It consists of just 2 chips, an edge connector and the 20 way header for a CF to IDE adaptor.
I used a 70ns 128k flash chip as that's actually cheaper than a 150ns 8k eeprom, however any reasonably modern flash/eeprom/eprom will do as long as it's 8k or more and faster than 350ns or so. The rom chip is wired as is it were an MTX rom board. A0 to A12 and D0 to D7 on the chip is wired to their respective points on the edge connector. GROM from the edge connector connects to both *OE and *CS. Any spare address pins from A13 upwards get wired to 0v. the *WE pin is connected to 5v.
The 74HCT138 does the I/O decoding. with A7 connected to the active high enable, and IORQ and A6 to the two active low enables. the A,B C selects are connected to A3,A4 and A5. That will decode the Y6 output as port #B0-B7 and Y7 to #B8-BF.
The IDE header needs Do to D7 connecting, D8 to D15 are left unconnected, as CF has an 8 bit access mode that most other IDE devices don't support. The 3 address lines A0,A1 and A2 provide register select when combined with the Y6 and Y7 outputs from the '138
The teas board has provision for a 2nd manually switched rom image to allow CPM, but on a 40 column screen CPM isn't great, and the 62 column display from CFX is hard on the eyes, so in the end I've only built an 8k rom for the SDX basic extensions.
Here's what it looks like. The PCB is a £0.50 - 70mm x 50mm mini matrix board of which there are numerous sources on Amazon and probably Ebay too. Because all control signals required are close to the rom signals, a fill sized edge connector isn't needed. Anything longer than 20x2 will do. This one happened to be 25x2 but I had to cut it down to fit the board, as that only has 24 holes on the long side.
- Just a ROM and one 74HCT138
- Component side.jpg (108.31 KiB) Viewed 35270 times
- The edge connector only needs to be more than 20x2
- wiring side.jpg (122.06 KiB) Viewed 35270 times
- Sitting on the left of the keyboard
- Mounted.jpg (77.14 KiB) Viewed 35270 times
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