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The Memotech MTX Series

Martin Allcorn's Games ROM

Prototype

The finished product (V1.0)

Building MAGROM

As you can see in the photos above, Martin built the "development" versions of MAGROM on prototyping board. This is obviously a viable option for anyone who wants to build their own, but as a number of other Memotech users seemed to be interested in having one, I thought it would be worthwhile getting some PCBs professionally made. Having never done this before, this also seemed an ideal opportunity to develop the skills that I would need to complete my MTXPlus project.

To that end, with Martin's help, I have created a board design using KiCAD.

To give maximum flexibility, I wanted the option of being able to fit MAGROM to either the internal or external MTX bus connectors, in a similar way that Andy has done with his MTX Memory Card. To support this, Martin modified the operation of MAGROM to allow it to be bypassed, or rather, to require it to be selected for use when resetting the MTX - this allows the board to be left connected when not being used, even when mounted externally, this feature will save wear & tear on the edge connector - they are not meant for frequent connect/disconnect operations.

As Andy points out, there is very little clearance between the bottom of the MTX keyboard PCB and the surface of the computer board and any expansion cards. Therefore, for the board to be suitable for internal installation, the components need to be placed as far up the board, towards the rear of the case, as possible.

I modified Martin's KiCad schematic to include the second edge connector.

Note, although shown on the schematic in the same orientation, the J0 (internal) connector is the mirror image of the J10 (external) connector.

(Click on the image to open the PDF)

Once we decided that the design was mature enough, I took Martin's schematic and designed a PCB for manufacture by iteadstudio in China (they are the company that Andy uses to manufacture his boards, including REMEMOrizer).

As iteadstudio have a price point on PCB size at 10cm x 10cm, it made sense to try to keep the board within these limits.

Combined with the limited space inside the MTX, this means that the board layout is pretty congested at the top, this was necessary to ensure that the ICs were mounted in the deepest part of the MTX case, i.e. at the rear.

KiCad 3D view of the PCB, with components placed.

The edge connector profile is not accurate, but you can see the the edge connectors are mounted on the reverse side of the board. This is necessary for the board to align with the MTX PCB as well as allowing the components to fit sit in sockets, low enough on the PCB to suit the height limitation of the MTX case

Although this was my first attempt at committing a board design for manufacture, I took the plunge and placed an order with iteadstudio on 6th February.

 

I was resigned to having to wait a few weeks for delivery, but they processed the order and shipped it very quickly - the boards shipped on the 11th and arrived in Aberdeen on the 19th, less than 2 weeks - more than acceptable.

The boards looked great - at least they did before I set to work with a soldering iron! Visually, they appeared to be perfect and having done a point-to-point continuity check against the schematic, I felt confident enough to start construction. In this photos, you can see that I have mounted all of the passive components, apart from the ROM socket as I was waiting for one to be delivered.

I forgot that the MTX PCB internal edge connector does not have cut outs adjacent to the pins like the external one, so I  needed to cut slots in the ends of the J0 connector - this would have been done more easily before I fitted the connectors!

While I was waiting for the socket to arrive, Martin was able to press ahead and assemble his board, shown here connected to the external cartridge port, ready to go.

A higher resolution picture of the Martin's finished board - with a single edge connector fitted.

If you display the full size photo, you can see that, apart from the flash chip, Martin has used 74HCT chips throughout.

The solder side of Martin's  finished board.

His soldering skills are ever so slightly better than mine !

It works!

Martin's board running Software Version 1.0 - the photo records the time & date of this momentous event :-)

MAGROM proved to be very popular and the initial run of boards was soon snapped up. Although there is probably not a huge demand for additional boards, we have done a second run - shown here freshly assembled by Martin.

They are functionally the same as the first run, but I took the opportunity to add identification labels to the machine selection jumper in the top left corner.

(Do you want one ?)

 

Bill of Materials

 

These are indicative prices

- for example, the ICs are based on prices from RS Components for tubes of 25, but they provide a good "ball-park" figure.

Passive Components
C1 100nF tbc
C2 100nF tbc
C3 100nF tbc
R1 4.7k 5% or 1% 250mW (W) or 0.6W metal film tbc
R2 4.7k 5% or 1% 250mW (W) or 0.6W metal film tbc
R3 4.7k 5% or 1% 250mW (W) or 0.6W metal film tbc
R4 4.7k 5% or 1% 250mW (W) or 0.6W metal film tbc
R5 4.7k 5% or 1% 250mW (W) or 0.6W metal film tbc
P3 1 way 0.1" pin header tbc
P4 3 way 0.1" pin header tbc
- 0.1" jumper shunt (link) (for P3/P4) tbc
J0 Edge connector, 30x2 way right angled type tbc
J10 Edge connector, 30x2 way right angled type tbc

Passives Total   

tbc
RS Prices - HCT RS Prices - LS
U1 74HCT32 tbc U1 74LS32 tbc
U2 74HCT00 tbc U2 74LS00 tbs
U3 74HCT30 tbc U3 74LS30 tbc
U4 29F040 tbc U4 29F040 tbc
U5 74HCT374 tbc U5 74HCT374 (preferred) tbc
U6 74HCT138 tbc U6 74LS124 tbc
Semiconductor Total tbc Semiconductor Total tbc

Total (ex.VAT)   

tbc

VAT   

tbc

Grand Total   

tbc

 

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