New wiring, power test

Posted on September 17, 2014

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Vnevoa:

Due to the new I²C communications bus scheme, I had to make new cables to connect the Raspberry Pi controller to Njay’s power bridge controllers. This cable is a simple “Y” shaped common bus made from CAT-5 network cable. The network cable is cool for the job because it’s already got 4 twisted pairs of wire and we need 3 for this connection: one for 3.3V power, one for the Clock signal (SCL), and one for the Data signal (SDA). It may seem overkill to have 3 ground wires inside the same cable, but the twisted pairs reduce the inductance of the wires and so preserve the edges of the digital signals. As a bonus they reduce the electromagnetic radiation that would emanate from them if they were straight or had just one ground wire. It also reduces the chances of picking up electric noise from the surroundings, which is a big preoccupation for us.

As usual, everything starts with a drawing. In this one I laid out the adapter board where I'll solder the cables to the plug that goes into the RasPi.

As usual, everything starts with a drawing. In this one I laid out the adapter board where I’ll solder the cables to the plug that goes into the RasPi.

20140912_new_adapter

And… done. Neat enough for me. 🙂 There are 3 cables going into the RasPi case: 2 for I2C towards the Power Bridges, and 1 for 5V power coming from the 24V DC-DC converter.

Next up, comes the testing. After making sure with the multimeter that all the connections worked as they were intended to, I connected all the 3 boards and turned on the power. Here’s some video with LEDs (geeks love this kind of stuff, especially when it’s something we created with our own minds and hands – the first time a circuit is turned on and the lights blink, it feels like a baby was born – it compensates all that time and effort that was put into it.  😉 )

Every little light has its own meaning of course. The large red lights confirm that the I²C bus is powered. Why 2? Because the add-on board has a special chip that insulates the two sides of the bus (Master and Slave), in order to avoid any current circulating on the bus cable. So, since there is no current passage from the RasPi to the Bridge controller, the RasPi powers the Master side of the bus (left LED) and the Bridge powers the Slave side (right LED).

20140902_ready_bridge

There are also some tiny faint green LEDs on that board, one pair on each side, to signal that SDA and SCL lines are active. But that will be better seen later, when I show you the communications test.

(to be continued…)Lisbon_MMF_logos_Banner_big

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