ST506/412 Interface
In the early years of computers (1978-1983) IBM decided to rely on industry standard interfaces for their systems. Consequently, the ST506/412 interface was one of the first interfaces used for the IBM-XT computer system.

First introduced in 1980 by Seagate Technologies, the ST506/412 was first used in the ST506 drive. This drive was a full height, 5-1/4 inch case with a formatted capasity of 5 MB. By today's standard ST506 drive was an antique dinosour.

  • In 1981 Seagate developed the ST-412 drive which added a feature called "buffered seek" to the interface. The ST-412 had a total formatted capasity of 10MB.

  • In 1983 IBM added "ROM BIOS Support" to the ST506/412 interface for the then current XT systems. Motherboards at that time had no support for hard drives.

  • Also in 1983, When IBM introduced the AT style computer system, ST506/412 support was added to the ROM BIOS located on the motherboard. Since then, any system that is compatabe with IBM systems, has an enhanced version ST506/412 interface built into the motherboard ROM BIOS.

Characteristics of this interface are:

Encoding - Encoding was done on a separate controller Card.

  • MFM Coding - The original encoding scheme or method was Modified Frequency Modulation or known as (MFM). This method packed data on a disk rather loosely which allowed a disk size of 5 to 100 MB. This method looks at data on a disk one bit at a time. The data transfer rate of the ST506 with MFM is 625K bytes per second.

  • RLL Coding- As the size of programs increased the need for larger drives became apparent. A new encoding scheme called Run Length Limited was developed which packed data on a disk much tighter. The chief difference between MFM and RLL is that the RLL method looked at data in 16 bit chunks. This allowed a kind of compression that allowed about 50% more data on a disk.

    The problem is that with the RLL method, you need a higher grade media (disk surface) and the timing is more critical. The data transfer rate of the ST506 with RLL is 937K bytes per second. The RLL method is still in use today on ESDI,SCSI, and IDE drives.

Cables and Connectors

  • Drive Select Jumpers - are located on the hard drive circuit card and are numbered 0-3 or 1-4.   If two drives are installed in the system, and a twisted cable is used. I a two disk system both drives should have a jumper for the second drive select.  This will be either pin 1 or pin 2 depending on how the jumper block is labeled.

  • One 20 pin data cable is required for each drive.

  • Terminating Resisters - The drive at the end of the data cable (Drive C:) should have the termination resister installed. (This is at the end of the daisy chain) The second drive should have the resister removed.

  • Cables - Two drives are connected to a single controller card through 2-3 cables. A single 34 pin controller cable is used by both drives. Some connectors are keyed on the end where they connect to the controller card. The drive control signals are reversed in the cable by twisting five of the wires between the drive connectors. (Simular to Floppy Disk Cables except floppy cables have 7 twisted wires)

    Reference
    For a complete description of the ST506/412 interface and complete configuration and installation instructions see "Upgrading and Repairing PCs" (Fifth edition) by Scott Mueller, Chapter 15,"Hard Disk Interfaces" (ISBN 0-7897-0321-1) $49.99.