Hard Disk Primer This abbreviated document is published as an informational reference for the numerous hard disk drives encountered in the PC environment. The information is intended to identify the different characteristics required when installing, formatting and servicing hard drives. Disk Drive interface types. MFM ST412/506 17 SPT RLL ST412/506 26 SPT SCSI Small Computer System Interface. SCSI-2 SCSI w/fast (10mbs) and/or wide SCSI options. SCSI-MAC Apple Macintosh SCSI interface. ESDI Enhanced Small Device Interface. SMD Storage Module Drive interface. SMD-2 20 MHZ SMD interface. IDE/AT 16 bit Integrated Drive Electronics. IDE/XT 8 bit Integrated Drive Electronics. PS/2 IBM PS/2 interface. IPI Intelligent Peripheral Interface (IBM) COMMON DISK DRIVE INTERFACES The most common disk interfaces found in the PC world are, ST412/506, SCSI-(I&2), ESDI, IDE and EIDE. Other less common disk interfaces are SMD (Storage Module Drive) and IPI (Intelligent Peripheral Interface). IPI is an IBM System 38/AS-400 bus interface. SMD is typically used on larger 8" and 14" mini and mainframe disk drives. ST412/506 INTERFACE ST506 was once the most most common interface in the PC world. It is a bit serial interface that runs at 5 Mega-Bits or 625 KB (Kilo-Bytes) per second data rate. It can be identified by the dual cable interface from the drive to the controller. The data cable has a 20 pin connector and the control cable has a 34 pin connector. The interface required a separate data cable for each drive and a common dual connector control cable for both drives. ST506 is the original mainstream device interface designed for the Seagate 5MB ST506 drive when small hard disks were a new technology. The ST506 interface can only handle drives up to 140MB due to design limitations. Up to 4 drives may be attached to one controller. MFM Recording The data recording technique on ST506 drives is called MFM, (Modified Frequency Modulation). The data stream from the controller to the drive is an 8 bit per byte serial interface. MFM controllers can correct single bit data errors from the disk. MFM drives have 17 sectors per track, with 512 bytes per sector. RLL Recording RLL (Run Length Limited) is an encoding technique that limits the number of adjacent 0's in the recording data stream. The code for the byte being recorded is calculated by the controller, based upon the previous byte and the next byte. The code generated for recording each byte is less than 8 bits per byte. RLL 2,7 insures that the sequence of zeroes in any byte will range from 2 to 7. When the data is read, the controller expands the encoded data back into the correct 8 bit byte pattern. By using this encoded byte recording technique, approximately 50% more data can be recorded onto the same disk area with less error probability than MFM recording allows. RLL drives have the same dual cable, 20/34 pin connector interface as MFM drives. RLL drives have 26 sectors per track with 512 bytes per sector. NOTE: MFM drives should not be used on RLL controllers. The RLL recording frequencies exceed the limit of MFM heads and media. A drive that was reading/writing data fine may suddenly cease to function due to these stresses and total data loss will occur. It is not physical damage but the inability to sustain those high data rates indefinitely. If the drive is installed on an MFM controller and reformatted it will generally function properly. ST412 interface The ST412 interface added "buffered seeks" to the ST506 interface to accommodate the 10MB ST412 disk drive. The data recording techniques remained the same. ESDI INTERFACE ESDI (Enhanced Small Device Interface) is an improvement on the ST506 interface. As computer speeds increased, the need for a faster interface emerged. ESDI is an intelligent controller that can handle hard drives, floppies and tapes, and handle direct transfers between these devices. ESDI use a bit serial data transfer the same as ST506. Data transfer rates on the ESDI interface can be as high as 24 Mega-bits/sec but usually are 10 Mega-bits/sec (2 mega-bytes). Data separation occurs on the drive for greater data integrity. ESDI uses the dual cable 20/34 pin interface from controller to drive. ESDI controllers and hard drives must be matched to insure compatibility. Up to four drives may be attached to one controller. ESDI soft and hard sectoring. ESDI drives can be soft sectored or hard sectored dependent upon the controller used. Hard sectored controllers use 34 SPT and soft sectored controllers can use 34, 35 or 36 SPT. The drive must be jumpered for