Fullscreen HTML5Med Res (??MB)HTML4

COMPUTER BITS BY BYRON MILLER The inside story on hard disc standards Confused about hard disc drives? Here we sort out the differences between today's competing disc drive standards. There is a battle raging. It is a battle to assume the role of standard bearer for the PC hard-disc drive interface. The venerable ST-506 served the PC industry well during its first decade, but as we move off into the 90s with increasing reliance on high-performance 386, 486 and 586 systems, users are demanding ever-greater speed, capacity and ease-of-use. Three technologies - ESDI, IDE and SCSI - are vying to become the next standard. But how does the prospective buyer choose from these? In this article, we will examine the basic ideas and history behind each, compare and contrast their strengths and weaknesses, and point out situations where each would be useful. Reprinted with permission from RadioElectronics magazine, June 1992 issue. Copyright©Gernsback Publications, Inc., 1992. Because each of the three new driveinterface standards represents, in some way, a response to the ST-506, let's begin with a little history and Background background on the development of that standard. Properly speaking, the ST-506 was the model number of a hard-disc drive that Seagate Technology introduced in 1980. The capacity of that drive was a whopping five megabytes (5Mb)! Several years later, Seagate introduced a 10Mb monster (the ST-412) with a similar electrical interface, and a new feature called buffered seeking. This feature allowed the drive to "collect" sequential seek commands and then move the read/write head across the surface of the disc in one quick, smooth motion. These drives recorded data on the .disc platters using modified frequency modulation (MFM). The combination of recording method and electrical interface limited the maximum rate at which data could be transferred to and from the drive to five megabits per second (5 Mbps). By encoding the data on the drive in the run-length.limited (RLL) format, designers could increase the data transfer rate by 50% to 7.5 Mbps. The capacity also increased by 50%. Early standards This 85Mb hard disc drive from Western Digital has an IDE interface. It also features 960 cylinders, 10 heads & 17 sectors per track. 92 SILICO N CHIP The market continued to demand greater performance and so, in early 1983, an ad hoe committee formed and produced the first draft of a specification for a new drive interface. This later became known as the Enhanced Small Device Interface (ESDI). By 1986, ESDI became a proposed ANSI standard and early in 1990, it became officially recognised·as ANSI X3.1701990. Development of the Intelligent Drive Electronics (IDE) interface began in 1984 when Compaq got together with Western Digital to develop an ST-506 controller that mounted directly on a hard-disc drive. The following year, Compaq worked with Imprimis (now a part of Seagate) to integrate Western Digital circuitry onto a Wren disc drive. Compaq subsequently shipped the first PC with an IDE drive and other manufacturers followed suit shortly thereafter. The appeal of IDE is that it eliminates one PC board and most of the interface electronics required between a system bus and a hard disc, thereby significantly lowering cost. Today, IDE has pretty much displaced ST-506 as the standard drive interface for desktop PCs. The Small Computer System Interface (SCSI) can be traced back to the Shugart Associates System Interface (SASI), which was developed by the same company (Shugart Associates) and the same designer (Al Shugart) that developed the ST-506. In fact Shugart developed SASI around the same time as the original ST-506. From the beginning, the SASI interface was designed to be more general than the specialise9- interfaces previously developed for personal computer peripherals. Rather than using specialised signals to control various lowlevel hardware functions, SASI/SCSI included from the beginning a general-purpose 8-bit parallel bus and several control signals. The hope was (and still is) that a general-purpose bus would attract designers of various types of peripherals. SASI supported several important features, including daisy-chaining drives and issuing high-level commands via a command block. Vendors quickly adopted SASI and began to add features and functionality; eg, support for Write Once Read Many (WORM) drives and other types of devices. Similarly, vendors increasE)d the maximum number of devices from two to seven. They also added the ability to service several devices at once. After some evolution, the SASI interface became so popular that in 1986 the X3T9 .2 ANSI working group adopted it as standard ANSI X3.1311986, or SCSl-1 for short. An enhanced version, SCSI-2, was finalised in 1990; it provides for wider bus widths and other performance-enhancing features. With that background in mind, let's now look at each type of interface in more detail. ESDI basics ESDI is a disc-controller interface TABLE 1: ESDI AND ST-506 SIGNALS ESDI Signal ST-506 Signal Pin No. Head select Reserved 2 Head select Write gate Head select Write gate 4 6 Config/status data Seek complete 8 Xfer Ack Track 0 Attention Write fault Head select Head select Sector Pin 7 on data cable Head select 10 12 14 16 18 20 22 24 26 28 30 32 34 Head select Index Index Ready Ready Xfer request Step Drive select Drive select Drive select Drive select Drive select Drive select Read gauge Command data Drive select ' Direction in that is like an enhanced ST-506. For starters, ESDI uses a similar cable and connector scheme - a 34-conductor control cable that is daisy-chained from drive to drive, and a separate 