 in (21.8mm) .075 in (1.9mm) 2 PLACES 5.76 in (146.4mm) 3.12 in (79mm) 3.12 in (79,2mm) 5.49 in (139mm) 2.36 in (60mm) 2.08 in (53mm) 1.81 in (45.9mm) 5.87 in (149.0mm) 0.197 in (5mm) 1.7 in (43mm) 0.51 in (13.0mm) Cabling and connectors The drive provides standard, single-ended SCSI transmission.
Chapter 3 Installation The cable characteristic impedance should not be less than 90 ohms nor greater than 140 ohms. A cable characteristic impedance of greater than 100 ohms is recommended. To minimize noise and ensure even distribution of terminator power, the minimum recommended conductor size is 28 AWG (0.08042 mm 2). Electrical characteristics This section lists measurements of various electrical signals in relation to the singleended SCSI connection.
Installation Chapter 3 Single-ended SCSI connector The STT20000 SCSI minicartridge internal drive provides a 50-pin, right-angle, dualrow connector on the main PCB at the rear of the drive. The pin assignments for this single-ended connector are listed in the following table. Note: STT20000N Product Manual All odd pins except pin 25 are connected to signal ground at the drive. Pin 25 is left open. A signal name or abbreviation preceded by a –(dash) indicates that the signal is active-low.
Chapter 3 Page 26 Installation Pin Assignment 42 –MSG 44 –SEL 46 –C/D 48 –REQ 50 –I/O 1 The +5V drive supply is available on the SCSI connector as a terminator power option. This pin is connected to the +5V through a diode. The option is a hardware jumper on the rear PCB of internal drives with terminator power disabled as a factory default. 2 ANSI defines -RST as a bi-directional pin. On the drive, -RST is input only.
Drive operation and maintenance Chapter 4 Drive operation and maintenance 4 Introduction This chapter describes important operational procedures for the STT20000 SCSI minicartridge drive. It covers the following topics: • Operation of the front panel LED • Using cartridges • Loading revised firmware (updating flash EEPROM) • Drive maintenance and troubleshooting Front panel LED operation The front panel of the Travan drive contains the cartridge opening and one amber, light-emitting diode (LED).
Chapter 4 Drive operation and maintenance Using Travan cartridges The minicartridges recommended for use with the STT20000 SCSI minicartridge drive are listed in Chapter 2. This section describes some operations using the cartridges. Loading and unloading tape cartridges Your tape drive has a flip-up door that covers the cartridge opening when a tape cartridge is not installed in the drive. Once a cartridge is inserted, it is held firmly in place by the drive's positive locking mechanism.
Drive operation and maintenance Chapter 4 Setting the write–protect switch Travan minicartridges feature a write-protect switch located in the upper left corner of the cartridge. You can set the switch to keep data from being written on the tape. Use this switch when you want to make sure that important data on the tape will not be overwritten. Figure 4-4 shows the cartridge with the switch in the non-protected (read/write) or unlocked position.
Chapter 4 Drive operation and maintenance 2. Place the firmware upgrade cartridge record switch to the nonrecord position. Insert the firmware upgrade cartridge in the drive and observe the amber LED light on the front of the drive. 3. Once the upgrade cartridge is inserted, tape motion begins. The drive LED flashes on and off. Approximately 15 seconds later, tape motion stops, and the LED continues to flash. 4. The LED flashes on and off at a steady rate as the firmware upgrade continues.
Drive operation and maintenance Chapter 4 bulk erasers. Do not lay cartridges on the computer monitor or on the base unit of the computer. • Keep the cartridge away from direct sunlight and heat sources, such as radiators or warm air ducts. • Keep the cartridge free of moisture. Do not wet or submerge a cartridge in any liquid. • Do not expose the cartridge to temperature extremes. Allow the cartridge to assume room temperature slowly.
Chapter 4 Drive operation and maintenance Figure 4-6 Cleaning the drive head and capstan Capstan Tape Head Troubleshooting If a power interruption occurs during a backup or restore operation, start over when the power is restored. If the drive appears to fail during a backup or restore operation, try the following steps: 1. Remove and replace the cartridge and try again. Make sure you are using the correct type of tape cartridge. 2. Turn off all power to the computer and drive.
