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PapaLink
PapaLink (previously OpenLink) is a physical interface and a communication protocol that does data and power in the same cable. It is developed by DEV Joni to be used in various open-source devices targeted for industrial and scientific use.
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Motivation
Most modern connectors (for example, USB and HDMI) are not feasible to build from ground up because they depend on factory-made parts. We believe that open-source devices have to be accessible for everyone. We designed an open-source connector that anyone could put together in their garage - provided soldering tools, M3 screws and nuts and some metal strips. The result is a reliable connector with a size ideal for most industrial and scientific applications.
Data lines
Data-transfer wise, we did not want to reinvent the wheel. The data lines run the industry standard I2C protocol that offers relatively fast data transfer speeds, supports up to 127 devices, requires only two additional wires, and most importantly, is supported by virtually all micro-controllers and single-board computers. We added a few formalities on top of this protocol to make it easy for all the PapaLink devices to communicate together.
I2C has two data lines: SDA for data and SCL for the clock signal. The lines are in an open-drain configuration meaning that each connected device can only pull the line down, to 0 V ground, or leave it be at the pull-up voltage. The pull-up voltage in PapaLink is 3.3 V, making PapaLink compatible both with 3.3 V and 5 V logic level circuits.
Power lines
We decided to set 12 V (with allowed range of 8V from 14V) as the power line voltage because most micro-controllers and single-board computers have integrated voltage regulators that can handle up to 12 V or so. It is possible to to use lower or higher voltages if all the devices connected in the bus can handle them.
The default connector and cables can handle up to 10 A continuous current safely. Multiple power-sources and circuit-separators can be used to drive many high-power devices. When a single device needs more than 10 A, it seems reasonable to design heavy-duty versions of the connectors and cables that can pass perhaps up to 8 times more current safely.