I
was thinking about programming VAX MACRO-32 Assembler language for a
long time without actually writing anything, then in a totally natural
diversion, I bought a clone of the original Motorola Education Computer
Board (MECB) off Ebay. My nostalgic assembler trip was finally
realised...
The original Motorola Education Computer board was first produced in
1982 as a cheap ($495) way of evaluating the CPU and as an introduction
to the assembly language. The shining star was the Tutor ROM code that
included a single line assembler and disassembler. I first came across
this board at Hatfield Polytechnic (now University of Hertfordshire)
during my degree course. It was housed in a large metal case (think
Research Machines 380Z) with a single line vacuum flourescent display.
Interaction was with a ADDS serial terminal.
Clone Board
My clone board was purchased off eBay in February 2006. I'm not sure
how many have been/will be made. This is a picture of it (click for
larger image ~330k):
The clone board provides the same functionality as the original with the following exceptions:
The RAM has been increased from 32K to 64K.
The ROM size is selectable (via soldered jumper link) between 64K and 128K and at a different base address.
There is no tape interface (see below for project to replace this).
There is no prototyping area and the address bus brought out to a header is restricted to A1..A18.
The two serial ports are restricted to 9600 or 19200 baud.
My post to the Easy68K forum contains a discussion about various aspects of this board.
Corrected ROMs
The ROMs supplied with the clone board have an assembly error embedded
which corrupts the TRAP #14 function table. The ROM code is an exact
copy of the original MECB Tutor V1.3 code, with the exception of a
relocation of the ROM base. I found a version of the MECB Tutor V1.3
code that had been ported to the Easy68k assembler. Relocating and
assembling this code resulted in a corrected version of the ROMs.
The ROM images below are the necessary high/low images required for burning to 27C256 EPROMs.
The original MECB board included hardware to interface to a cassette tape recorder driven by a
couple of lines from the parallel input/output chip. I extracted the
interface schematic from the original documentation and have produced a
replacement PCB. I've used the tool diptrace
(size limited but fully functional version is free) to create the
schematic
and lay out the PCB. I have created the interface board and fully
tested it by saving and loading programs and memory to/from a computer
wav file and a Sony Minidisc. Just don't ask me why you would want to
do it (well, maybe later)!
