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TMS9995 Max

Information here is about a TMS9995 pcb board that has made use of NVRam for the full 64k of memory and a TMS9901 peripheral chip.

Pcb view layout (top).



Schematic of the board

There was major mistake in this design where the address lines are reversed on the CPU (however also reversed on the NVRAM chip).
To test viability of the design a plugin board was used with an eprom and ram chip to mimic original Stuart's design.




This is the pld code

Name            9995 EMVBUG + Cortex extra ram space;
Partno          0001;
Revision        Ver 1;
Date            27/10/17;
Designer        mc;
Company         mcoz;
Location        oz;
Assembly        manual;
Device          g22v10;

/** Inputs **/

pin [1..11] = [a0..10] ;
pin [13..17] = [a11..15];
pin 20 =nmemen;
pin 18= nwe;

/** Outputs **/

pin 22 = rom;
pin 23 = mce;
pin 21 = !sce1;
pin 19 = !cruclk;

/** Declarations and Intermediate Variable Definitions **/

field ioaddr= [a15..0];

sce1_eqn   = (ioaddr:[0000..003F]);

/** Logic Equations **/

mce = !a15 # nmemen ;
rom  = a15 # nmemen;
sce1  = sce1_eqn & nmemen;
cruclk   = nwe;

In above case the CS for the PIA(pin 5) was redirected to EPROM (pin20)

Running the TEST3 code in eprom (from Stuarts website) ..here is the output ...


Also loading the Monitor and cortex Basic into eprom (the rom vesrsion) ...

As you can see it runs with 22K free RAM space
now the trick is to find out why NVRAM does not cooperate

Here is updated version of the board with EPROM option  .. that does not need any mods on the board



Updated view of a PCB ...of above circuit