ST506

The ST506 interface was created in 1982 by the company Seagate for its 5 ¼ -inch disks ST506 (5 MB) and ST412 designed (10 MB) and has long been regarded as the de facto standard.

It is a further development of the floppy disk interface and is very deep in the hardware. So the heads over step pulses in the cable are controlled and the recording method (MFM or RLL) is determined by the controller in the computer.

With the ST506 up to four drives can be addressed and they used two different cables. The A cable ( 34 -pin) is from drive to drive -connected ( daisy chain) and must be terminated (on the last disk ) at the end. It is used for addressing and control of the drive or the head. To distinguish between the two drives, some wires of the ribbon cable are twisted between the two HDD connectors.

The B Cable ( 20 pin) is a point-to -point connection between the controller and hard drive and transfers the read and write data ( bit-wise ). For the operation of four hard drives so five connections are required on the controller (one A and four B). But in PC no more than two hard drives due to BIOS and operating system limitations were common.

Addressing the data on the hard disk was over head, track and sector. As MFM encoding was used with 17 sectors per track or RLL with usually 26 sectors per track. Controller and hard drive had to be designed for each of the encoding. The data transmission rate between controller and hard drive was 5 Mb / s at MFM and 7.5 Mb / s at RLL. From the rotation speed of 3600 min -1 and usually the number of sectors per track to a usable data rate of about 522 or 799 kB revealed / s

IBM launched from its own interface called ESDI, which was only mechanically compatible. With 34 sectors per track the capacity and data rate could be increased.

Successor of this interface is Integrated Drive Electronics (IDE).

Pin assignment

The pin assignment of the two cables is loud ST506/ST412 OEM Manual: ( The symbol " ~ " denotes a logically inverted signal)