The Main Printed Circuit Board

The old joke about the two new heads, and three new handles? Almost. There are rows of chips piggybacked to others, and a birds nest of wiring running on both sides of the boards, with ZIF sockets for EPROM firmware and character generators.

Custom Modifications

This computer was the guinea pig for all future modifications that I wanted and needed. Most of the final designs were marketed via User Groups, and local electronics magazines. This meant that every track cut, rewire, and additional ICs were all added to this computer, and in many cases removed, after prototype boards were tested.

Speedon:

Location, Top Right:

Small PCB attached to the main board. A sophisticated board I designed to control the clock input frequency to the Z80 CPU. Allowed reliable speed shifting during all I/O.

Graphics:

Location, Plug-in Board with 24 pin ribbon cable next to Speedon:

Small PCB plugs into the original character generator, and extends the socket position to the 24 pin ZIF socket on the face of the keyboard. Used a 2716E EPROM. This allowed for lower case characters to be displayed. More on generator later.

Donmon:

Location, Center of board. Plug in 24 pin header cable:

Extends the system ROM via the header cable to the DONMON PCB, hanging out the back of the computer. This was normally fitted inside the case, but as this unit was the prototype for all modifications, it had to be removed on a regular basis, and ended up living outside the case. More on Donmon later.

From Level I to Level II:

Location, Center of board. Plug in 24 pin header cable, and the ROM to the right of this:

Level I was 4K ROM. Level II was 16K in three ROMs. I had to purchase the original upgrade, which was a simple adapter with header cable, and small PCB, to add the extra ROMs. Eventually this was discarded when Donmon came along, and the extra ROMs were placed on the Donmon board, along with the Donmon circuitry.

64K DRAM:

Location, 8 chips running horizontally under Donmon cable. See image above.

These started off with the original 4K of 4114s, and were replaced with 16K of 4116s when these were first released. Later, additional 4116s were piggybacked three high, to give 48K of DRAM. When the 64K 4164s came onto the market, I replaced the three high piggyback with 8 by 4164 chips. However the Level II firmware would only allow for a maximum DRAM size of 48K.

Original keyboard wore out, so I fitted a new one from a Super-80 computer. (Dick Smith kit).

Reset button for Donmon is located top right of the keyboard.

Can't remember why I ended up with two ugly holes in the bottom of the case. No doubt to bring cables, and components out during various stages of R&D :-)

Donmon went through a number of board and firmware revisions.

Close up of the main board I.D.

Printdon 779 (Gendon spin-off)

A lower case character generator for TANDY LINE PRINTER ONE, and CENTRONICS 779 PRINTERS.

Having used my old upper case only line printer for some time, I became envious of the new Japanese plastic printers and their capabilities. I noticed an AD in a US magazine for a small kit that added lower case for this type of printer for $95. At this point in time, I was a little cheesed off with US mail order companies, so I decided to have a go at rolling my own. Wish I hadn't looked, as it developed into a real game of adventure. I'm sure Centronics mapped their original ROM to just to put me off the trail to lower case success. After spending a lot of many hours into this project, I was able to come up with the full 128 ASCII character set, and plus all those up and down arrows etc., that I had been missing.

NOTE **** Wasn't able to have descenders, as there was only 7 print pins (height) on these printers.

My first printer was a WWII Baudot Model 15 Teletype that I interfaced to my TRS-80. This was later followed by a much lighter Siemens Halske Teleprinter Model 100.

Donmon Screenshot

Donmon was memory mapped from 3000H to 37DFH and slipped in between the keyboard and video memory maps.

Screen shot has the address and phone number masked. Why? These haven't changed in 35 years and:

• People actually do call in at 23:00 and think it is quite OK to do so.
• People actually do phone at 03:00 from the US and think it is quite OK to do so.

The top left corner text alternates between "POWER UP" and "RESET TO" as a system alive indicator.

This monitor program was entered by powering up the system, or by pressing reset on the keyboard.

The idea was to have Donmon completely invisible to all operating systems, have the ability to jump to the Donmon monitor program, and return at any time to the program that was interrupted, when the reset button was originally pressed.

Once in the Donmon monitor program, the following drivers were available:

• Screen print facility, keyboard driver with shift lock, flashing cursor with control, keyboard beeper, control characters, auto repeat on all keys, lower case video driver.

And the following features were available:

• Display character set, ROM and RAM check, ASCII and hex display of memory, edit memory, deposit data byte in full block of memory, goto hex address, move blocks of memory, reset memory size without destroying BASIC text, overwrite a BASIC new command, write a system tape, etc. A string floppy version was later written for F800H.

The Donmon short form kit consisted of 2 EPROMs, a bare printed circuit board, and manual at $40AUD and had a 90 day refund offer if not satisfied.

My other Z80 spin-offs

ZLOAD

This was a development board based on PBUFF, that allowed a fast download of assembled Z80 machine code to the target machine. Required no EPROM burning, and started instantly. Similar development systems of the day, started at several hundred dollars.

Reference:

• Electronics Australia Magazine Apr-94
• Dontronics Z80 Files

Z8TBASIC

I recompiled to 8080 Tiny BASIC into Z80 code, then added the I/O routines to make it tick on my ZLOAD development board. Meant you could write a BASIC language program, test it, then burn it into an EPROM when completed.

Reference:

• Dontronics Z80 Files

TRI-colour LED moving message board

Reference:

• Dontronics Z80 Files
• Silicon Chip Magazine Mar-89 Part 1
• Silicon Chip Magazine Apr-89 Part 2
• Silicon Chip Magazine May-89 Part 3
• Silicon Chip Magazine Jun-89 Part 4