ST506 interface
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The ST506 interface is a hardware interface . It was designed in 1980 by the Irish company Shugart Technology, now Seagate , for their innovative 5¼-inch hard drives ST506 (5 MB) and ST412 (10 MB) and has long been considered the de facto standard.
It is a further development of the floppy disk interface (sometimes also called Shugart bus) and starts very deep in the hardware. The head carriage is controlled by step pulses and the heads are selected by signals in the cable. The recording method ( MFM or RLL ) is determined by the controller in the computer.
Up to four drives can be addressed with an ST506 controller. He uses two different cables. The A cable (34-pin) is looped through from drive to drive ( daisy chain ) and must be terminated at the end (on the last hard drive) . It is used to address and control the drive or head. In order to be able to differentiate between the two drives, some wires of the ribbon cable are twisted between the two hard disk connections.
The B cable (20-pin) is a point-to-point connection between the controller and the hard disk and transmits the write and read data (bit by bit). To operate four hard disks, five connections are required on the controller (one A and four B). Due to BIOS and operating system restrictions, however, no more than two hard drives were common in the PC .
The addressing of the data on the hard disk took place via header, track and sector . The coding used was MFM with 17 sectors per track or RLL with usually 26 sectors per track. The controller and hard disk each had to be designed for coding. The data transfer rate between the controller and the hard disk was 10 Mbit / s, which corresponds to a user data rate of 5 Mbit / s for MFM and 7.5 Mbit / s for RLL. The rotation speed of mostly 3600 min −1 and the number of sectors per track resulted in a usable data rate of about 522 or 799 kB / s.
IBM derived its own interface called ESDI , which was only mechanically compatible. With 34 sectors per track, the capacity and data rate could be increased.
The successor to this interface is Integrated Drive Electronics (IDE).
Pin assignment
The pin assignment of the two cables is according to the ST506 / ST412 OEM manual: (The symbol "~" denotes a logically inverted signal)
| signal | Pin code | Pin code | signal |
|---|---|---|---|
| Dimensions | 1 | 2 | ~ HD SLCT 3 (or ~ Reduced Write Current) |
| Dimensions | 3 | 4th | ~ HD SLCT 2 |
| Dimensions | 5 | 6th | ~ WRITE GATE |
| Dimensions | 7th | 8th | ~ SEEK CMPLT |
| Dimensions | 9 | 10 | ~ TRACK 0 |
| Dimensions | 11 | 12 | ~ WRITE FAULT |
| Dimensions | 13 | 14th | ~ HD SLCT 0 |
| Reverse polarity protection (no pin) | 15th | 16 | reserved |
| Dimensions | 17th | 18th | ~ HD SLCT 1 |
| Dimensions | 19th | 20th | ~ INDEX |
| Dimensions | 21st | 22nd | ~ READY |
| Dimensions | 23 | 24 | ~ STEP |
| Dimensions | 25th | 26th | ~ DRV SLCT 0 |
| Dimensions | 27 | 28 | ~ DRV SLCT 1 |
| Dimensions | 29 | 30th | ~ DRV SLCT 2 |
| Dimensions | 31 | 32 | ~ DRV SLCT 3 |
| Dimensions | 33 | 34 | ~ DIRECTION IN |
| signal | Pin code | Pin code | signal |
|---|---|---|---|
| ~ DRV SLCTD | 1 | 2 | Dimensions |
| (not connected) | 3 | 4th | Dimensions |
| (not connected) | 5 | 6th | Dimensions |
| (not connected) | 7th | 8th | Reverse polarity protection (no pin) |
| (not connected) | 9 | 10 | (not connected) |
| Dimensions | 11 | 12 | Dimensions |
| + MFM WRITE | 13 | 14th | -MFM WRITE |
| Dimensions | 15th | 16 | Dimensions |
| + MFM READ | 17th | 18th | -MFM READ |
| Dimensions | 19th | 20th | Dimensions |