20conductor data cable for each drive. ESDI controllers typically support only two drives, even though the specification allows a maximum of seven. The signals on ESDI and ST-506 cables are similar but by no means identical, so you cannot run an ESDI drive on an ST-506 controller, nor an ST-506 drive on an ESDI controller. Electrically, all signals are TTL compatible; the maximum length of an ESDI drive cable is about three metres . Table 1 compares signals from both of those systems. Another similarity between ESDI and ST-506 is that ESDI is a devicelevel interface. In other words, its control signals ·direct low-level actions such as selecting a drive head and moving it to a desired track on the disc. As we'll see, SCSI and IDE devices contain high-level interfaces in which the operating system issues commands like: "Give me a block of data, as quickly as you can, and don't bother me with the details!" The biggest difference between ESDI and ST-506 drives is the data transfer rate, which for basic ESDI drives runs at twice the ST-506 rate (10 Mbps) , -- and which reaches its maximum at 24 Mbps. As for disc format, ESDI drives typically put about 34 sectors on a track (versus 17 for a standard ST-506 drive), and they run with a 1:1 interleave. In operation on a PC, most ESDI controllers emulate standard ST-506 controllers (eg, the ubiquitous WD1003), so no additional software drivers are required. IDE drives also emulate the WD1003 but SCSI drives always require external software drivers. IDE The IDE interface strongly resembles the AT I/O expansion bus , as shown in Table 2. There are some important differences and there is also some inconsistency in the way different manufacturers use some signals. For example, IOREADY can appear on pin 21 , pin 27 or on both, depending on the disc drive manufacturer. Many new system boards contain a built-in IDE interface, so there's no need to waste an expansion slot on a disc controller. Inexpensive IDE adapter cards are also available for older systems. If you're not buying a preconfigured system, you must check to ensure compatibility between your intended controller and hard disc drive(s). Electrically, an IDE drive connects to the O C /'Olll-:H -1992 93 TABLE 2: I0E AT 1/0 BUS SIGNALS AT 1/0 Signal IDE Signal Description CS1FX- N/A Chip select for ST-506 compatible 1/0 CS3FX N/A Chip select for ST-506 compatible 1/0 DA0-DA2 SA0-SA2 Drive address bus lines DASP N/A Drive active I Drive one percent 000-0015 SD0-S015 Drive data bus DIOR- -IOR Drive 1/Q read DIOW- -IOW Drive 1/Q write DMACK- -DACKx DMAWQ acknowledge DMARQ DRQx OMA request INTRO IRQ14 Drive interrupt IOCS16- -I/OCS16 Drive 16-bit 1/0 IORDY IOCHRDY 1/0 channel ready PDIAG- N/A Passed diagnostics RESED- RESET Reset; on AT bus is opposite polarity SPSYNC N/A Spindle sync. Produces clock for slave drives . controller with a 40-conductor ribbon cable. Like ESDI, the IDE interface emulates a standard IBM hard-disc controller, and an IDE drive masquerades as one with a corresponding value in the host system's BIOS drive table. Internally, an IDE drive typically has 34 sectors per track, although translation can make it appear to have 17, to match a BIOS table value. In addition, IDE drives usually operate at a 1:1 interleave. You cannot change interleave, perform a low-level format, or run low-level disc utilities. The controller electronics reside at standard disc-drive I/O port addresses (IF0-IF7 and 3F0-3F7) and respond to all standard commands (format track, read sector, write sector, etc), as well as enhanced commands that allow for more efficient operation. For example, commands C4 and C5 allow the system to read and write multiple sectors, respectively. However, most AT BIOS's do not yet support the enhanced disc-drive commands. The IDE interface has evolved rapidly since 1984, occasionally with different vendors creating incompatible enhancements. Hence, in 1988 a Common Access Method (CAM) commit94 SILICO N CI-IJP tee formed to define standards. By early 1989, the committee had produced a draft of an AT Attachment (ATA) interface standard. That document has evolved quite a bit over the years and is now well on its way to becoming an ANSI standard, by way of the X3T9 .2 working group. Like the ST-506, the IDE standard allows a maximum of two devices on its shared bus. Drive 0 functions as the master and drive 1 as the slave. Maximum cable length is only 45cm, so the drives must be situated close together, SCSI SCSI is an intelligent system-level interface that, in theory, can connect a variety of devices through a common parallel 8-bit bus, including disc drives, optical scanners, printers, tape drives, network adapters , and various types of optical drives . It is an unfortunate fact of life that, in practice, you'd probably end up installing a different SCSI host adapter for each type of device in your system. And it is difficult if not impossible to use a SCSI device intended for one system (eg, a DOS-based PC) on another (eg, a Macintosh) system. The SCSI bus consists of eight data bits, a parity bit, nine control lines, and a line for terminator power, as shown in Table 3. The bus can be driven with either single-ended or differential line drivers. In both cases, the bus has a total of 50 lines. A single-ended system alternates grounds with signals; in a differential system, even and odd pins form differential signal pairs. Maximum cable length is six metres for single-ended systems and 25 metres for differential systems. SCSI devices on PCs and Macintoshes usually follow the single-ended standard. A host device issues a command to a SCSI device via a 6-byte command descriptor block, which