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"MTX Plus+" CPU Board (Version 2.3)



My original design for the CPU board was based on the use of discrete logic chips, like the original MTX. The design was more or less finalised when I decided to use a CPLD in the design to reduce the congestion on the somewhat cluttered circuit board and to make it easier to modify the glue logic should any changes be necessary.

The Wikipedia entry for CPLDs uses a photo of an Altera EPM7128S as an example of a typical CPLD. Coincidentally, Lez Anderson had included this device in the bag of "free stuff" that he generously sent me when the MTXPlus+ project was first mooted, so this seemed like an obvious device to use. Given that I have absolutely no knowledge of using and programming a CPLD, this will delay the production of a working CPU board but will, hopefully, lead to a better design.

The other advantage of using an Altera device is that I already had a very basic introduction to Altera's Quartus II Web Edition software when I loaded the software for Andy Key's REMEMOTECH project onto my Altera DE1 development board as described on the My REMEMOTECH page. That page also describes how to obtain a copy of this free software from the Altera web site.

Altera EPM7128S Abridged Specifications (PLCC)
Useable logic gates 2,500
Macrocells 128
Logic array blocks 8
User I/O pins 68

The basic specifications of the EPM7128S are shown in the table, for more detailed information on this CPLD and my attempts to use it can be found on my CPLD page.

The Version 2 CPU board uses a Z80 CPU in a DIP-40 package and the Version 3 board uses a Z8S180 CPU in a PLCC-68 package.

Hardware - Control Bus Signals

The backplane distributes a number of control signals to the rest of the boards in the system, the majority of these are standard Z80 control bus signals, generated by the CPU, but there are also a number of other signals generated or used on the CPU board that are distributed over the backplane. The source of some of these signals differs between Version 2 and Version 3 of the CPU board.

Name Description Source Direction
Clock Clock source for the CPLD X1 To CPLD
PHI4 4MHz clock signal CPLD (v2)
or  X2 (v3)
To Backplane

System clock signal

CPLD (v2)
or CPU (v3)
To Backplane
PHI8 Additional 8MHz clock signal (Deleted) (Deleted)
SER01 Clock for serial port 1 CTC To Backplane
SER02 Clock for serial port 2 CTC To Backplane
DIAG Signal to the Diagnostic board to latch 7-segment displays based on logic in the CPLD.

By reprogramming the CPLD and switching the position of jumper J5, simple on board status conditions can be shown using the on board "DEBUG" LED.

CPLD To Diag. board
/VDPINT Interrupt signal from VDP VDP From Video board


Hardware - Real Time Clock Sub-system

The CPU board includes an RTC using the Dallas DS12887 that Lez included in his "kit of parts". As described on my Notes page, I found this really helpful article written by a Wilf Rigter around 1996. The article demonstrates the use of the DS1287 (an earlier version of the DS12882) with a ZX81 and included examples of Z80 assembler and BASIC programs to access the chip. Martin was able to use this information with the DS12887 and modified address decode logic to make a working RTC that will be included on the MTXPlus+ CPU board and has posted the details on the Memotech forum in this thread.


Status Summary

Development of the Version 2 and Version 3 CPU boards are described in more detail on their own pages. The Version 2 board had pretty much been proven to meet original design intent for MTX compatibility - at least, when running at 4MHz like the original. However, as things progressed during the build, a number of changes were made to correct a couple of weaknesses in my design. Added to this was some degree of "scope creep" that led to other modifications and enhancements.

These "design developments" were originally documented on this page, but the number and magnitude of the changes probably justifies their description on a separate page that I've called, oddly enough, Design Development. The main items are now just bulleted below, for details of the changes, click on the links to open the new page.

In addition to the enhancements to the Version 2 Z80 board, an enhanced board, based on the Z8S180 is currently in development.

Design Development


Version 2.21 of the CPU board design, this version of the board was used to prove the base functionality of the system and test development of the more advanced features.

This version incorporates TTL buffer chips on the Z80 Address, Data and Control lines that were not required and removed from the final design.
The "final" version of the Z80 board used to produce the CPU PCB.

This version incorporates the "Mystery" upgrade, and due to its increased complexity, no longer includes the option to implement the glue logic in GALs; the board is based on a CPLD only solution.
The KiCAD 3D model of the Z80 CPU PCB
The assembled PCB 

I seem to be getting better - just one minor routing error on this one!



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