This is the Mainboard for the replica Atom, it closely follows the original architecture although with the consolidation of memory chips this has allowed address decoding to be simplified.
Memory
the board carries a full 32K of RAM for 'Text' space in IC20-52 (replacing 12 2114 RAMs)
video RAM is provided by an 8K RAM IC32-43 (replacing the 12 2114 RAMs)
a 16K EPROM, IC20-21 replaces the original IC20 and 21 and carries COS, BASIC, BASIC Floating Point and DOS for the optional Floppy Disk Controller (FDC)
a 16K EPROM, IC24 replaces the original Utility ROM allowing selection of up to four different ROMs including NFS (this was a 4K 2732 EPROM on Issue 1 PCBs)
Video
the video signals at PL4 and SK2 have been retained
the three main video signals Y, Pr and Pb have been 'beefed up' with the addition of a 3-channel video driver, IC56 (Q1 is still required to invert Y)
the modulator has been dropped
a YPrPb jack socket, SK4, has been added
the 'Alan Knowles' screen noise eliminator has been incorporated (IC105 and half of IC55)
Econet
Lack of board space meant that the bus buffers and DIN41612 expansion connector had to be omitted however a full Econet interface has been included on the board.
As far as possible original component references have been used, 100 series refs are used for the Econet interface and 200 series for the FDC interface
Links
| Ref | Name | Function |
| LK2 | 6522 (IC1) IRQ | Not usually fitted |
| LK3 | 6854 (IC101) IRQ | Fit only if using Econet |
| LK4 | Invert | Normally fitted |
| LK5 | CSS | Normally fitted in 0V position |
| LK6 | Utility ROM (IC24) page select | allows selection of one of four images from IC24 |
Connectors
| Ref | Name | Type | Function |
| PL4 | Colour Output | 2 x 5 header | various video signals as original Atom |
| PL5 | Printer | 26-way box header | 'Centronics' parallel printer output |
| PL8 | 'PL8' | 2 x 10-way header | Originally for Econet interface |
| PL9 | Keyboard | 26-way box header | For external keyboard |
| PL10 | Speaker | 2-way header | For speaker (use Electron speaker) |
| PL11 | FDC Interface | 26-way box header | Connects to optional FDC board |
| PL12 | 6522 Spare | 10-way header | Unused 6522 I/O pins |
| PL13 | Aux Power | 2-way header | For e.g. Floppy Drive |
| PL14 | I/O Decode | 8-way header | I/O decodes for optional use |
| SK1 | Video | RCA socket | Composite video output |
| SK2 | Cassette | 7-pin DIN socket | Cassette output as original Atom |
| SK3 | Power | 2.1mm power jack | +5V power input |
| SK4 | YPrPb | 3.5mm 4-pole TRRS jack socket | Colour outputs |
| SK100 | Econet | 5-pin 180o DIN socket | Econet interface |
Schematic
Bill Of Materials
Blank PCBs
