MTX ROM Card

Design

I used to KiCad to prepare this circuit diagram. When placed and routed, and then the board manufactured and populated, you get :-

Observe the fact the edge connector sticks out of the reverse side of the circuit board.

The card will accept the following ROM sizes, as controlled by the red jumpers J1 and J2 :-

Chip	Size	Subpages per slot	J1	J2	Comments
27C512	64KB	4	5V	any	Place chip so that pins 1,2,31,32 are empty
27C010	128KB	8	any	5V
27C020	256KB	16	SP3	5V
27C040 or 27E040	512KB	32	SP3	SP4

The card provides ROM for two adjacent ROM slots, as controlled by the black jumpers J3 and J4 :-

Slots	J3	J4	Comments
0 and 1	0V	0V	Don't use this combination, the system ROMs are in these slots
2 and 3	0V	5V	ROM cards normally sat in slot 2, older SDX ROMs sat in slot 3
4 and 5	5V	0V	Boot ROM and SDX ROMs normally sat in these slots
6 and 7	5V	5V	I've never seen anything in slot 6, slot 7 was intended for games

The content of the ROM chip can be viewed as a sequence of 8KB chunks, which appear in this order :-

• slot N, subpage 0
• slot N+1, subpage 0
• slot N, subpage 1
• slot N+1, subpage 1
• slot N, subpage 2
• ...

Note that the .COM to ROM program can be used to make ROM image files for programming into this "dual ROM slot" board. Be sure to use LOADER2.BIN or LOADER2A.BIN.

Also note that the Disk to ROM program can be used to convert CP/M filesystem images into ROM Disc Images. You can combine suitable CP/M boot ROM and SDX BASIC ROM images with these ROM Disc Images to produce a system which can access ROM content as disk, from both CP/M and SDX BASIC.

The board in the photo has the J1 and J2 red jumpers set for a 27E040, and the J3 and J4 black jumpers set for ROM slot 4 and 5.

The circuit uses a GAL16V8 to decode when the ROM slots are accessed and when the subpage register is written, and a 74LS273 to act as the subpage register.

Usage

These are many possible uses :-

• Put a small game (8KB) into a small ROM
• Put a larger game, like SMG (36KB), onto a small ROM
• Put the PASCAL and NewWord ROMs into a small ROM, and set the jumpers for slots 2 and 3
• Put something like the MAGROM menu, and a load of games into a larger ROM
• Put the original Video Wall ROM content into one slot, using a >= 256KB ROM. This relied on being able to copy the <= 120KB of disk image to RAM Disc.
• Put a special boot ROM, a special SDX ROM, and then a load of filesystem data into a larger ROM, and set the jumpers for slots 4 and 5. The special ROMs would use ROM subpages as the boot disk. This is an easy way to get (64-2)*8=496KB of ROM Disc. The reason this ROM card supports two ROM slots is specifically to allow it to provide both boot ROM and SDX ROM. This is a superior way to implement a Video Wall ROM based solution. It is also a convenient way to deliver games or other programs that are runnable from CP/M, or from SDX BASIC.
• ... more I'm sure

The ROM card is plugged in to the left-hand-side edge connector. This obviously means you can't use an SDX or REMEMOrizer at the same time, although if you had a REMEMOrizer, you probably wouldn't need a ROM card.

Cost

If you shop around :-

Item	Cost
PCB	£2
ROM	£3
GAL16V8	£1
MTX edge-connector	£2
Passives, headers, sockets, etc...	£2
Total	£10

I sometimes have spares on my shelf, which I sell for £10. Email me if you're interested.

Download

Full package downloadable from here.