BeebSCSI Hardware Guide

The BeebSCSI board


The BeebSCSI schematic (board revision 7_5) is split over two sheets. The first sheet contains the CPLD and AVR microcontroller:

BeebSCSI Schematic 7_5 Sheet 1

The routing between the 1 MHz bus IDC connector, the terminating resistor networks and the CPLD is designed to minimise the required board space as much as possible. The routing between the CPLD and the AVR is also optimised in the same manner as much as is possible. Note that the on board 5V serial port does not provide power; it is designed to work with a USB based converter that is self-powered from the USB port of the host computer.

The design uses a mix of 3V3 and 5Vs with the AVR running at 16 MHz at 5Vs and the CPLD running at 3V3. Since the SD card is driven by the AVR, it requires a line-level conversion to provide 3V3 signals to the SD card.

The second sheet contains the SD card socket, line-level conversion and the 3V3 power regulation circuits:

BeebSCSI Schematic 7_5 Sheet 2

The schematic diagrams are drawn using the Eagle CAD application (version 7.7) and can be freely downloaded from the BeebSCSI project website. Note that the 3 pin power connector is designed so the power connector can be connected in either orientation without the opportunity to provide a reversed polarity to the supply rails.

The SD card circuitry includes a 3V3 to 5V line-level converter IC and a MOSFET power control (allowing the SD card to be powered up and down in software) – this design is to ensure maximum compatibility with a wide-range of SD cards.


The BeebSCSI PCB design is a double-sided board designed so that it can be both hand-printed and professionally manufactured. The PCB measures approximately 80mm x 50mm. The PCB CAD files can be freely downloaded from the Domesday86 project website.

The PCB design ensures that no soldered connections are located under a component and component pins are not used as vias (since this is not possible to do when making your own PCB using a method such as UV transfer). The board vias are also large to allow side-to-side vias using wires rather than via plating available on professionally made PCBs.

The bottom-side of the PCB is primarily used for the power rails and decoupling and the top-side contains most of the signal routing. The exception is the SD card socket that is mounted on the bottom of the board due to space considerations.

The following image shows the top-side of the PCB (rendered with components in Eagle CAD):

BeebSCSI PCB 7_5 Top

The following image shows the bottom-side of the PCB:

BeebSCSI PCB 7_5 Bottom

Note: In the 7_5 version of the board there is an error in the silkscreen for the CPLD JTAG – the pins are labelled in reverse order. 3V3 is the pin nearest the IDC and TMS is furthest from the IDC.  This will be corrected in the next board revision.

BeebSCSI and the Master’s Internal 1MHz bus


The internal 1 MHz bus of the BBC Master is provided via a 20 pin SIL connector (Single In-Line) and has a different pin out from the external 1 MHz bus. The internal bus also only supplies 4 address lines. Electronically the internal bus is also different to the external as it used CMOS levels rather than TTL.

This generates an issue for BeebSCSI as the CMOS level signals are quite noisy and the noise peaks can cause the board to read bytes incorrectly. To use BeebSCSI with the internal bus it is necessary to use an adaptor board that provides CMOS to TTL level conversion for all control signals as well as converting the 20 pin SIL to the 34 way IDC used by the external bus. The adaptor board also provides a 5V signal on to the internal/external detect pin of the CDPL allowing the CDPL to detect that it is connected to the internal bus (and therefore switch logic to the AIV Host Adapter).


The schematic for the BeebSCSI internal bus adapter is shown in the following diagram:

BeebSCSI Internal Bus Adapter

The adapter board provides CMOS to TTL line-level conversion and a 5V signal to indicate that BeebSCSI is connected to the internal bus. The adaptor board also contains a 500mA PTC (a self-resetting fuse) to protect the Master’s PSU (and BeebSCSI) from incorrect wiring. The adaptor board also provides a 5V power output (with the same pinout as BeebSCSI) to allow BeebSCSI to be powered from the adapter board (and, therefore, the BBC Master PSU).

Important note: Since the internal bus adapter card provides a 5V signal on (what should be) a 0V ground it should not be used to connect any other 1 MHz bus device other than a BeebSCSI board.  The PTC fuse should protect the Master against any such use, but it’s better not to rely on the fail-safe!


The BeebSCSI Internal Bus Adapter PCB design is a double-sided board intended for profession PCB manufacture. The PCB CAD files can be freely downloaded from the BeebSCSI project website.

The adapter provides a 34 way IDC connector for connection to the BeebSCSI board. In addition there is a 3 pin header for supplying power from the adapter to the BeebSCSI board. As BBC Masters have two different types of connectors for the internal bus, the adapter board supplies a set of 20 connections that can be terminated in either a male or female connector.  The BeebSCSI VFS adapter (1_4) is shown in the following picture:

BeebSCSI VFS Adapter 1_4