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Introduction
The Z88 Flash/RAM card has been working for several years now. When OZ 4.6 was introduced, allowing applications to be run in RAM, some games failed to run.
| Standard Version | Modified Turbo Version |
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This was found to be due to an intermittent timing error from the decode chip CD74HCT139 which switches between the RAM and Flash chip.
Although this is one of the fastest decode chips, taking 10nS, the software speed improvements pushed it outside these limits.
This documents highlights the steps taken to rectify this. The detail of the steps taken are in subsections, allowing the reader to go into detail if required.
Object
The object of this exercise is to see if by redefining the design on a small PCB, with the same footprint (SMD SO16) of the existing decode chip, a faster decode could be achieved by
- Using high speed NAND gates, to see if then if that was successful
- Use a decode chip from the same family.
- Using the knowledge gained, add more components to make a high speed 74xx139 card.
The Z88 only uses one half of the dual functionality of the full chip.
Adding 3 more of these decode chips allow full functionality using four 3mm wide high speed chips.
NAND Gate Build
Building the circuit on a breadboard externally
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The Games played without crashing. The PCB could now be designed.
Printed Circuit Board
2-to-4 Line Decoder or SMD SO16 - DIL Convertor
Provisional Layout
There are two functions for this card. The SO 16 pads have holes through them allowing wires to be soldered to SO 16 pads on another card.
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Z88
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Turbo Card
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produced
It has been proved that the decode chip in the Z88 Flash/RAM card is too slow. Existing users who wish to use OZ v4.6 (or later) run applications in RAM will need their cards modified with the chip being replaced with this card.
Full 74xx139 Version Build Concept
Tiny url http://tinyurl.com/zf9haly
A new opportunity or a waste of time?
The decoder chip above works. It changes the timing from 10 to 3nS and replaces 1/4 of the 74139 in the Z88 design. Is there a demand for a full 74xx139 replacement card?
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| 16 pin DIL | 16 pin SO 16 |
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Using the PCB above with pins Z88 Turbo Card fitting pins or wires to connect the PCB 74xx139 Card piggybacked below both footprints may be accommodated. This has proved to be impractical. Another design needs to be considered if required perhaps using a slot across the middle to connect the SO16 footprint to the card.
Build 74xx139 on a 16 pin DIL footprint
This should be possible, 3 chips on each side of the PCB with 2 rows of 8 pins on either side. The PCB will be designed to see if this is possible on a 2 layer board.
Demand or not?
Once the PCB design has been completed, it can be costed and this concept can be floated.
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It will definitely not be able to compete on price, as it will cost more than 10p.
Building the circuit on a breadboard externally
Needs to be done
A test needs to be done to test the functionality of the NAND gate using the /CE signal.
The four signal and power lines can be seen connecting the card to the breadboard.
Results
The results need to be reported.
Printed Circuit Board
74xx139 DIL Card
To be done
There are two functions for this card.
- The DIL 16 pads have holes through them allowing wires to be soldered to DIL 16 pads on another card.
- When used with the Z88 Flash/RAM card, it can be used with a SO 16 footprint. 16 pins or wires need to be soldered so that the DIL chip or converter card can be piggy backed on top with 2 off SIL sockets.
74xx139 to be produced or not
Depends on response.