Dragon 32 & 64

Please read the other sections to learn more about the Dragon 32 and Dragon 64.

Dragon 32

Overview

The Dragon 32 is based around a design specification for the MC6883 Synchronous Address Multiplexer (SAM) chip (as also is the case for Tandy's CoCo computer), which was responsible for memory management and peripheral control, and the CPU is a MC6809E CPU clocked at 0.89 MHz, an 8-bit CPU that was capable of some internal 16-bit operations, giving it an edge over rival computers. A few other computers around the same time used the 6809, including the Hitachi S1 series, Canon CX-1, and Fujitsu FM7, FM8 and FM-11. Using a 6809 did have its disadvantages, as the 6502 and Z80 (commonly used in other computers at the time) already were very familiar with users and developers, and CP/M (popular with businesses) ran on the Z80, a big disadvantage for non-Z80 computers.

In addition, the Dragon 32 has 32KB of DRAM (hence its name), and a MC6847 Video Display Generator (VDG) chip with additional circuitry as to generate a video image compatible with European TVs.

Let's look at more detail of the Dragon 32 by going over my own Dragon, which I bought from ebay in 2021 for £50 (~$69 USD) as 'dead, not powering on', in the hope I could get it working. Firstly, here is the box which is not in the best of condition and has an early 'logo' sleeve instead of the later design you'll typically see online showing off a family huddled around the Dragon computer.

Next we have a photo showing the contents of the box which are the Dragon 32 itself, the power supply (top right), a Meteoroids cartridge (bottom right), a Dragon Mountain cassette (underneath the cartridge) and various documentation (under the computer). The Introduction to BASIC programming user guide is missing but that is viewable online; please see the Technical section on this page for the link.

Here is a close-up of the Dragon Mountain game cassette:

Note that (presumably) the owner wrote 'A' on the cassette to designate side A.

With my Dragon were a number of leaflets which I assume came with the computer originally and give a clue the software that may have been present when bought new.

The power supply is rated for input 240VAC 50/60Hz and output A 8VAC @ 1.5A, output B 28VAC 250mA, and is unusual as it uses a female DE-9 type connector, which seems a poor design choice as such a connector was typical used for joysticks/serial ports. Using a DE9 connector instead of the more usual barrel type, along with the unusual AC voltages, can make sourcing a replacement power supply difficult.

Now for a look at the top view of the Dragon 32 showing its beige case, the colourful name badge and keyboard consisting of 53 black keys:

The keyboard layout is somewhat strange as there is no '£' symbol (considering it's a Welsh computer) and while there are four arrow keys - two either side of the keyboard - they do not function as you would expect them to although the left arrow key as a backspace key, which makes some sense. Other keys of interest are the CLEAR key which clears the screen and returns the cursor to the top-left hand corner and the BREAK key which interrupts the current running BASIC program. As typical of microcomputers of the time for people today is can be tricky to use old keyboards due to the slightly odd layout, such as having '&' above the 6 instead of the 7.

On the right hand side of the Dragon is the cartridge expansion port (not labelled) consisting of a 40-pin connector (20x2 edge female) which was typically used for cartridge software:

Cartridge software has the advantage of instant access instead of the slow loading from cassette or floppy but of course cartridge software is read only and at the time of the Dragon's release, very expensive. The case of the Meteoroids cartridge game I have for the Dragon 32 unfortunately is cracked (funny as seen, my Dragon has a cracked power board). To take apart the cartridge, remove the single screw and then carefully prise the two pieces to separate them, revealing the circuit board inside. The PCB uses a socketed UV EPROM, type 2532 (4KB). You would expect to see a mask ROM, however, for low quantity production UV EPROMs were common back in the day and for us today such a choice means we can easily put in our own ROM. The cartridge board has provision for a second ROM and there is also a 7400 quad 2-input NAND gate IC.

You can see photos of the internals at:

http://archive.worldofdragon.org/index.php?title=Meteoroids

On the left side of the Dragon is the RF TV port, the reset button, the 'L' joystick port, the tape port, the 'R' joystick port and the P I/O Centronics parallel port (20 pin).

The bottom side of the Dragon can be seen next and note how the security seals have been broken, which alerted me that someone had previously been inside before I bought it.

At the back of the machine we see the power supply connector (DE9 male), the power switch and the composite monitor connector.

The internals of the Dragon can be viewed below and consists of the keyboard (left), main board (middle) with Centronics, joystick, tape and cartridge connectors, and reset switch, and the power board (right) which has the RF, video and power connections as well as the power switch. Notice has the power board is cracked in two places, quite the surprise for me when I opened up the computer and something I address in more detail in the repair section.

