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Digital Logic question: | by MrTrick | 2003-11-01 21:13:18 |
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I've read this several dozen times | by Egaeus | 2003-11-01 21:21:16 |
| oops, my bad. |
by MrTrick |
2003-11-01 21:33:40 |
Basically, I have a device (electronic LED banner) from Tandy opened up and ready to be dissected.
My assignment is to replace the keyboard system with circuitry, and have the circuitry communicate with a computer through a serial cable.
The keyboard is a simple touch-switch type, with a rubber overlay. The wires leading in and out were described before, and I've started experimenting with different combinations.
Each 4 control line can connect to 1 of 16 data lines. Touch a control line and a data line whilst the device is in "input" mode, and a letter will appear on the screen. The problem is that the logic levels for the keyboard are too low (for less power usage I suppose) to use CMOS or TTL voltages.
Ideally, I'd have 64 transistors - 1 connected to each valid permutation of a combination (C&E), and connect the 64 bases to a big demux. Now I'm told that you can't simulate a single transistor in a CPLD, only logic levels. Is there any logic component that either provides a short or open circuit across two pins, depending on the logic level of a third?
I don't know if that's any clearer. |
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[ Reply ] |
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Much better | by Egaeus | 2003-11-01 21:44:36 |
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I don't *have* 64 transistors | by MrTrick | 2003-11-01 21:48:11 |
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What kind of logic levels are you looking for? | by Egaeus | 2003-11-01 22:17:20 |
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