Tiny url http://tinyurl.com/jdp3qqr
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Some games that run in RAM, particularly when in combination with OZ v4.6 and later - running in slot 1, fails to run on the 512K/512K Flash/RAM Card. This may be due to a timing error which is introduced by the decode chip CD74HCT139. Although this is one of the fastest decode chips, it is too slow for our purposes.
The Z88 only uses one half of this dual decode chip. This circuit may be made with
- 3 inverters
- 2 NAND gates
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Object
The object of this exercise is to see if by redefining the design, a faster decode could be achieved by
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This ignores A1 GND line.
The Z88 only uses one half of this dual decode chip. This circuit may be made with
- 3 inverters
- 2 NAND gates
The inverters are slower than the NAND gates, so it was decided to use 5 NAND gates using 3 of the gates as inverters.
Logic Lab Test v Truth Table
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Building the circuit on a breadboard externally
Components required
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10 sot to IC adaptors were obtained in addition to the 5 NAND gates.
The circuit could now be bread-boarded and tested outside the Z88 card case.
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The four signal and power lines can be seen connecting the card to the breadboard.
Plan view showing all the connections.
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| 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 | ||||
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