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Reged: 05/11/13
Posts: 3
Loc: Italy
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Re: simulation of a digital circuit
12/31/13 12:01 AM

thanks for the support I really appreciate it.
I'm indeed trying again with Proteus, the only problem was that I can't find any model for the DRAM and for the ADC.

here there is my schematic (I don't attach it here since is really big):

so far, what I undestood is (hoping that I'm not wrong, but it's possible):

1) the heart of the circuit is the 74LS624 VCO, reading the Service Manual of my unit, it should produce 64kHZ as clock, but this can be modulated (varying the VCO input voltage).
2) On the top of the page, analog signal passes through a Sample and Hold circuit, and then through another set of switches (IC12) (don't know what for..), but this condition is stored in 1 bit of DRAM (IC34).
3) analog signal is then converted using a SAR+DAC, clocked by 74LS624, and the parallel 12bit output is stored in 12x DRAM 1bit 64k chips, and then read back using a DAC.
4) other "user inputs" include:
- bit PC0, don't know what's for.
- 10 bits (PA0,..,PA7 + PB0, PB1) on the bottom of the page. I think these 10 bits sets the delay time (incoming audio is stored in DRAM and then read back after x milliseconds), since the delay time can be anything from 0 milliseconds to 1023 milliseconds, with 1 millisecond step, and 10 bit ^2 = 1024 possible values. This 10 bit value is then converted to useful DRAM address by all that logic. This logic should also take care of the refresh procedure for the DRAM.
5) RAS and CAS bits looks like they got the same value all the time..since there's a bunch of inventers but logically they are useless (maybe they used them to buffer the bits when long pcb traces are used?)
6) Everything becomes overwhelming for me when I think that VCO input voltage is not fixed so that it always produces 64kHZ clock: indeed (if the user wants) it could be modulated like this:
VCO input voltage is a triangle wave (2V min, 4V max) with frequency from 0.1hZ to 16hZ. That would produce a variable clock that ranges from 40kHZ to 78kHZ according to the Service manual.

are my assumptions correct? and what else could be said about this circuit?
many thanks again

Entire thread
Subject Posted by Posted on
* simulation of a digital circuit Dimitree 12/26/13 05:15 PM
. * Re: simulation of a digital circuit RetroRepair  12/29/13 04:32 PM
. * Re: simulation of a digital circuit Dimitree  12/31/13 12:01 AM
. * Re: simulation of a digital circuit sz72  12/31/13 08:49 AM
. * Re: simulation of a digital circuit Dimitree  01/04/14 03:56 PM
. * Re: simulation of a digital circuit sz72  01/05/14 10:24 PM
. * Re: simulation of a digital circuit italieAdministrator  01/04/14 05:28 PM
. * Re: simulation of a digital circuit Phil Bennett  12/29/13 03:02 PM

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