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.
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-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.
Each device must have a 7-bit address that essentially is just an integer number between 0 and 127. PapaLink devices come with a randomized address but with many devices it is possible that these addresses overlap and cause issues. But no worry, it is easy to change the devices address for example by using the PapaLink USB Controller. Purposeful address overlap can be useful as it allows you to control identical devices truly simultaneously.
The default frequency of SCL is 100 kHz and thus the speed of data transfer is 100,000 bit/s. However the I2C protocol comes up with some overhead. A typical write message to a device consists of a 7-bit address frame to select the right device, 1 read or write bit, and three 8-bit data frames (8-bits for a key and 16-bits for its value). You also have maybe 2 ACK/NACK bits and then bits for the start and stop conditions. So for each 3×8=24-bit of actual data, we have to transfer 36-bits of data so there's ~33% overhead. This 36-bit message we can write at a rate of 2.7 kHz or with a delay of 370 µs. There are some ways we can improve these numbers:
1) More data frames per message (if feasible) will decrease the overhead and increase the overall data transfer rate but will decrease the rate we can switch between devices, thus causing larger delays. 2) When using many data hungry devices it is possible to split the data bus in two or more and use multiple controllers to get more bandwidth (the limit then comes in total USB 2.0 bandwidth available by the host PC when using the PapaLink USB Controller). 3) It is possible to increase the SCL frequency from 100 kHz (standard mode) to 400 kHz (fast mode) and up to 1 MHz (fast mode plus). This increase the transfer speed and reduces delays but may make the bus less stable when running in electrically noisy environments with long or shielded cables, and may require stronger pull-up resistors for the controller. The bus capacitance also should not exceed 400 pF or 550 pF.
Overall, PapaLink with the 100 kHz frequency fits most of the scientific and industrial speed and latency needs for device control and can be pushed further if really needed. However, it is not enough alone for big data such as high-resolution images and videos or buffered +MHz data acquisition.
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.