General
The ST506-Interface was designed in 1982 by the company Seagate for their 5 ¼-inch drive ST506 (5.4 MB) , ST412 (10.1 MB) und ST225(20.4 MB) and every well-known computer manufacturer was using this technology.

The ST506-Interface is working based on the MFM( Modified Frequency Modulation ) recording method, simular to the DEC RL interface (Section 1.1.C).

Implementation
My design is using a FPGA and currently the ST506, ST412 and ST225 disks ar supported . I decided to use the BeMicro-CV Development Board, equipped with an Altera Cyclone® V FPGA, type: 5CEFA2F23C8N. This board has 128 MB memory and thus it is also possible to simulate MFM drives bigger than 100 MB. In my design the MFM data is completely decoded in Real -Time, effectively "on the fly" and stored on a Micro-SD card. Therefore a simple HEX-Editor for example can be used for viewing the received data. Many other design's store only the time intervals of the MFM signals.

Problems, open issues
The interface and the corresponding signals were described in detail by the company Seagate and were widely respected. It looks quite different at data and timing format. Everything here is incompatible. Each manufacturer has guaranteed implemented his own track and data format which was genarated with their own low-level format program. The following differences exist:
>> CRC algorithm is different, such as different preset value.
>> Track format: ID AM differently.
>> Track format: DATA AM differently.
>> SYNC character differently.
Even the same manufacturer, for example, DEC. There were different formats used . A disk , formatted with the RQDX-1 controller Disk could not be used in a RQDX-3 environment. Furthermore, in my design the byte order is important, big-endian (eg Motorola) or little endian (eg DEC PDP11) and accordingly it can be selected with a DIP switch. To be able to save the data of a still functioning disk, the Clone Mode was also implemented. The data from the disc will be 1:1 transfered and then saved to the SD-Card. In the Emulator mode the data will be read from the SD card and the disc is simulated.

In collaboration with Computer-Museum Muenchen