We will now take a closer look at the main board:

Of particular interest is the MC6809 CPU, the MC6847 video chip, the 74LS783 (a.k.a. MC6883) SAM chip, the ROM (has a sticker with 'A7' on it) and two MC6821 Peripheral Interface Adapters (PIA), one of which has a red sticker on it.

Looking at the power board in more detail the two cracks are more evident:

As well as being home to the video, RF and power connectors, and the power switch, the power board of course also contains the power supply circuit which is a linear type. There are three voltage regulators; LM309 (REG 1) +5V, 7812 (REG 2) +12V, and LM7905 (REG 3) -5V. Note that REG 3 is not attached to the heat sink which may seem a mistake as the heat sink has provision for it to be mounted but looking online it's normal for it to not be attached (as it provides little current and thus shouldn't get hot) and should not be attached to the heat sink unless electrically insulated. The +5V, +12V and -5V are actually used, amongst other things, by the DRAM chips and it's a disadvantage of older computers that RAM was used requiring three different voltages.

As can be seen below, the computer boots in text mode allowing for 32 x 16 characters and the screen appears with a large black border, light green background and dark green characters (usually referred to as black by documentation). The colours appear darker than they should either because of my particular Dragon 32 needs adjusting or perhaps due to my capture card (turning up the brightness in the capture card's settings causes the border to be bright when it's supposed to be black so I left it at the default value).

As you may notice when using the Dragon, the characters aren't that clear - unfortunately composite doesn't give the best quality signal (but better than RF) - and some symbols are rendered not as you would expect, such as '*' which shows as a diamond.

The Dragon uses Microsoft BASIC V1.0 and the '16K' in the image above refers to the 16KB ROM that the BASIC interpreter is stored in. As the Dragon uses Microsoft BASIC you have to use American spelling so, for example, the COLOR command must be written exactly as that, using the American spelling.

To highlight one of the Dragon's shortcomings: to enter lower case mode press SHIFT and 0 but text on screen will show as inverse upper case (green on black) as there aren't any graphics for lower case characters although an attached printer will output lower case. Press SHIFT and 0 again to return to upper case mode. Note that BASIC keywords must be entered in upper case.

The BASIC is powerful and has support for sound generation, text and high resolution graphics and that is significant as the Commodore 64, released in the same year as the Dragon, lacked BASIC keywords to take advantage of its advanced features although the computer still ended up being highly successful unlike the Dragon. The Dragon does allow for some shorthand such as the PRINT keyword, which can be abbreviated as '?'. There is also a form of PRINT, PRINT @ (or ? @), which lets you place text at character positions on the screen. For e.g.:

PRINT @ 32, "HELLO"

Will place 'HELLO' at the start of the second line (in the default text mode there are 32 characters in a row).

Another useful keyword is CLS, which clears the screen but can also set the background colour by specifying a number between 0 and 8. E.g. CLS 0 sets the background to black. The colour values are as follows:

0 black
1 green
2 yellow
3 blue
4 red
5 buff
6 cyan
7 magenta
8 orange

Note that buff is supposed to be a light brownish-yellow colour but actually appears more like white/grey (at least on my Dragon 32).

If you specify a number greater than 8 for CLS then the screen clears to green and '(C) 1982 BY MICROSOFT' appears at the top of the screen, which is an Easter egg (and is also present in the TRS-80 Color Computer) and you can see it in the image that follows:

To get the amount of memory available for BASIC you use ? MEM. With no BASIC program entered you should get 24871 on a Dragon 32 as some memory is used by the screen and for other reasons. If you get a much lower value on the Dragon 32 when first switched on then possibly the RAM is faulty; please see the Troubleshooting section for help.

Text is always displayed in green (or black for lower case) which seems like a serious oversight, however, you can at least display special characters with a colour of your choosing. The special characters have ASCII values 128 to 143 and you can use PRINT CHR$() to display any of those characters (as well as the other character of course). To display a special character in a different colour add a multiple of 16 to the ASCII value to select the colour as follows:

+16 yellow
+32 blue
+48 red
+64 buff
+80 cyan
+96 magenta
+112 orange

As mentioned, the Dragon is capable of high resolution graphics and a nice feature is that after running a BASIC program that uses a graphics mode the computer automatically returns to text mode so you can continue to edit your program.

