Contents
Overview
The Acorn AIV Master computer used by the Domesday project was fitted with a specific SCSI adapter card in order to connect the Philips VP415 LaserDisc player to the computer. As there were a relatively low number of Domesday systems sold (and very few survive) these cards can be difficult (and expensive) to source.
This project provides a drop-in replacement for the original AIV SCSI board which both functions as and resembles the original.
Hardware
Schematic
The schematic for the AIV SCSI Host Adapter is split into several parts as shown by the following high-level schematic diagram:
Internal 1 Mhz bus
The connection to the Acorn Master’s internal 1 MHz bus is shown in the following schematic:
SCSI adapter logic
The SCSI to host adapter logic is shown in the following schematic:
SCSI connector and termination
The connection to the SCSI interface and required signal termination is shown in the following schematic:
Video clock
The adapter board also provides a more accurate video clock for the Acorn Master. The video clock circuit is shown in the following schematic:
PCB Design
The following picture shows the overview of the PCB design:
The PCB is designed to be professionally produced as a two-layer board. The following specifications are recommended:
- Layers: 2
- PCB Thickness: 1.6mm
- PCB Colour: Green
- Surface Finish: HASL
- Copper Weight: 1oz
- Material Details: FR4-Standard Tg 140C
Connection to the Internal 1 MHz bus
Connection to the Internal 1 MHz bus requires a custom SIL cable and connector to be made (as the original SIL cables are no longer available for sale). The suggested method is to use a 14cm length of tubing (such as antenna tubing for RC cars) which is capped at one end. Using 24 AWG hook-up wire, push the hook-up wire into the tube and snip the wire once it is pushed all the way to the end-cap; this way you can cut 20 equal lengths of wire very quickly. If required, a different colour of wire can be used to mark the first pin of the connector to help prevent reversal.
A 20 pin plug-connector header can be used to connect to the Master’s internal connector. The suggested connector is a Preci-dip part number 380-10-120-00-001101 as shown in the following picture:
The hook-up wires can be soldered into the connector and heat-shrink tubing applied for protection against shorts (and to improve the look of the connector). The SCSI board end of the cable can be directly soldered to the PCB or (optionally) another header can be used if the cable is required to be removable.
Power and video clock connections
The AIV SCSI board has 3 additional connections, 2 for power and 1 for the video clock. The power connections are provided by two 7cm cables terminated with two fully-insulated 6.3×0.8mm blade connectors and should be coloured red and black according to the silkscreen of the PCB (Single core PVC 0.5mm2 (AWG20) stranded wire is recommended). The clock connector is a 8cm yellow cable (24AWG hook-up wire is suggested) and terminated using a 2 pin Dupont female connector.
Bill of materials
The required materials are shown in the following table:
| Ref | Qnty | Value | Footprint | Description |
| A6, A5, A4, A3, A2, A1, A7 | 7 | 33nF | Capacitors_THT:C_Disc_D5.0mm_W2.5mm_P5.00mm | Unpolarized capacitor |
| C2, C1 | 2 | 22uF | Capacitors_THT:CP_Radial_Tantal_D5.0mm_P5.00mm | Polarised capacitor |
| C3 | 1 | 100pF | Capacitors_THT:C_Disc_D5.0mm_W2.5mm_P5.00mm | Unpolarized capacitor |
| C4 | 1 | 10uF | Capacitors_THT:CP_Radial_Tantal_D5.0mm_P5.00mm | Polarised capacitor |
| D2, D1 | 2 | 1N4148 | Diodes_THT:D_DO-35_SOD27_P10.16mm_Horizontal | 100V 0.15A standard switching diode, DO-35 |
| IC1 | 1 | 74AS373 | Housings_DIP:DIP-20_W7.62mm | 8 bits Latch, 3 state Out |
| IC2 | 1 | 74LS240 | Housings_DIP:DIP-20_W7.62mm | 8 bits BUS Buffer (Inverter) 3 State out |
| IC3 | 1 | 74LS373 | Housings_DIP:DIP-20_W7.62mm | 8 bits Latch, 3 state Out |
| IC4 | 1 | 74HC138 | Housings_DIP:DIP-16_W7.62mm | Decoder 3 to 8 (active low outputs) |
| IC5 | 1 | 74LS07 | Housings_DIP:DIP-14_W7.62mm | Hex Buffer (inverting) with open-collector outputs |
| IC6 | 1 | 74LS14 | Housings_DIP:DIP-14_W7.62mm | Hex Inverseur |
| IC7 | 1 | 74HC74 | Housings_DIP:DIP-14_W7.62mm | Dual D FlipFlop, Set & Reset |
| IC8 | 1 | 74LS279 | Housings_DIP:DIP-16_W7.62mm | |
| J1 | 1 | +5VA Spade | Additional_footprints:PCB_Blade_Terminal_6.3mm | Generic connector, single row, 01×01 |
| J2 | 1 | 0VA Spade | Additional_footprints:PCB_Blade_Terminal_6.3mm | Generic connector, single row, 01×01 |
| J3 | 1 | Wire/Red | Connectors:Pin_d1.4mm_L8.5mm_W2.8mm_FlatFork | Generic connector, single row, 01×01 |
| J4 | 1 | Wire/Yellow | Connectors:Pin_d1.4mm_L8.5mm_W2.8mm_FlatFork | Generic connector, single row, 01×01 |
| J5 | 1 | Wire/Black | Connectors:Pin_d1.4mm_L8.5mm_W2.8mm_FlatFork | Generic connector, single row, 01×01 |
| OSC1 | 1 | VTCXO-14 | Oscillators:Oscillator_DIP-14 | Voltage-Controlled Crystal Clock Oscillator, DIP14-style metal package |
| PL1 | 1 | 50 Way IDC Plug | Connectors_Multicomp:Multicomp_MC9A12-5034_2x25x2.54mm_Straight | Generic connector, double row, 02×25, odd/even pin numbering scheme (row 1 odd numbers, row 2 even numbers) |
| PL2 | 1 | 20 Way SIL | Pin_Headers:Pin_Header_Straight_1x20_Pitch2.54mm (Note: optional) | Generic connector, single row, 01×20 |
| R1, R2, R3 | 3 | 2K2 | Resistors_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal | Resistor |
| R4 | 1 | 2R2 | Resistors_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal | Resistor |
| R5 | 1 | 100R | Resistors_THT:R_Axial_DIN0207_L6.3mm_D2.5mm_P10.16mm_Horizontal | Resistor |
| RP1 | 1 | Bourns-4116R-3-221/331 | Housings_DIP:DIP-16_W7.62mm | Resistor array |
Files for download
The KiCAD files for the AIV SCSI adapter board can be found in the Github repository along with the Gerber files required for PCB manufacture:
Acorn AIV SCSI Adapter repository