SCSI Interface Chapter 5 SCSI Interface 5 Introduction The STT20000 SCSI minicartridge drive provides an embedded Small Computer System Interface (SCSI) controller for communications between the host computer and the drive. The drive supports the SCSI-1 (ANSI X3.131, 1986) and SCSI-2 (ANSI X3.T92) interfaces. The factory default interface at power up is for the SCSI-2 interface. The STT20000 SCSI minicartridge drive provides a single-ended SCSI connection.
Chapter 5 SCSI Interface SCSI-1 message codes Code Description Direction* 00h Command Complete In 02h Save Data Pointer In 04h Disconnect In 05h Initiator Detected Error Out 06h Abort Out 07h Message Reject In/Out 08h No Operation Out 09h Message Parity Error Out 0Ah Linked Command Complete In 0Bh Linked Command Complete with Flag In 0Ch Bus Device Reset Out 80h Identify (When sent by host, disables Disconnect/Reconnect) In/Out C0h Identify (Enable Disconnect/Reconn
SCSI Interface Chapter 5 SCSI-1 commands Code Type Command 00h O TEST UNIT READY 01h M REWIND 02h V REQUEST BLOCK ADDRESS 03h M REQUEST SENSE 05h E READ BLOCK LIMITS 08h M READ 0Ah M WRITE 0Ch V SEEK BLOCK 0Dh V PARTITION 10h M WRITE FILEMARKS 11h O SPACE 12h E INQUIRY 13h O VERIFY 15h O MODE SELECT 16h O RESERVE UNIT 17h O RELEASE UNIT 19h O ERASE 1Ah O MODE SENSE 1Bh O LOAD/UNLOAD 1Dh O SEND DIAGNOSTIC 1Eh O PREVENT/ALLOW MEDIUM REMOVAL 3
Chapter 5 SCSI Interface ANSI X3.131 CONFORMANCE STATEMENT (SCSI-1) General features • Supports arbitration • Disconnect/reconnect. • Single-ended drivers. • Termination power supplied to cable (jumper option). • Supports both single and multi-initiator systems. • Fixed and variable block transfer lengths. • Hard reset. • Synchronous data transfer. • Parity implemented (switch option). • Space blocks, filemarks, sequential filemarks, and EOD. • Supports third-party reservation.
SCSI Interface Chapter 5 Messages Save Data Pointer Linked Command Complete Disconnect Linked Command Complete with flag Message Reject Identify Initiator Detected Error Synchronous Data Transfer Request Abort Message Parity Error Bus Device Reset Command Complete No Operation Vendor-unique commands • Seek Block • Partition • Request Block Address SCSI-2 interface The SCSI-2 interface for the SCSI Minicartridge drive conforms with the ANSI X3.T92 standard.
Chapter 5 SCSI Interface SCSI-2 message codes Code Description Direction* 00h Command Complete In 02h Save Data Pointer In 04h Disconnect In 05h Initiator Detected Error Out 06h Abort Out 07h Message Reject In/Out 08h No Operation Out 09h Message Parity Error Out 0Ah Linked Command Complete In 0Bh Linked Command Complete with Flag In 0Ch Bus Device Reset Out 80h Identify (When sent by host, disables Disconnect/Reconnect) In/Out C0h Identify (Enable Disconnect/Reconn
SCSI Interface STT20000N Product Manual Chapter 5 Code Type Command 08h M READ 0Ah M WRITE 0Ch V SEEK BLOCK 0Dh V PARTITION 10h M WRITE FILEMARKS 11h M SPACE 12h M INQUIRY 13h O VERIFY 15h M MODE SELECT 16h M RESERVE UNIT 17h M RELEASE UNIT 19h M ERASE 1Ah M MODE SENSE 1Bh O LOAD/UNLOAD 1CH O RECEIVE DIAGNOSTIC RESULTS 1Dh M SEND DIAGNOSTIC 1Eh O PREVENT/ALLOW MEDIUM REMOVAL 2Bh O LOCATE 34h O READ POSITION 3Bh O WRITE DATA BUFFER 3Ch O REA
Chapter 5 SCSI Interface M = Mandatory Command O = Optional Command V = Vendor Unique Command ANSI X3.T92 conformance statement (SCSI-2) SCSI-2 general features • Disconnect/reconnect, arbitration (required in SCSI-2). • Single-ended drivers. • Termination power supplied to cable (jumper option). • Supports both single and multi-initiator systems. • Fixed and variable block transfer lengths. • Hard reset. • Synchronous data transfers • Parity implemented (jumper option).
