Circuit design for identity mode

Thread Starter

Alejo1104

Joined Jun 21, 2020
3
Hello,
I am trying to design a circuit that allows me to connect pin A9 of the Eprom 27c256 memory to use the identity mode but I have a problem this mode needs a voltage of 12.5 V and pin A9 is assigned to a data bus from another circuit that allows me to select the address of the Eprom memory for the programming and reading mode, then I came to the conclusion that this circuit must have TTL inputs and outputs 0v, 12.5v and hi-z (it will allow me to disconnect the circuit so that it does not interfere with the data bus).
I think that in totem pole circuit maybe it would work but I don't know how to do the complete circuit.
I'd appreciate your help

output totem pole circuit:

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Irving

Joined Jan 30, 2016
815
Here's a quick circuit that does close to what you want, based on something I did ages ago. Caveat - this specific version based on memory, not tried in anger, not checked against data sheet for rise/fall/hold_off/etc. etc. timing.

The logic truth table is trivial. Worries me that you are designing a complex digital system, yet that beat you. If you can't explain why/how the analog side of this works, be prepared for a load of pain. Suggest you breadboard it and examine with a decent scope. PCB layout could have some interesting side effects too, but will probably be OK at 1KHz (simulation here at 500Hz).

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I'd also point out that, many moons ago, when I was a junior design engineer, a certain major supplier of EPROM programmers got the design wrong and over-stressed a specific EPROM that then failed in the field, expensively (I was on the investigating team for the computer manufacturer who used the chips). I admire an entrepreneur, but if you don't have the skills and knowledge to test thoroughly across 100s of devices from many suppliers and unless you have deep pockets, beware! Fitness for purpose could come back to bite you.
 

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