Based on the issue 2 schematic and photos of an issue 2 board. Not quite a replica as I have made the following changes:
the display is replaced with more available 7-segement LED display
the keyboard is replaced by either 'D6' or OMRON B3F tactile switches
additional links added for cassette backplane connections
The board has two configurations:
as the Keyboard, Display & Cassette Interface for a System 1 Computer
as the Cassette Interface for a System 2+ Computer
The links have the following function:
LK1 - connects NHITONE to B14
LK2 - connects B20 to 0V
LK3 - connects B19 to CASIN
LK4 - connects B20 to CASSOUT
LK5 - connects B21 to NHITONE
System 1 Computer
When part of a System 1 Computer the board is populated with most of the components with the exception of the 64-way DIN connector and connected to the 6502 CPU Board with a length of 0.2" pitch 20-way ribbon cable.
Only link LK2 should be fitted which connects 0V to the board.
System 2+ Computer
The board can also be used as a Cassette Interface only for a System 2+ or 6809 Computer, just the cassette interface components are fitted i.e. omit the display, key-switches and IC8.
A 64-way DIN41612 connector is fitted but with most of the pins removed (A2 to A31, B1 and B20 to B31).
Normally fit links LK3,4 and optionally LK5, see below.
Cassette Backplane Wiring
The System 2 Technical Manual suggests connecting the three cassette interface signals and O2 to pins on side B and 'wire-wrapping' them across:
| O2 | CPU A29 | connects to | B11 |
| CASIN | CPU B19 | connects to | B12 |
| CASOUT | CPU B20 | connects to | B13 |
| NHITONE | CPU B21 | connects to | B14 |
This doesn't work for me as I use backplanes where all of side B is tracked and I want to be able to plug any card into any slot (except for 6502 CPU Board on a Buffered Backplane of course).
Also if you were to use a 6502A CPU Board then the connections are pins B3 (CASIN), B4 (CASOUT) and B5 (NHITONE).
(Pretty sure a 6502A board will not work anyway with standard COS as it uses a 6522 rather than an 8154 ?)
The Acorn 6809 CPU Board uses pins B19 (CASIN) and B20 (CASOUT).
B19 and B20 are not allocated on any other boards however B21 carries a 1MHz clock on the 6502A CPU Boards.
Therefore I have opted for this wiring:
| O2 to A29 | standard backplane connection |
| CASIN to B19 | by fitting LK3 |
| CASOUT to B20 | by fitting LK4 |
| NHITONE to B14 | or alternatively B21 |
On this board:
| O2 | is wired to both A29 and B11 |
| CASIN | is wired to B12 and B19 via LK3 |
| CASOUT | is wired to B13 and B20 via LK4 |
| NHITONE | is wired to B14 via LK1 or B21 via LK5 |
This means that when a 64-way DIN connector is fitted certain pins must be removed to break the unused connections i.e. B1 (+5V), B12, B13, B14.
If you are using an original Keyboard, Display & Cassette Interface Board then these connections would have to be wire linked on the board.
Schematic
Bill Of Materials
PCB
Front Panel Drilling (System 2+ Only)
