Acorn AIV SCSI Host Adapter Replacement

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.

Acorn AIV SCSI Adapter replacement board

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:

Acorn AIV SCSI Adapter – High-level

Internal 1 Mhz bus

The connection to the Acorn Master’s internal 1 MHz bus is shown in the following schematic:

Acorn AIV SCSI Adapter – Internal 1 MHz bus

SCSI adapter logic

The SCSI to host adapter logic is shown in the following schematic:

Acorn AIV SCSI Adapter – SCSI to host logic

SCSI connector and termination

The connection to the SCSI interface and required signal termination is shown in the following schematic:

Acorn AIV SCSI Adapter – SCSI termination

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:

Acorn AIV SCSI Adapter – Video clock

PCB Design

The following picture shows the overview of the PCB design:

Acorn AIV SCSI Adapter PCB – Top

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:

Bus connection header

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