"Disc" storage on a budget

Modern, Memotech inspired, hardware projects
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Martin A
Posts: 406
Joined: 09 Nov 2013 21:03

"Disc" storage on a budget

Post by Martin A » 06 Aug 2018 23:30

Here it is - just about the smallest, simplest IDE/Compact flash interface possible.

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.
Component side.jpg
Just a ROM and one 74HCT138
Component side.jpg (108.31 KiB) Viewed 643 times
wiring side.jpg
The edge connector only needs to be more than 20x2
wiring side.jpg (122.06 KiB) Viewed 643 times
Sitting on the left of the keyboard
Mounted.jpg (77.14 KiB) Viewed 643 times

Martin A
Posts: 406
Joined: 09 Nov 2013 21:03

"Disc" storage on a budget - part 2

Post by Martin A » 06 Aug 2018 23:43

Booting from the SDX rom produces a familiar looking screen.
Boot Screen.jpg
Simplified CFX boot screen
Boot Screen.jpg (91.2 KiB) Viewed 641 times
Here's the component layout of the test board. For a single 8k image leave off the 3 pin jumper on A13 and wire to 0v. 100nf capacitors can be added between 5v and 0v on the 2 chips and the IDE header
The layout of the test board
(223.55 KiB) Downloaded 42 times
The forum wont allow the rom image to be posted as is, so this is the 8k image with a fake extension!
microCFX Build 4.pdf
Not a PDF, but the 8k rom image
(8 KiB) Downloaded 44 times

Bill B
Posts: 161
Joined: 26 Jan 2014 16:31

Re: "Disc" storage on a budget

Post by Bill B » 07 Aug 2018 18:38


Interesting as always.

Do you ever sleep :)


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Posts: 555
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: "Disc" storage on a budget

Post by 1024MAK » 09 Aug 2018 20:12

@Martin - nice 8-)
Bill B wrote:Do you ever sleep :)
When a hardware geek gets an idea in their head, sleep takes a back seat while the iron is warm... ;)


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