A connector for the I2C bus

At this time I know of no real "standard" but there have been several recommended ways to arrange I2C signals on a connector.

Desirable features for a bus standard might be:

• A cannonical bus voltage, either 3.3V or 5V
• A cannonical logic suppy voltage, either 3.3V or 5V
• An additional higher voltage for peripheral devices
• An "INT" pin for devices that signal activity via an extra pin
• One or more "address" pins typically wired low or high

The purpose of address pins is so a board with multiple connectors could have each connector coded to a different address, and if two identical devices were connected one would assume the "0" address and the other the "1" address. This trick is used in memory modules in order that the ID EEPROMS can be associated with specific slots.

ACCESS.bus

4 way "modular" connector. I've seen this described as RJ10, but the latch appears different, the images I've seen appear to show two latches to the sides of the connector

• Pin 1: GND, Black
• Pin 2: SDA, Green
• Pin 3: +5v, Red
• Pin 4: SCL, White Shield: ground

Advantages:

• In the event of it being reversed the likelihood of damage is low (though not eliminated) as a VDD-GND swap is not possible.
• If common twisted-pair cables are used then each signal gets twisted with one power rail, and this is true for both a 1-2, 3-4 twist or a 1-4, 2-3 twist. Given that I2C is NOT differential this is a good compromise.
• Unambiguous 5V supply
• Specifies wire gauges, power: 26AWG data 28AWG

Disadvantages

• obsolete
• A.b is a subset of I2C, strictly speaking only "write" transfers are allowed requiring all devices to be capable of "mastering"

Sparkfun Qwiic

4-pin JST connector, PCBs use connectors in pairs to allow chaining

• Pin 1 Ground, Black
• Pin 2 3.3V, Red
• Pin 3 SDA, Blue
• Pin 4 SCL, Yellow

Advantages

• "Ecosystem" of existing products
• Support for daisy-chaining
• Unambiguous 3.3V supply
• Pin order is compatible with 1-4, 2-3 twisted pair configuration

Disadvantages

• Poor availability of JST connectors in hobbyist quantities
• No 5V support
• No addressing
• No provision for "int"
• No separation between SCL and SDA

MCC corp Tyco/AMP 4-1761206-1

• Pin-out matches ACCESS.bus, this appears to be by design as MCC appear to have adopted the ACCESS.bus connector specifications too

Advantages

• Using an existing configuration gives it some track record
• Same electrical advantages as ACCESS.bus, though maximum range may be reduced

My Preferance The arrangement I'm using is almost certainly copied from MCC/ACCESS.bus

Molex KK or equivalent 2.54mm pitch 4 way connector

• Pin 1 Ground, Blue wire (preferably)
• Pin 2 SDA, Green (or white) wire (preferably)
• Pin 3 +5V, Red wire
• Pin 4 SCL, Yellow wire

For clarification as there is ambiguity in connector orientation: If the connector is above a PCB, with the lugs facing you, then pin 1 is to the left.

Obvious disadvantages are:

• I have assumed the availability of 5V. In a 3.3V system I would put 3.3V on pin3.
• I have made no provision for other DC supplies
• I have made no provision for address pins
• I have made no provision for "int"

OSEPP Electronics I2C

Connector unknown, 2.54mm system resembling C-grid

• Pin 1: Power
• Pin 2: I2C Data
• Pin 3: Ground
• Pin 4: I2C Clock

• Similar to ACCESS.bus sequence but with power REVERSED!!

MIKRO-bus

PCB Daughterboard connector

https://download.mikroe.com/documents/standards/mikrobus/mikrobus-standard-specification-v200.pdf

Advantages

• Ecosystem of existing products
• Supports 5V and 3.3V supplies
• Includes "INT" signal

Disadvantages

• PCB-only, does not define a cable
• No support for address pins (this is surprising)
• No specification for bus supply voltage to use when both 5V and 3.3V parts are present. I am assuming that the host microcontroller's supply is normally used, but this could be 3.3V OR 5V depending on the specific microcontroller in use.