MTX Series Memory Control

Overview

The circuit diagram shows the original memory control logic for the MTX 500/512/RS128 computers (changes were made for the MTX512S2), specifically, the 4000-04 computer board. The diagram is extracted from the Phoenix MTX Operators Manual and is the same as that shown in the Memotech MTX500/512 Service Manual, it has been corrected to show that the chip in board position 2B is a 74LS04 (Hex Inverters), rather than the 74LS14 (Hex Schmitt Trigger Inverters) shown in the manuals.

A Programmable Array Logic (PAL) chip enables custom logic to be programmed into a single chip, rather than using discrete logic chips. Memotech used PAL14L4s to control certain memory operations and address decoding for different models of MTX computer having differing RAM and ROM configurations. (Further details of Memotech's use of PAL chips can be found on my PAL Reader page.)

The Service Manual describes the purpose of the computer board links associated with the different RAM options.

Link LK7 was intended to identify the installed RAM as either 32kbytes (Link 7, position 2 for +5v) or 64kbytes (Link 7, position 4 for 0v). In the PAL equations documented in the Phoenix manual, this input is referred to as I2H4L (2High / 5v, 4Low / 0v). (This input is likely redundant - as described here.)

Link LK6 is used to select which half of faulty 64KB RAM chips had been selected for use as 32KB RAM in an MTX500.

The PAL14L4 has 14 inputs and 4 outputs, in the MTX, there are :-

• Two functions associated with the output enable logic for the ROMs, and
• Two functions associated with the RAM.

The ROM Enable functions are described on the Next Page.

Functions associated with the RAM are described on page 5.

Note : The Service Manual describes changes made to the Timing Chain from October 1984 to resolve "occasional faults occurring after about 2 hours of use - and poor fire up response. (especially when running in conjunction with F.D.X.). The diagram below shows what components were changed and to what values:-

R14 680 Ohm resistor to be replaced by a 330 Ohm resistor.

C6 68pf capacitor to be replaced by a 33 pf capacitor (or left out completely).

Memotech noted that "You may feel the need to replace the timing chain with the modified circuit if your MTX is suffering from intermittent fire ups (e.g. 1 fire up failure in less than 5 attempts constitutes a possible fault in the timing chain circuitry). However, the RAM logic description in these pages is based on the original circuit design which will be found in machines shipped before October 1984.

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