Building an Acorn System 1 replica - Plans, Schematics and PCB foils (30/03/2022)
Here I will list the files available for each board, along with schematics, PCB foil plans and complete Gerber file sets for download.
** NOTE ** I've included all boards that are work in progress and not yet completed. If you are intending to build one of these systems for yourself, please make sure you ONLY download plans for boards that are marked as COMPLETED. Only completed boards will contain the PCB foils and Gerber files.
If available, clicking on the board image will take you to a dedicated page for that board.
Boards marked as "IN PROGRESS" usually mean I'm either waiting for them to be delivered, or to to prove they work as expected.
** NOTE ** I've included all boards that are work in progress and not yet completed. If you are intending to build one of these systems for yourself, please make sure you ONLY download plans for boards that are marked as COMPLETED. Only completed boards will contain the PCB foils and Gerber files.
If available, clicking on the board image will take you to a dedicated page for that board.
Boards marked as "IN PROGRESS" usually mean I'm either waiting for them to be delivered, or to to prove they work as expected.
Note for those wanting to build a true Acorn System X replica.
With the differences I've already outlined in previous pages, the CPU and KeyPad boards are compatible with the existing Acorn System 1 design. Using these you should be able to go on and build System 2's, 3's etc. They should happily plug into an existing Acorn backplane and work.
However, I personally have no interest in going down the System X route so some of the additional boards that I will include below will not necessarily be compatible within the Acorn universe. The backplane is an example. You will not be able to build my backplane, plug in an existing Acorn VDU board and expect it to work. I'm following a different philosophy so cannot guarantee compatibility. That said, they may work... I'm just not going to be testing them. The schematics and PCB foils are included with those boards that are at a suitable stage in their life cycle so you can download, examine and judge for yourself.
However, I personally have no interest in going down the System X route so some of the additional boards that I will include below will not necessarily be compatible within the Acorn universe. The backplane is an example. You will not be able to build my backplane, plug in an existing Acorn VDU board and expect it to work. I'm following a different philosophy so cannot guarantee compatibility. That said, they may work... I'm just not going to be testing them. The schematics and PCB foils are included with those boards that are at a suitable stage in their life cycle so you can download, examine and judge for yourself.
CPU Board
This is the main Acorn System 1 CPU board.
Revision 1.2.1 is a fully working design and uses a 6522 VIA chip instead of the legacy INS8154 chip.
Revision 1.2.1 is a fully working design and uses a 6522 VIA chip instead of the legacy INS8154 chip.
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Revision: 1.2/2
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Status: COMPLETE
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Schematic layout file & PCB foil in DipTrace format
Schematic file in .png format .ZIP file containing files for PCB fabrication A copy of the BIN file used to program a 2764 EPROM with the standard Acorn System 1 Monitor. A scanned copy of the original Acorn System 1 user manual. | ||||||
The revision 1.2.2 schematic is identical in functionality to the 1.2.0 version but has some component numbers corrected and some slight readability enhancements
Some things to note about this version of the CPU board.
This board will ONLY accept the W65C02 CPU from the Western Design centre. This variant of the CPU will run to a maximum of 14 MHz which opens up some interesting possibilities.
If you do want to want to run the CPU faster than 1MHz, you need to be careful which version of the 6522 you have.
The original 6522 came in either a 1 or 2MHz variant. However, if you want to run your system faster than this, you will need to replace the 6522 with the Western Digital W65C22 chip. This can also run up to a maximum of 14 MH, but running the system this fast could cause access problems for the RAM and EPROMs. 14MHz equates to around 70ns access time, so you would need suitable RAMs and ROMs. 8MHz seems perfectly stable using more common EPROMS and RAMs.
** NOTE **
IC15 is labelled as a 74LS04. I actually had a couple (and it was only a couple) of system crashes on an earlier prototype. This could have been because I was messing around at the time. However, I've actually been running my boards using a 74LS14 since and I've had no issues at all all the way to 8 MHz.
Some things to note about this version of the CPU board.
This board will ONLY accept the W65C02 CPU from the Western Design centre. This variant of the CPU will run to a maximum of 14 MHz which opens up some interesting possibilities.
If you do want to want to run the CPU faster than 1MHz, you need to be careful which version of the 6522 you have.
The original 6522 came in either a 1 or 2MHz variant. However, if you want to run your system faster than this, you will need to replace the 6522 with the Western Digital W65C22 chip. This can also run up to a maximum of 14 MH, but running the system this fast could cause access problems for the RAM and EPROMs. 14MHz equates to around 70ns access time, so you would need suitable RAMs and ROMs. 8MHz seems perfectly stable using more common EPROMS and RAMs.
** NOTE **
IC15 is labelled as a 74LS04. I actually had a couple (and it was only a couple) of system crashes on an earlier prototype. This could have been because I was messing around at the time. However, I've actually been running my boards using a 74LS14 since and I've had no issues at all all the way to 8 MHz.
