Using 5 NAND Gates to prove that there is a hardware problem was very useful, but for a production run using a single chip to replace these is the next step.
Choosing a Decoder chip
Looking through the data sheets for a Texas Instruments VC1 chip found the following timings:-
| Part No | Chip | Manufacture | Speed nS | Package | Price | |||
|---|---|---|---|---|---|---|---|---|
| Min | Typ | Max | ||||||
| 1741279 | SN74LVC1G139DCTR | Ti | <2.5 | SM8 | $ 0.69 | |||
| SN74LVC1G139DCUT | Ti / Farnell | <2.5 | VSSOP | £0.404 | ||||
The 2-to-4 Line Decoder display fast times.
Here is the chip that has been chosen:-
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Full Data Sheet for SN74LVC1G139 2-to-4 Line Decoder & Timing
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This chip on its own provides the logic for the Z88 application.
Circuit Description
There are 2 inputs,
- A19 - Selects either the top half of the 1M memory space for the flash chip or the bottom half for the RAM using the A input.
- /CE - Selects this chip.
Truth Table of Decode Chip
| Inputs | Outputs | ||
|---|---|---|---|
| B /CE | A A19 | /Y1 /CE1 FLASH | /Y0 /CE0 RAM |
| 0 | 0 | 1 | 0 |
| 0 | 1 | 0 | 1 |
| 1 | X | 1 | 1 |
Circuit Diagram using 2-to-4 Line Decoder
Updated 3/10/2016
Building the circuit on a breadboard externally
Components required
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2 MSOP-8 AND 1 VSOP to IC adaptors were obtained in addition to the 5 2-to-4 Line Decoder chips.
This circuit could now be bread-boarded and tested outside the Z88 card case.
The four signal and power lines can be seen connecting the card to the breadboard.
The Games play without crashing.