When I was developing the PDP-11/Hack I was also evaluating several options to add some sort of disks to the system. The ultimate solution would be some sort of MSCP controller. However at that stage I the system was still under construction and so I decided to look for the simplest disk controller interface on a PDP-11 which turned out to be a RLV12. The RLV12 has the advantage of supporting 22-bit memory addresses and it also is supported by almost all PDP-11 operating systems.
First I built a RLV12 emulator for the PDP-11/Hack. The idea behind the design is to have a microcontroller does all the processing and have some simple interface logic between the Q-Bus and the microcontroller. The data is stored on a SD-Card which is attached via SPI to the microcontroller. Because the bus protocol of the PDP-11/Hack was based on the real Q-Bus I then later decided to convert the design to a real Q-Bus design and make it my first Q-Bus interface card. In fact the only difference is that the Q-Bus version makes use of Q-Bus bus receivers and transmitters so at the end not only the MCU runs the same code but also the CPLD design is pretty much the same.
The RLV12 emulator is an all-in-one solution. It consists only of the board and plugs into the system bus (Q-Bus or Q-Bus64), it emulates four RL01/RL02 units and stores the data on a SD-Card. The data on the SD-Card is either stored in partitions that have the exact size of a RL01/02 unit or it can be a normal disk image file stored in a FAT-32 partition.
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