Keypad and Display board
The Keypad and Display board connects via a 26-way ribbon cable to the System 1 CPU board. This board then allows the user to interact with the Acorn Monitor firmware, enabling the entry, modification, execution and debugging of machine code programs.
The board doesn't have the cassette interface electronics embedded, rather it has an expansion socket where an interface module could be attached if required. There are some other slight differences from the original. Tactile push button switches are used instead of the original spring dome switches, there is a power LED onboard, the NMI and IRQ buttons have been moved from the CPU board to this keypad board, and there is space for two additional push buttons if required. Because the CPU board now uses a 6522 VIA chip, the four additional control signals from the VIA chip are routed to this board and could be connected to these spare switches if required.
The board doesn't have the cassette interface electronics embedded, rather it has an expansion socket where an interface module could be attached if required. There are some other slight differences from the original. Tactile push button switches are used instead of the original spring dome switches, there is a power LED onboard, the NMI and IRQ buttons have been moved from the CPU board to this keypad board, and there is space for two additional push buttons if required. Because the CPU board now uses a 6522 VIA chip, the four additional control signals from the VIA chip are routed to this board and could be connected to these spare switches if required.
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Revision: 1.2/0
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Status: COMPLETED
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ZIP File contains Diptrace format schematic and PCB foil.
It also contains the .beam files for a Beamo laser cutter to make the key pad dust shield. | ||
Be carful with any of the boards from here on down. They may NOT be compatible with the Acorn System X universe. The backplane certainly isn't !!.
Blank Euro Card PCB
This is a blank Euro card PCB template that can be used as the basis for your own designs. Don't change the board dimensions or move the board outline as it won't then fit in a 19" rack card frame.
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Revision: 1.0/0
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Status: COMPLETE
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| blank_euro_card_v1.0.0.dip |
9-Slot Passive Backplane
This backplane board is designed to be mounted into a 19" rack enclosure and leaves enough room to also install a standard 500 watt PC PSU within the frame.
The backplane is passive so contains no active electronics. Only some 100nF bypass capacitors are required; one per slot.
*** THIS BOARD IS NOT CAMPATIBLE WITH THE ACORN SYSTEM X UNIVERSE. !!! ***
The backplane is passive so contains no active electronics. Only some 100nF bypass capacitors are required; one per slot.
*** THIS BOARD IS NOT CAMPATIBLE WITH THE ACORN SYSTEM X UNIVERSE. !!! ***
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Revision: 1.0/0
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Status: IN PROGRESS
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Provision is provided to connect multiple backplanes together if required. Not all slots need to have a connector. This would allow for larger (wider) modules to inserted and more economical use of the sockets. Whilst it's a simple task to solder in additional sockets as required, removing them intact and without damaging the PCB is almost impossible. Be warned !
Euro size development board
This board makes it simpler to prototype and create your own plug in expansion cards. The board accepts a standard Euro 96 way connector and all the major signals are labeled on the board for easy identification. Final designs can remain on these boards or then transferred to a PCB if required.
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Revision: 1.1/0
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Status: IN PROGRESS
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The initial revision 1.0.0 board was updated to better support higher pin-count/wider IC packages.
INS8154 Piggy back replacement module
This module is designed to plug into an Acorn System 1 CPU board (original or replica) and allow for the legacy INS8154 IC to be replaced with a more easily obtained 6522 or a modern W65C22 IC.
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Revision: 1.2/01
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Status: COMPLETE
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Two additional wires need to be attached to the CPU board to carry the clock and R/W signals.
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ZIP file contains schematic, PCB CAD file, pictures and the full Gerber/Drill files so you can order the PCB's yourself.
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Extender Euro Card
The extender Euro card allows for simpler debugging of device cards whilst still plugged into the backplane when mounted within an enclosure. The card can also be used to link the system backplane so a breadboard via a ribbon cable to aid card development.
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Revision: 1.1/0
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Status: COMPLETE
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The ZIP file contains the PCB foils for the main extender card and the two interface head options.
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Segment Address Decoder - SAD
The SAD (Segment Address Decoder) is used to reduce the burden on expansion cards to perform their own address decoding. IT saves adding a lot of duplicated decode logic, address switches or jumpers on each expansion card, and saves board space, complexity and cost.
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Revision: 1.0/1
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Status: IN PROGRESS
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This was born from an idea I had recently to reduce the requirements of every expansion board needing to decode the address bus itself. It's a proof-of-concept design that if works as hoped, will reduce the complexity of each expansion board thus saving board space and component count, but with the added benefit of allowing each devices address to be easily changed centrally as required.
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The .ZIP file contains the schematic and PCB foil.
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8 x LED Binary Output Card
A basic card that can display a single byte of data on 8 x LEDs.
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Revision: 1.2/0
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Status: COMPLETE
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The .ZIP file contains the schematic and PCB foil.
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