This project is a recreation of the SA2120 CPU Mainboard for the 1982 Dragon Data Dragon 32
Everything (excluding modifications) is taken from the SA2120 schematics. Component positioning is as close to the original too. The DRAM positions are as close as possible to the 1982 board but in preference use 4164s instead and set the memory configuration jumpers appropriately, this avoids transplanting a riser board to use the 16x16k configuration. The 16x16k should still work of course...
The two ROMS will need to be sourced from original boards or an adapter used to allow the use of modern, available EPROM or EEPROM components
The variable resistor (RV1) and motor relay (RL1) are adjusted for modern, available components. The cartridge port is based on a modern design but may still be hard to source
The scans on the original SA2120 schematics are not always clear and it has taken a fair amount of careful study to get everything correct but there is no guarantee at this time that the board design is as intended
The values of passive components are taken from observation of an original production board, in most cases those values do match the schematics, but not always
Revision 1.x of this board is effectively untested and unproven
In the current state (2.0) basic operation is tested working on CPU, RAM, Keyboard, Joystick and Video.
Audio appears to be good but not fully tested
Cassette input/output is untested. Cassette motor relay is untested. Parallel port is untested in either mode
The two joystick ports provide a second fire button capability wired to consecutive keyboard rows, as per the Tandy CoCo. Unless a two button joystick is used this make no difference to the operation of the computer. It also requires the use of 6-pin DIN sockets, these are pin compatible with the 5-pin originals so a regular single button joystick can still be used.
The two extra buttons can also be disabled by omitting L3 and L4
An extra jumper has been introduced to bypass the logic inverter on the STROBE line. When bypassed the parallel port should operate as a communication port for DRIVEWIRE
The board is configurable between 32k and 64k ram addressing using jumper 13/14. Position 13 is for original 32k ram configuration, position 14 is for an upgraded 64k ram. Note that this then requires removal of some additional passive components and assumes that suitable 64k x 1 ram chips are fitted in IC1 to IC8.
Jumper 9/10 has been added for consistency with some of the other early designs and for compatibility with the 64k upgrade. It is important to avoid populating C50-57 when using this option, along with C12, C58, C59 and R66.
See [World of Dragon: Dragon 32 - 64k Upgrade] (http://archive.worldofdragon.org/index.php?title=Dragon_32_-_64K_Upgrade)
Many of the original components are simply unobtainable or just very hard (and expensive) to locate. The transistors will need modern substitutes as the BC141, BC212 and 2N2369 are all obsolete but thankfully these are fairly run-of-the-mill transistors and shouldn't cause any particular issues
The video op-amp SFC2318D at position IC10 is especially hard to find and this one component can easily double the price of populating the board if an original is used... I've identified a Harris HA-5111 as a likely candidate, it is an audio op-amp rated for upto 40MHz so plenty of bandwitdh for a simple PAL/NTSC video signal. There are of course lots of other single channel op-amps out there in a dip-8 format but they tend to be expensive (as in really expensive) possibly cheaper than a real 2318D but not by much, some are stupidly expensive (nearly 100x that 2318D). The HA-5111 should be about £0.90
The CPU, SAM, PIAs and VDG are all fairly easy to obtain online and possibly even new (MC6821s are still made and there are lots of compatible alternatives). A and B rated components should all be usable but make sure the processor is an "09e". The Hitachi HD6309 is usable but may cause crashes in some software due to lazy coding
The 4000 series logic chips are particularly tricky - some have modern equivalents. The MC14053 and MC14050 can be replaced with modern CD4000 series chips. The MC14529 was replaced with a CD4000 series logic chip but even that is now obsolete with no valid replacement
The LM1889 is another hard to find chip but still reasonably cheap online when found
The two ROMs are genuinely unavailable except from donor boards
As these parts become harder to find it is inevitable that replacements and redesigns will be needed. Abandoning the original composite video output circuits and moving to a (more) modern RGB or VGA output instead removes a big chunk of the more troublesome hardware, notably the LM1889 and SFC2318 along with a number of other common logic chips. If a composite output is still required an ADR724J can be used to generate the necessary signal, all in one chip
A cheat replacement for the 6847 exists, using a raspberry pi pico but this could easily be reduced even further an FPGA (assuming such chips are readily available again)
The ROMs can be replaced with a single EPROM/EEPROM, even opening up the option of switchable ROM images given the size of a modern EPROM's addressable memory. I have a mini conversion board that fits into a single socket with a jumper to the E pin of the second socket. (see https://github.com/jimbro1000/Dragon32RomFix)
The one thing that isn't so quickly removed from the equation is that MC14529 - a dual 4-channel data selector. Onsemi and Harris used to made the CD4529 that replaced the original MC14529 and it is only fairly recently that the chip became obsolete so there is stock out there but the chip is somewhat specialised and as such really isnt easily substituted even with a modern SMD package.