SCSI Interface Chapter 5 Read Block Limits Write Data Buffer Read Data Buffer Write Filemarks Read Position SCSI-2 messages Save Data Pointer Linked Command Complete Disconnect Linked Command Complete with flag Message Reject Initiator Detected Error Identify Synchronous Data Transfer Request Abort Message Parity Error Bus Device Reset Command Complete No Operation SCSI-2 vendor-unique commands • Seek Block • Partition • Request Block Address STT20000N Product Manual Page 41
Chapter 5 SCSI Interface Command set changes to Incorporate RWW and HDC The following changes have been implemented in both the SCSI and ATAPI command sets to support of Read While Write and hardware data compression. Included are methods for detecting the presence of each feature. Read-While-Write Log Sense Write Error Counts page code (02h) Log Sense page code 02h (Write Error Counts) has been added to the Seagate tape drives that have read while write (RWW) capabilities.
SCSI Interface Chapter 5 Hardware data compression Mode Select / Mode Sense Data Compression page code (0Fh) Mode Select / Mode Sense Data Compression page code 0Fh has been added to Seagate tape drives that support hardware data compression. A Seagate tape drive that supports hardware data compression can be detected by first requesting the available Mode Sense pages (Mode Sense page code 3Fh).
Chapter 5 SCSI Interface Parameters that control data compression A Data Compression Enable (DCE) bit of one indicates that data compression is to be enabled. When this bit is set, data sent to the Drive by the Host shall be processed using the ALDC compression algorithm prior to being written to the medium. A DCE bit of zero indicates that data compression is disabled.
Tape format Chapter 6 Tape format 6 Introduction The STT20000 SCSI minicartridge drive conforms to the QIC-3220-MC recording format standard. This format is for streaming magnetic tape in a minicartridge that is to be used for information interchange among information processing systems, communication systems, and associated equipment. This chapter provides an overview of the tape format used by the SCSI Minicartridge drive.
Chapter 6 Tape format Track numbering All even numbered tracks, and the Directory Track, are recorded in the forward direction (the direction from the BOT marker to the EOT marker). All odd numbered tracks are recorded in the reverse direction (the direction from the EOT marker to the BOT marker). All even numbered tracks are located below the Directory Track; all odd numbered tracks are above it. Track format Each track is recorded sequentially beginning with track 0, then track 1, and so on.
Tape format Chapter 6 Figure 6-1 General track layout Frame N Frame N+1 Frame N+2 Frame N+3 Frame N+4 Frames are numbered indirectly using the 26 most significant bits of the Physical Block Address. Frame operation is controlled by the drive and generally invisible to the host. Frames are used primarily as a means to control the error correction operations. Frames can be overwritten with new data frames or an end-of-data (EOD) frame. Append operations can only begin at EOD.
Chapter 6 Tape format Blocks Information in the Block Control byte determines the type of block being recorded except for ECC blocks which are recognized by their block numbers. Also, information about the number of user data bytes available in each data block is recorded in the Block Control byte of the block. The Data Field of the blocks always contain 512 bytes. However, the number of valid data bytes in the block may be less than 512.
Tape format Chapter 6 Write equalization The drive uses the technique of write equalization. Write equalization inserts short pulses in the write current to break up the long spacing intervals. These pulses are too short to be detected in the read process but result in significant improvement in read-back resolution. On 1650-Oersted media, write equalization provides the benefit of compatibility with Magneto-Resistive (MR) heads.
Chapter 6 Tape format Notes Page 50 STT20000A Product Manual
Theory of operations Chapter 7 Theory of operations 7 Overview The STT20000 SCSI minicartridge drive is a low-cost, high-performance TR-5 minicartridge tape drive that is ideal for standalone users or small network computer systems. With capabilities of up to 10 to 20 Gbytes, depending upon media and data compression, the drive offers quick, efficient backup and restore operations.
