Drive Controller Types for the PC
Want to learn about old hard drives? The first hard drive came out in 1956 and was called the IBM 305 RAMAC, (Random Access Method of Accounting and Control). It required 50 24-inch magnetic disks to store five megabytes of data, weighing in at a ton. Then in 1973 IBM introduced the IBM 3340 hard disk unit, known as the Winchester, it stored 30MB. Seagate introduced the ST-506 in 1980, the first 5.25-inch HDD, with a capacity of 5 megabytes in late 1979 to early 1980 for the personal computer (micro-computer).
MFM
- Modified Frequency Modulation
The first IBM XT computers used MFM hard drives. MFM allows a transfer rate up to 5MB/s and were only available in sizes from 5MB to 110MB. MFM uses a twin-cable hard drive interface. The data cable was 20 pin and another 34 pin cable. These drives fit into a 5.25” bay. Almost all MFM drives had 17 sectors per track.
It was not uncommon to have as many as 10 or more bad sectors with the drive still fully functional despite the areas that were marked. The drives had to be parked before the PC was transported to prevent damage from the drive heads impacting the disks.
Floppy disk drives also use MFM technology even on current systems with a standard floppy controller.
RLL
- Run Length Limited
The successor to the MFM drive, externally appearing the same as an MFM drive. This encoding method can store 50% more data on the disk than MFM. The RLL drive connected to the RLL controller with two cables just likes with MFM. The RLL encoding method was still used on IDE drives although RLL drives are not. Until the mid 1990's all IDE drives used RLL encoding.
If a 10MB MFM drive formatted with a RLL controller could store an additional 5MB (making it 15MB) and perform 50% faster. RLL drives put 26 or 32 sectors per track, unlike the MFM 17 sector per track formatting. However, the early practice of retrofitting RLL controllers to MFM drive systems resulted in reliability problems.
RLE is not a type of drive, but a way to format a drive. Whether or not the drive becomes MFM or RLE is determined when it is low level formatted and attached to either type of controller. Early MFM drives would often fail when formatted to RLE, until later RLE certified drives were introduced. You could get more out of your MFM drive by attaching to an RLE controller, but you risked reliability if your MFM drive was not RLE certified.
EDSI
- Enhanced Small-Device Interface
A somewhat obscure enterprise level drive technology dating back to 1983. Maxtor company introduce drive technology for extreme high end systems. EDSI was considered a direct competitor to the later dominate SCSI drive technology.
SCSI
- Small Computer System Interface
SCSI busses support up to seven additional devices attached at one time, an eight counting the adapter itself. The SCSI bus structure has two ends, with devices connected between them. On each end of the SCSI bus, you must install a terminator pack to help reduce the amount of noise on the bus cable. Therefore, a SCSI bus has two terminators, one on each end.
SCSI was first introduced in 1986. The SCSI-2 standard came out in 1990 and SCSI-3 in 1996.
- Fibre Channel Protocol (FCP) is the interface protocol of SCSI on the Fibre Channel.
SCSI drives use a processor for executing commands and handling the interface and a separate processor controlling the head positioning through servos. ATA drives use a single processor these functions. This can lead to an efficiency gain when using SCSI drives in a high demand setting, such as a web server. SCSI drives tend to outlast IDE/SATA since there is less physical movement needed to retrieve data under certain conditions. However, new high end SATA RAID adapters can prove to be equally as efficient for most applications.
SAS
- Serial Attached SCSI
SAS are drives using the SCSI command set and are basically the next generation of SCSI technology. A SAS controller can be used to connect SAS drives and SATA II drives. The SAS bus is point-to-point while the SCSI uses a multidrop bus. Many more devices can be attached to a SAS controller than a SCSI controller. NL-SAS, or Nearline SAS, are a class of enterprise level SAS drives.
Since SATA II includes performance improvements that challenge the advantages of SCSI, SAS technology has been introduced to extend the capabilities of SCSI, offering SAS-harddisks with spindle speeds of 22,000 RPM. SAS offers data transfer rates in excess of 3 gigabits per second (Gbps) with potential rates of 10 Gbps or more. SAS will replace regular parallel SCSI bus technology and offer compatibility with SATA II drives. SAS drives do not have to be terminated like parallel SCSI, have higher speeds, but still use the SCSI command set.
IDE
- Integrated Drive Electronics
Standard hardware interface for hard drives and CD/DVD drives in a computer. Introduced in 1986 with 20 megabytes of storage, the IDE drive soon became the standard for personal computers. IDE storage grew into the hundreds of gigabytes. The successor, SATA, reached terabyte capacities.
Since IDE technology uses a parallel data channel from the motherboard to the disk drives, it is also referred to as PATA, or Parallel ATA. The usage of the acronym PATA became prevalent for describing ATA, IDE, and EIDE after the introduction and adoption of Serial ATA (SATA) drives in the industry.
IDE drives use an embedded controller, unlike MFM/RLE. When MFM/RLE the controller was on the controller card. It is somewhat of a misnomer when the term IDE controller was introduced, since the controller has always been built into the IDE drive, so the card is actually just a host adapter. Prior to Pentium and laster 486 systems, a host adapter card (referred to as the IDE controller card) was placed in one of the card slots on the motherboard. The IDE host adapter (controller) became standard integration onto the main board post 486 systems.
EIDE
- Enhanced Integrated Drive Electronics
IDE with the ATA-2 extension which provides three enhancements to traditional IDE. Data transfer is faster, maximum drive capacity is higher, more drives per channel. A member of the PATA family of drive technology.
The inclusion of ATAPI, AT Attachment Packet Interface, which defines device characteristics for IDE devices like CDROM drives by converting the SCSI command set to an IDE interface command set.
PRML (partial result maximum likelyhood) succeeded RLE encoding for IDE drives in the mid 1990's. This is part of the new self monitoring S.M.A.R.T technology introduced in 1995 and implemented on all IDE, SATA, SSD and SCSI hard drives. SMART data is accumulated and stored inside a hard drive to evaluate the performance and history of the drive. Problems and events stored on the SMART drive can be displayed using many different software utilities. PRML is Seagate’s proprietary method for statistical analysis of the data.
ESDI
- Enhanced Small Device Interface
SATA
- Serial Advanced Technology Attachment
SATA or SATA 1 or SATA I - Communicates at a rate of 1.5Gbit/s. It can only handle a single pending transaction at a time while SCSI disks can handle multiple outstanding requests, allowing the drive targets to re-order the requests to optimize response-time.
SATA technology was introduced in 2002 and like PATA drives, these are IDE. SATA is still a type of IDE drive, unlike SCSI.
SATA 2
- Serial Advanced Technology Attachment Two
SATA 2 or SATA II - Communicates at a rate of 3Gbit/s. The term SATA II has grown in popularity as the moniker for the SATA 3Gb/s data transfer rate even though it is a misnomer. SATA II was the name of the organization formed to author the SATA specifications, now known as the Serial ATA International Organization, or SATA-IO. 3Gb/s has become synonymous with the term SATA II.
SATA 3
- Serial Advanced Technology Attachment Three
SATA 3 or SATA III - Communicates at a rate of 6Gbit/s.