specifies an op code, a logical unit number and block address, a length control byte, and a control byte. The control byte has a feature that allows multiple SCSI commands to be sent in a single block. Every SCSI command returns a status byte, each bit of which has a specific meaning (good, busy, etc). Most devices currently on the market adhere to the SCSI-1 standards. However, many new devices conform to SCSI-2, which offers much greater potential performance. Whereas SCSI1 allows a maximum of four million transfers per second, SCSI-2 allows 10. In addition, SCSI-2 increases maximum bus width from the 8-bit SCSI-1 standard to an optional 16 or 32 bits. The X3T9.2 committee completed the SCSI-2 specification in August 1990; after editorial polishing, it should be published some time this year. SCSI can communicate with several different devices simultaneously. For example, a SCSI host can disconnect from a target device after issuing a command, connect to a different target device , give it a command, disconnect from it, and then reconnect back to the original device. By contrast, IDE operates in a master/slave mode in which the interface can issue only a single command at a time. BIOS-level software drivers are re• quired to use a SCSI device in a PC, typically added through an adapterbased EPROM or a device driver loaded at boot time. The Macintosh has a built-in SCSI manager. SCSI compatibility is still a problem. Although electrically identical, SCSI peripherals from different vendors may be dissimilar. In other words, a SCSI drive from vendor A may work TABLE 3: SCSI BUS SIGNALS Slgnal(s} 080-7 8-bit bidirectional parallel data bus Data bus parity line (optional) ATN Attention. Used to send message to target when it has control of the bus BSY Busy. Indicates that the bus is unavailable for use ACK Acknowledge; used by initiator for handshaking RST Reset. Used to initiate a bus free phase MSG Driven by target to indicate that current transfer is a message CID REQ 1/0 NEW Explanation DBP SEL whets Used by initiator to select target before command execution. Also used by target to reconnect when the re-selection phase is implemented Control/Data. Used during information transfer phases to transfer commands, status, messages and data over the bus Request by target during information transfer phases. Handshakes with ACK to envelop data Input/Output. Determines direction of transfer during information transfer phases in Speaker Design ? " "i) . ~~:~k~r $149* ~~ Designer Australia 's latest complete speaker design environment, includes, enclosure, crossover & optimisers, zobel, room placement & much more ! CALSOD l .20H - $119*# Australian and powerful LEAP - modular trom $449# LEAP - evaluation $149# LMS Ver 3.0 - update $195*# Pnces based an AUSSl=USS0.75 fine with a given SCSI adapter, while a SCSI drive from vendor B does not. This is due to variations in the interpretation and implementation of the SCSI command set. Hundreds of commands are available, some of which work differently with different types of devices. For example, one form of the write command can be used for writing to a Direct Access Device (DAD) and another for a Sequential Access Device (SAD). One vendor can interpret a disc drive as a DAD where another would interpret it as a SAD. Sending a SAD write command to a DAD device will not work. In response to that dilemma, the CAM committee has defined a standard subset of SCSI commands that performs basic functions (read, write, etc). The resulting eleven commands are known as the Common Command Set (CCS) and are part of the SCSI-2 standard. Compare & contrast Like ST-506, ESDI is an unintelligent device-level interface that transfers data serially from drive to controller, which compiles serial bits into 8-, 16- or 32-bit chunks of data and presents them to the host. IDE and SCSI devices, by contrast, build up data bytes on the drive and present them to the system in 8-, 16or 32-bit chunks. The advantages are several: less-expensive controllers and adapters, less cabling, improved reliability, and higher performance. IDE drives (even with an adapter, if required) typically cost less than SCSI and ESDI drives of comparable capacity and performance. However, a given system can hold a maximum of two IDE drives , whereas seven SCSI devices can be handled directly, and theoretically thousands indirectly. ESDI controllers typically allow only two drives and there is no pretence of supporting other types of devices. Both IDE and SCSI drives suffer from various types of compatibility problems that make system integration trickier than it should be. * demo disk available # comprehensive data available ME Technologies ( an ME Sound Pty Ltd subsidiary ) P.O. box 50, Dyers Crossing NSW 2429 '!I' 065 50 2254, fax 065 50 2341 Silicon Chip Binders Recommendations Selecting a drive interface depends on your performance needs, capacity needs, budget, and future system plans. If cost is the main determinant, you'll probably want to go with IDE. If performance is paramount, ESDI or SCSI will be your choice. Remember that performance you don't need right now may become necessary in the future. Sometimes a little added expense turns out to be a good investment. If you need a really large drive, ESDI or SCSI will also be required. If you hope to share a single interface card among multiple peripherals. SCSI may eventually help you realise that goal. SC These beautifu lly-made binders will protect your copies of SILICON CHIP. They are made from a distinctive 2-tone green vinyl & will look great on your bookshelf. Price: $A 11.95 plus $3 p&p each (NZ $6 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Qcromrn 1992 95