Physical Design Standard

SOR Community Project Physical Design Standard

Physical Size

Pcbs should be multiples of 32.5mm x 32.5mm. ~1.25 inches
The target size for a module should be 64mm x 64mm ~(2.5 inches x 2.5 inches).
32.5mm x 32,5mm, 32.5mm x 64mm, 64mm x 64mm & 64 x 96.5mm are all acceptable sizes.
This is done to allow for compatibility the the Axon and Axon II MCU boards.

Mounting Holes and Mounting Brackets:

PCBs should have a 3.5mm (1/8 inch) hole in each corner allowing for M3 (4Gauge) screws. The screw hole should be .1 inches to center from the edges of all four corners of the board.

Chassis components should allow for M3 (4Gauge) screw to be fitted in a pattern that will allow for
32.5mm x 32.5mm square PCBs to be fitted adjacent to each other.
Screws can then be omitted as required to allow the fixing of larger boards.

On-board Voltages:

Standard power bus should be 5V DC.
Modules operating at less than 5V should have on-board voltage regulator and buffer circuitry to allow 5V I/O.
Modules accepting greater than 5V (eg. 12V) which contain regulators to supply 5V to circuitry on board should also be able to supply other boards with 5V power to a minimum of 200mA.
Modules accepting greater than 5V (eg. 12V) to power high voltage components (eg, DC motor drivers) do not need regulators for modules 5V circuitry. They may draw 5V from another module.

Other electrical Specifications:

The I2C bus should run at 100kHz, choosing reliability over speed. However supplied source code should be written in such a way that the end user can easily modify individual modules to run at different speeds.
There should be space for i2c pull up resistors on all i2c master boards although only one set should be installed per bus. Also any modules designed to be power supplies for all 5 volt circuitry should also contain optional pull up resistors for the I2C bus.

If large amounts of interference are expected from an inductive load to be connected to a module (eg, large DC motors) efforts must be made to protect the i2c and power bus from adverse effects.

Connector Type:

For logic level (ie. 5V) connections standard 2.54mm (.1 Inch) pitch, single row header pins.

These should be placed near edges of board whenever practical to enable the use of straight or right angled header pins.

Male header pins should be used except where PCBs are going to mate with other PCBs where one should be a female type header.

Inexpensive crimp connectors should be used for connecting cables.

For higher voltage connectors a larger connector type should be used. (Exact specification TBA.)


Standard Connectors:

The standard connector for i2c and power bus is a 5 pin header like this: Vcc, GND, SCL, SDA, (NC).
The NC (Not Connected) pin is to prevent accidental damage from reversing the connector.

All boards should be fitted with one of the i2c/power connectors at either side to allow modules to be daisy chained together.

UART header pins should also bring power in a 5 pin header like this: Vcc, GND, TX, RX, (NC). This is to provide debug capability to master modules and need not be available on slave modules.
Power here is for level shifters or other limited circuitry connected to the UART.

Standard Position for Connectors:

No set position for general purpose I/O pins although it is recommended to put them near the edge of boards to allow the use of right angled header pins if required.

The 5 pin i2c/power connectors should be mounted one at each side of the board to enable boards to be stacked easily.... (Exact position TBA.)