Notes: DEC RQDX3

The RQDX3 Disk Controller is a Digital disk interface module for the Qbus. It was used in microPDP11 and microVAX computers. Digital offered a series of ST506/ST412 interface disks using MFM encoding. These disks had an RD identifier and varied from 5MB to 156MB. The controller was smart and, using a T-11 microprocessor, implemented the MSCP protocol that abstracted the disk as an error free string of blocks. The host computer did not have to deal with the details of geometry or work around bad blocks. This was all handled by the disk controller.

ST506/ST412 interface disks are not very common today and after 30 years have long passed the 5 year design life. The disks have about 8k bytes per track, between 4 and 16 heads and up to 2048 cylinders. The data was written to the disk using MFM encoding at a bit rate of 5 MHz. Today it is possible to simulate the disk using a microprocessor and flash memory.