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Old 31st Oct 2019, 6:18 am   #1
circuitryboy
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Join Date: Jun 2012
Location: Bristol, UK.
Posts: 12
Default A More Useful MK14

It's been suggested that I post the mods I made (way back) to convert my MK14 to a more useful system.
So here goes.

As supplied, MK14 has a 12-bit address bus. So entering any address, eg XF20, selects the same ONE location (of 4K), whatever the value of X.

Issue 5 original address decoding is shown in (A). Gates are shown with SoC IC number. No unused gates remain.
This puts PROMs at X000-X1FF. And leaves X200-X7FF unused.
(An earlier MK14 modified per SoC instructions will have different pin numbers.)

All MK14s have X800-XFFF occupied. Half of this is 'repeats'.
In particular, address lines connected directly to the '8154 present it at X8xx, XAxx, XCxx, and XExx.

With some basic Boolean and a few extra gates the memory map can be completely tidied.
(This was my first 'hands on' micro system; I avoided loading the unbuffered address bus further.)
Additional circuit is shown in (B), taking inputs from original circuit at A,B,and C. Gates are shown with their 74LS- number.

1. Break original decoding at D-E and insert the new circuit D-<AND>-E.
This puts Keyboard/Display at XDxx (used by SCMPKB and SCIOS). X9xx is free.

2. Output I - to CS1 of '8154, pin 34. A8 still to CS0, pin 35. A7 still to M/IO, pin 33.
This puts RAM/IO at XExx. X8xx, XAxx, XCxx are now free.

3. Output H - to Extra RAM enable. Now moved to 0Cxx. 0Bxx free.

4. Now two 1K blocks are available, X400-X7FF and X800-XBFF. Outputs F and G enable these.
PCB positions IC2 & IC3 and IC4 & IC7 can be modified to take 2114s.
(System RAM has to be at XFxx - IC6 pairs with IC5 by sharing CE from IC17.)

5. X200-X3FF remains. Breaking X-Y in original circuit and connecting B-Y presents PROMs(twice) at X000-X3FF.
A 1K PROM/EPROM can then be used here for Monitor + utilities.
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Attached Files
File Type: zip A_Issue5 address decoding.zip (863.6 KB, 35 views)
File Type: zip B_Additional decoding.zip (637.6 KB, 39 views)
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