Chapter 7 Theory of operations Figure 7-1a Simplified block diagram of the STT20000N 256KB EEPROM 5VOLT 188uP 20MHz 40MHz CRYSTAL READ WRITE SCSI INTERFACE DIGITAL CONTROL ASIC TAPE HEAD STEPPER MOTOR MOTOR CONTROLLER CAPSTAN MOTOR 512KB DRAM SENSORS Figure 7-1b Simplified Block Diagram of the STT20000N-C Drive 256KB EEPROM 5VOLT 188uP 20MHz 40MHz CRYSTAL READ WRITE SCSI INTERFACE DIGITAL CONTROL ASIC STEPPER MOTOR MOTOR CONTROLLER ALDC-1 H/W DATA COMPRESSION RWW TAPE HEAD CAPSTAN MO
Theory of operations Chapter 7 Mechanics The mechanical package for the STT20000 SCSI minicartridge drive was designed to provide all the advantages of easy cartridge loading and unloading while maintaining the positioning accuracy necessary for high-density data recording. The orientation of the cartridge is the same as the majority of QIC minicartridge drives in the field.
Chapter 7 Theory of operations The 512-Kbyte DRAM (2Mbytes in the NS drive) data buffer allows the drive to maintain streaming when the SCSI host cannot provide data continuously at the tape streaming rate. The write driver receives data from the write formatter and generates write current, causing data to be written on tape by the tape head. The read channel amplifies and conditions the signal from the tape head and passes it back to the read formatter.
Theory of operations Chapter 7 The drive has an electro-optical sensor assembly comprised of a solid-state light source (LED) and a photo-sensor (photo-transistor) that sense the beginning-of-tape (BOT), end-of-tape (EOT), load point, and early warning holes of the cartridge. Media (Travan minicartridges) The drive is designed to use Travan TR-5 (or NS) cartridges, quarter-inch minicartridges, and QIC-Wide minicartridges. These small (approximately 2 inches x 3 inches x 0.
Chapter 7 Theory of operations Read-While-Write The STT20000N-C (NS-20) drive is able to read back the data it has just written, hence the term read-while-write check. A different head is utilized in the NS drive to accomplish this task along with some additional playback circuitry. The digital control ASIC is the same for NS and standard drives, but the NS drive enables the CRC checking circuitry necessary to perform the read-while-write check.
Glossary Glossary Appendix A A Azimuth—The angular deviation, in minutes of arc, of the mean flux transition line from the line normal to the tape reference edge. Backup—Copy of a file or collection of files on fixed disk, diskette, or tape. Ensures against data loss. Beginning of Media (BOM)—Equal to the physical beginning of the tape. Beginning of Tape (BOT)—Equal to the logical beginning of the tape. Bezel—Front panel of a drive. Bit—A single digit in the binary numbering system.
Appendix A Glossary Directory track—The track at the centerline of the tape, identified as track 254 by its Track ID frame. Disc Drive—A peripheral storage device that rotates the disk, writes data onto it, and reads data from it as instructed by a program. ECC—(Error Correction Code) Special drive generated information that can be used to correct bad blocks. ECC block—A block containing drive-generated ECC data in its data field and part of control field.
Glossary Appendix A RLL (Run Length Limited)—A data encoding method where data bits are encoded so that certain constraints are met with regard to the maximum and minimum distances between flux transitions. Streaming—A method of recording on magnetic tape that maintains continuous tape motion without the requirement to start and stop within an inter-block gap. Tape Drive—A peripheral storage device that records data onto removable tape cartridges. Used to back up a disk drive. (See also Disk Drive.
Appendix B Acronyms and units of measurement B Acronyms and units of measurement Acronyms and abbreviations Page 60 Acronym Meaning ANSI American National Standards Institute BIOS Basic Input Output System BOM Beginning of Media BOT Beginning Of Tape BPI Bits Per Inch CD Compact Disc CMOS Complementary Metal-Oxide Semiconductor CSA Canadian Standard Association DMA Direct Memory Access ECC Error Correction Code ECMA European Computer Manufacturers Association EEPROM Electronica
Acronyms and units of measurement STT20000N Product Manual Appendix B Acronym Meaning IEC International Electrotechnical Commission IPS Inches Per Second LED Light Emitting Diode LSI Large Scale Integration MTBF Mean Times Between Failures MTTR Mean Time To Repair OEM Original Equipment Manufacturer PCB Printed Circuit Board QIC Quarter Inch Cartridge Drive Standards, Incorporated RAM Random Access Memory RLL Run Length Limited SCSI Small Computer System Interface UL Underwrite
Appendix B Acronyms and units of measurement Units of measurement Page 62 Measure Meaning A Amp C Celsius or Centigrade cm centimeter dBa decibels, A-weighted sound power reference one picowatt F Fahrenheit ft foot or feet g acceleration of a free-falling body; equal to 32.17 feet per second2 Gbyte gigabyte Hz Hertz in.