Further reading

Information and photos regarding the Dragon 200 and 200-E:

https://archive.worldofdragon.org/index.php?title=Dragon_200

Details about the Dragon 64:

http://www.dragondata.co.uk/adverts/d-64-brochure-intro/index.html

Nostalgia Nerd's article on the Dragon 32:

https://www.nostalgianerd.com/the-dragon-32

Repair

I bought my Dragon 32 second hand from ebay as 'dead' but I thought it was worth the risk especially as it could be a simple fix. With old equipment it's always best to inspect the computer internally before powering on even if it is supposedly working. I saw that the security seals covering the four screws keeping the two halves of the Dragon 32 together had already been broken so clearly someone had already had a look inside or even attempted a repair. While the main board looked OK I was surprised to see that the power supply board is cracked in two places! I can only imagine that someone broke it perhaps while screwing the board back in or even when trying to get it out.

In the photo that follows you can see the power board that I took out of my Dragon, with one very obvious crack near the middle and a less visible crack to the left of the screw hole near the power switch at the bottom of the image.

The power board attaches to the main board via a 9 way cable (PL 10) and this needs to be unplugged in order to get the power board out along with the four screws that hold it in place (it is quite fiddly to remove the board).

The Dragon 32 schematic (see Technical section) has the pinout for the power board PL10 connector which I checked against my own board and have reproduced here in case it's useful for troubleshooting but please use the information with caution.

Pin 1 on the power board is the opposite end of the red marking on the ribbon cable (that is, pin 9 is marked with red) and is confirmed by the pin 1 marking on the main board. The pinout:

1 +12V

2 +5V

3 -5V

4 0V (power)

5 GND (video)

6 Video

7 Sound

8 +5V (from main board)

9 Modulator video input

Notes: pin 4 and 5 are connected together on the power board and main board. The modulator video input (pin 9) is a buffered version of the composite signal that goes to the video connector. Pins 2 and 8 are bridged on the main board.

One approach to repair the power board would be to scrape off the green coating on the tracks to reveal the copper underneath and then solder across the tracks to complete them, however, instead I soldered wires between the nearest components along each track. After the soldering I tested the power board with the original power supply but not connected to the main board and found that I got +5V, +12V and -5V on the appropriate pins. To further test I checked each power connection again but with a load resistor as to draw an appreciable amount of current without exceeding the maximum amount for each power rail. The Dragon schematic specifies the maximum current the power board can supply for each voltage as:

+5V 1400mA

+12V 250mA

-5V 50mA

Next I tested the Dragon with the repaired power supply and the computer booted and I was able to use it for a while before random characters appeared on the screen (see the image below) so I will have to investigate if there is some other fault such as bad RAM. Of course, the repair to the power board was temporary and a 'new' or alternative power supply needs to be installed for long term use.

Amazingly, the Meteoroids cartridge that I got with my Dragon, which has a cracked case and the contacts are very poor, actually works although because of the possible RAM fault with the Dragon the game did 'glitch up' even though it was playable.

Software

As well as the software released for the Dragon 32, thanks to the Dragon being mostly compatible with the Tandy CoCo, even more software was available for the machine.

To give a bit of an idea of some of the games that were available for the Dragon 32 here are a number videos showing YouTube user Simonwgb playing various Dragon games on an emulator:

Technical

The Dragon Data the Archive site has a lot of useful information about the Dragon computers:

http://www.dragondata.co.uk/index.html

A site covering the Dragon 32 BASIC guide variations and a link to the guide:

http://www.dragondata.co.uk/Publications/BASIC-MAN/BASIC-man.html

A direct link to the BASIC guide that came with the Dragon 32:

http://www.dragondata.co.uk/Publications/BASIC-MAN/DRAGON_32_BASIC_MANUAL_rel-v2.pdf

ERRATA TO An Introduction to BASIC Programming:

http://www.dragondata.co.uk/Publications/BASIC-MAN/Errata_To_BASIC.pdf

There were a number of different motherboard revisions which you can view on this site:

http://archive.worldofdragon.org/index.php?title=Motherboard_Pictures

Hardware details about the various main chips:

https://www.6809.org.uk/dragon/hardware.shtml

Inside the Dragon book which has advanced information for the Dragon user:

http://www.dragondata.co.uk/Publications/InsideTheDragon.pdf

Wiring your own transformer:

http://archive.worldofdragon.org/index.php?title=Replacement_power_supply_unit

Dragon 32 schematics:

http://www.dragondata.co.uk/tech/circuit-diag/index32.html

Various pinouts for Dragon 32/64:

http://dragon32.info/info/pinouts.html

Pinouts of power and tape connector, power supply internal wiring, and other useful info including links:

http://oldcomputer.info/8bit/dragon32/index.htm

How to upgrade the RAM on a Dragon 32:

http://archive.worldofdragon.org/index.php?title=Dragon_32_-_64K_Upgrade

While the Dragon computers support analogue joysticks it is possible to use digital joysticks:

http://vrai.net/retro/?p=112

Troubleshooting

General

A series of videos from YouTube user RMC - The Cave covering the history of the Dragon 32 and restoring the system:

Bad RAM

Diagnose and locate faulty 4116 chips in the upper 16K of RAM on Dragon 32 MKII boards:

http://www.dragondata.co.uk/tech/YT-Upper-RAM-Fix/Dragon-32-Upper-RAM-Fix.html

Dragon 64

Overview

In 1983 the Dragon 64 went on sale, bringing with it a number of improvements over its predecessor while still appearing very similar to the Dragon 32, having an almost identical looking case, although light grey, and the keys are dark grey but with the same layout as the Dragon 32 keyboard. On the underside of the machine is a sticker declaring it to be made in G.B., along with the serial number, and on the left side of the computer are the RF socket, reset button, left joystick, tape, right joystick, serial (RS232), and printer (labelled as 'P. I/O' on the Dragon 32) connections. A nice touch is that 'PRINTER', along with the ‘L’ and ‘R’ for the joystick ports stick out (that is, they are embossed). At the right side you will find the cartridge port only and at the back is the transformer input socket, power switch, and monitor connection. Toward the front on the underside we find printed: DRAGON DATA LTD © 1982.

The Dragon 64, not surprisingly has 64KB RAM, two 16KB ROMs, and a MC6809E running at 0.89MHz (0.888625 MHz to be precise, which is slower than the Dragon 32's 0.894886 MHz, which derived the CPU clock from the NTSC standard rather than the PAL frequency as is the case for the Dragon 64).

You can find a detailed summary of Dragon 64 differences at:

http://dragon32.info/info/d64difs.html

When the Dragon 64 is powered on it boots into 32KB mode as to maintain compatibility with Dragon 32 software and thus you will get the familiar looking boot screen:

When you first boot the Dragon 64 you can enter 64KB mode using EXEC, a command used to call a machine code program, but if no EXEC was previously called it will be the equivalent of EXEC 48000, which enters 64K mode by switching in the extra 32KB and copying the BASIC interpreter into the top 64KB. If EXEC was previously used for a different purpose then you need to use EXEC 48000 to enter 64KB mode; either way you will be greeted with a new screen after a few seconds:

As well as the new copyright date the other immediately noticeable difference is the flashing blue cursor. If you check how much free memory there is for BASIC using ? MEM you will get 41241, a big difference from 24871 when in 32KB mode. In 64KB mode cartridges cannot be used (which will disable an attached disc drive since that’s connected via the cartridge port) as the BASIC ROM now occupies what would be cartridge space. The full 64KB is available if using a machine code program that doesn't require the BASIC interpreter. Also, when in 64KB mode the keys will auto repeat, which is not a feature in 32KB mode, as to maintain compatibility with Dragon 32 software.

Technical

Teardown

Like with the Dragon 32, to open up the Dragon 64 there are four screws that need to be removed from the bottom, allowing the two parts of the case to be separated, revealing the PSU board and main board. The main board in my Dragon 64 is inscribed with 'P/N: CD41505 ISSUE 2A © DRAGON DATA LTD 1983'. It has HM4864P-2 RAM x 8 and EPROM x 2 (IC34 & IC37). There are two option links: LK1 (under keyboard) set in south position and LK2 set in south position.

Comparing to the Dragon 32

In my Dragon 64 the PSU board is issue 4, and is very similar to the issue 4 PSU in my Dragon 32 (it’s possible they never made a ‘Dragon 64’ PSU and just repurposed the Dragon 32 ones). Compared to my Dragon 32 PSU board the Dragon 64 PSU has R1 connected to R7 and theres no inline resistor between the RF modulator and PCB. It has the following (different) regulators; REG 2 SFC2812EC, REG 3 L7912, compared to 7812 and 7905 on the Dragon 32 PSU. This tells us that that the Dragon 64 PSU outputs -12V on pin 3 of PL10 instead of -5V that Dragon 32 PSU outputs on that pin. This change was made since the Dragon 64 needs +12V and -12V for RS232, although it still needs -5V for the video circuit (but not the RAM), the -5V supply is produced by the 79L05 (REG 1) on the mainboard. Because of the different voltage on pin 3, the Dragon 32 and 64 PSU boards aren’t interchangeable but the transformer can be used on either.

Useful links

Dragon 64 schematics:

http://www.dragondata.co.uk/tech/circuit-diag/index.html

All content of this and related pages is copyright (c) James S. 2021-2022