MasterSD, Acorn BBC Master SD Card Interface Cartridge

MasterSD Features

  • Quick installation, just insert into a cartridge slot
  • Opening up the computer is not required
  • Easy to access slot for a standard SD card
  • Excellent compatibility, no system memory used
  • Uses widely available .SSD disk image files
  • Sturdy 3D printed cartridge case
  • LOAD and SAVE your own programs
  • Uses the popular MMFS filesystem
  • Fast read/write transfer speeds
  • Not compatible with the BBC Master Compact

About the MasterSD

This project grew out of a desire to re-use parts of the ElkSD Plus 1 Rev 2 design, as the cartridge ports on the Electron/Plus1 and Master are electronically quite similar.

However, the differences were enough to necessitate significant changes from the ElkSD-Plus1. The Master lacks the Electron's 16MHz clock on the cartridge ports, it only presents the 2MHz CPU clock. This required a re-designed SPI interface to maintain performance in read/write operations.

Also, unlike the Electron the Master does not permit cartridges to access the I/O space at &FCxx and &FDxx by default. A bit in the ACCCON register needs to be set before I/O access and restored afterward. This causes a notable performance drop, which wasn't acceptable. So the MasterSD moves it's I/O registers into sideways memory space, implementing a three-way split ROM/RAM/IO sideways bank.

MasterSD FAQ

Q) Is installation really as easy as plugging in the cartridge?
A) Pretty much, yes. Plug in the cartridge, run the *MMFS command to select the SD card as the current storage device, and you're in business.

Q) Will the MasterSD work on a Master Compact?
A) No. The Master Compact does not have cartridge ports.

Q) Can the MasterSD be made the default storage device?
A) Yes. The *CONF. FILE command can be used to do this.

Q) How fast is the MasterSD?
A) It depends on the SD card being used, but 30K/sec reads and 25K/sec writes are about average.

Q) I have an OS switcher device with BBC Model B ROMs in my Master. Will the MasterSD work?
A) Yes. See the user manual for details.

Q) I've seen SD interfaces that connect to the user port. Why should I pay more for a MasterSD?
A) OK, time for a bit of a rant :)

Those user-port interfaces are one reason why I started work on the MasterSD. They have a whole heap of drawbacks. Just getting one installed involves flipping the Master upside down to get to the user port, then opening it up to install an EPROM chip. After that, depending on the exact interface design, you either have to flip over the computer to get access to the SD slot, or have it dangling on the end of a cable like a soap-on-a-rope. 

On the software side of things, these interfaces are far from optimal. Many of them ship with a filesystem that was never intended to work on the Master and/or has been abandoned and is no longer under development. This can cause a range of issues, most notably compatibility problems.

The MasterSD uses a version of the popular MMFS filesystem that has been adapted to offer maximum compatibility by using the MasterSD's on-board RAM rather then the Master's system memory, as user-port interfaces have to do as they lack any RAM.

MMFS can be updated on the MasterSD simply by copying the update to an SD card and running a single command. No need to burn an EPROM and mess about swapping chips.

Buying a MasterSD

The MasterSD is available to buy via EBay.co.uk. Click the Ebay logo to view the product page.

Downloads

Installation Instructions mastersd_install.pdf
User Guide mastersd_guide.pdf
Small Game/Demo Collection masterssds.zip