External upgrade for +D interface. Contains 128kB PEROM, 32kB SRAM and 8255 PPI (can be configured for joystick, network cable or IDE).
The interface has through slot to which the +D must be connected.
The 8255 is connected as in MB-02, also have the same DB37 slot configuration. In the #1F/#3F/#5F/#7F configuration 8255 can be used as Kempston Joystick interface (port #1F), and ZXVGS network cable (#3F/#5F). In #1B/#3B/#5B/#7B config, can drive IDE8255 interface for harddisk and CD-ROM.
The 128kB PEROM provides 8 ROM pages that replace the +D ROM. PEROM can be reprogrammed in-system. The pages can contain boot ROM that detects harddisk and floppy disk format, original G+DOS ROM, UniDOS ROM, patched G+DOS/UniDOS with access to diskimages on harddisk and ZXVGS PLD.
The 32kB SRAM can be paged as two separate 8kB RAM pages, or as two separate 16kB RAM. This allows to run CP/M (second screen of ZX128 is required or an ZX48 upgrade that allow page-out the screen).
The memory is switched with #EF port that sets 8 latches. Bits 2, 3, 4 are latch addres and bit 5 as data input.