hard/soft sectoring to match the constraints of the controller being used. (ie WD1005-WAH controller is hard sector only at 34 SPT) ESDI translation mode For systems with BIOS types that only support 17 SPT, the controller may be jumpered for translation mode. In ESDI translation mode, the controller converts the drive to a 17 SPT drive by logically doubling the number of physical heads. (see exmple below) Actual drive parameters 940 Cyl 4 hds 34 SPT BIOS selection if in 940 cyl 8 hds 17 SPT translation mode. A disk address of cyl 0, hd 5, sec 0, will actually R/W to cyl 0, head 0, sec 18 in translation mode. In translation mode the controller can convert 17 sector addresses or it can still R/W true disk addresses. The drive must be formatted in its real cyl, hd, sec configuration (940, 4, 34) even if it is jumpered for translation mode. SCSI INTERFACE SCSI (Small Computer System Interface) is a device interface that is designed to accommodate intelligent devices attached to the bus. SCSI controllers are intelligent controllers which contain the device software drivers for the various devices attached to the bus. The devices interpret and perform the SCSI commands sent by the controller. These devices can be hard disks, tapes, CD rom drives or any device capable of adhering to the standard SCSI command set. SCSI devices are daisy chained from device to device with the last device on the bus being terminated. Data transfer from controller to device is 8 bit wide, byte serial (not bit serial) and can range from 1.5 MB/sec in async mode to 5 MB/sec in burst mode. In async mode each byte is acknowledged by the receiving device. In burst mode acknowledgment is sent to the receiving device after receipt of the data block. SCSI supports device to device transfers on the bus without host computer interruption. The single ended SCSI bus has 9 signal lines and 9 data lines on a single 50 pin cable. Various manufacturers have implemented the SCSI standard differently. The spec did not require all elements of SCSI to be implemented in a device. This can result in not-quite -compatible devices from different manufacturers that do not operate correctly when daisy chained together. SCSI devices require a unique Logical Unit Number (LUN) from 0-7 which must be set for each device on the bus. The SCSI controller itself requires a LUN and (in the PC world) is usually assigned LUN 7 with the seven devices using LUN's 0-6. Up to 7 SCSI devices may be attached to one controller. NOTE!! In some mini computers, the controller is assigned LUN 0 and the devices LUN 1-7. It is a matter of how the SCSI driver SW was written, looking for a controller at either LUN 0 or LUN 7. SCSI-2 INTERFACE SCSI-2 is a new design standard intended to increase connectivity and compatibility. It uses an expanded common command set to accommodate compatibility, adds new device types to the interface and adds new device functions. SCSI-2 supports standard SCSI devices and SCSI-2 devices attached to the same bus. FAST SCSI Fast SCSI doubles the number of synchronous data transfers from 5 MB/sec to 10 MB/sec on the 8 bit bus by increasing the data bus clock speed from 5MHZ to 10MHZ. Fast SCSI is an optional mode. Both the controller and the device must be able to accommodate this mode. WIDE SCSI Wide SCSI increases the width of the data path to either 16 or 32 bits. 342 bit Wide SCSI requires a second cable type called a B cable which uses a high density 68 pin connector. The wide cable is used only for data transfer. With Wide SCSI and Fast SCSI implemented, data rates up to 40 MB/sec are possible. Wide SCSI is an optional mode. DIFFERENTIAL SCSI Differential SCSI is an interface which uses two wires per signal and the signal voltage reference is between the two wires and not to a ground reference point. Differential SCSI allows better signal to noise ratio's and longer cable lengths than single ended SCSI. Differential SCSI can be implemented with the standard SCSI cable as it uses 36 of the wires in the 50 pin cable. Single ended and differential SCSI devices CANNOT be mixed on the same cable. This is an either or decision. There is a different cable pin out for the two types of signalling and the controller and devices must match each other. IDE INTERFACE IDE is known as the Integrated Drive Electronics interface and also as the Intelligent Drive Electronics interface. On IDE drives, the disk controller is an integral part of the disk drive itself. Most IDE drives are 3.5" form factor and the SPT can vary from zone to zone on the media. Outer cyls may have 49 SPT, middle cyls 35 SPT, and inner cyls 26 SPT. IDE drives generally use RLL encoding. This combination of RLL recording and variable SPT allows greater storage densities on the same 3.5" media used with ST412/506 drives. IDE drives also incorporate automatic bad block sparing. IDE TRANSLATION TIPS Most IDE drives will not configure normally in the setup CMOS due to the 1024 track limitation and the variable sectors per track. The IDE design allows the drives to operate in a "translation mode" which makes the drive appear to the system as a 17 sector AT drive. In most drives the "translation" parameters are fixed and must be matched exactly. Some drives allow the cyl,hd,sec parameters to vary as long as they do not exceed the max storage capacity of the drive. Formula for calculating capacity in MB is: Cyls X Hds X Secs X 512 = disk capacity in MB. For drives with variable parameters, a drive type may be selected in the setup table that does not exceed the max capacity of the drive. For drives with fixed parameters, they must match up exactly with an entry in the setup table. If there is no exact match in the setup table, the drive must be user defined using its required parameters. After installing an IDE drive in a system, it will usually power up with a 17xx error if the drive setup is incorrect. PRIOR TO REPLACING A SYSTEM PCB ALWAYS CHECK THE CMOS SETUP FOR AN IDE DRIVE IF THE SETUP IS ACCESSIBLE!!! This will save much frustration when the drive documentation is unavailable. 528MB STORAGE LIMIT The original IDE specification allowed the system BIOS to address up to 1024 cyls, 16 hds and 63 sectors per drive. At 512 bytes per sector this allowed a total of 528MB of storage per disk. When systems came with 40MB and 60MB drives, this 528MB limit seemed sufficient. FORMATTING IDE drives are low level formatted at the factory and should not require low level formatting in the field. Performing low level formatting could lose some bad block information detected during marginal testing at time of manufacture. FDISK will partition, delete or repartition a DOS partition on an IDE drive but cannot create or delete a non-DOS partition. SpeedStor rev 5.11 or above and DiskManager rev 4.10 or above are disk utilities that must be used to create, modify and delete non-DOS partitions on IDE disks. IDE INTERFACE CONNECTIONS IDE drives interface to the system PCB in one of two ways. Newer PCs have a 40 pin dual in-line IDE connector and associated logic integrated on to the system PCB. A jumper is installed or removed or a CMOS entry is altered to enable or disable the IDE interface on the PCB. On system PCB's without the 40 pin dual in-line connector, an IDE controller PCB is installed in an available I/O slot and the 40 pin IDE cable connects to the IDE PCB. This adaptor PCB is simply a pass through of the I/O bus signals required by the IDE interface with some logic to control interrupt and priority arbitration. When two IDE drives are daisy chained they must be jumpered for a master/slave configuration. Refer to the manufacturers jumpering info on how to jumper a drive for a master mode or slave mode. Fast ATA Fast AT Attachment. (Some times called Enhanced IDE) ATA is a high speed IDE interface using standard IDE connections (single 40 pin data cable) to VESA local bus or PCI controllers. Data is transferred at up to 13.3 MB/s which is three times faster than standard IDE products and 30% faster than fast SCSI-2. The fast ATA interface supports PIO mode 3 at 11.1MB/s, DMA mode 1 at 13.3MB/s and multiple block reads and writes. The most common implementation of ATA is PIO mode 3. Fast ATA is backward compatible with existing IDE host connections and controllers. The drive electronics must support fast ATA operation and the IDE controller must also contain the fast ATA IDE chip in order to facilitate the high speed transfer. ATA and standard IDE drives can be mixed on an ATA controller as long as the appropriate jumpers are set on the IDE interface PCB on the drive. (I.E. An ATA master can have an IDE slave attached on an ATA controller. An ATA drive can be installed on a standard IDE controller and no PIO mode 3 operations will be attempted.) The EIDE spec also increases the available disk addressing space from 528MB to 8GB. ATA-2 Fast AT Attachment Mod 2. ATA-2 is an enhancement to the original ATA which will allow data transfers up to 16MB/s on standard IDE connections (single 40 pin data cable.) Fast ATA-2 supports all the functions of ATA and adds support for PIO mode 4 and DMA mode 2 to attain a maximum transfer rate of 16.6MB/s.