I want to use a shielded cable with SwitchBlox, where can I connect the shield of the cable to?

BotBlox Hardware
1

– Copied from customer email –

Is there a place on the 5 port switch to ground the shield?

2

Your question is simple, but the answer is nuanced.

None of our boards have shield or grounding points on the connectors. This was a decision we made early on in the design of our boards. There’s a lot of reasoning behind this, but in general the thought process was…

  • Shielding protects signal integrity when the environment is uncontrolled (imagine a long cable between two different buildings, you’ll want that shielded).

  • Shield is also useful for longer cables (eg longer than 10 meters), because longer cables have more chances to pick up interference.

  • However our boards are typically used in embedded systems, inside a chassis. Therefore the engineer has a lot more control over the environment the cable is in. (Also most connections to our switches are short).

  • Our customers care more about size, it’s the main motivating factor in their design, thus we decided that not including shielding pins on our boards was a reasonable tradeoff. (Remember, nearly all of our customer base are building compact, embedded systems).

This does however mean that it’s harder for the engineer if/when they do decide they want shielding. There are many reasons an engineer would want shielding, even on a short cable inside a chassis. The most common reasons are something like…

  • There’s a motor nearby that generates a lot of EMI that could interfere with the ethernet signals
  • There’s a sensitive radio transceiver nearby that could be affected by the ethernet signals (1GBASE-T fundamental frequency is 125MHz with harmonics higher up)

This does present a problem for customers that do need to shield on our boards… What does the shield connect to?!

In most cases, smart consideration of the overall system is required to find the best grounding point. Generally this is the chassis of the device, but it is often the negative terminal of the power supply. In most cases, our customers will connect to the negative terminal of the power supply, which means our shielded cables need a drain wire that can be tied down to the power ground. Of course, this does add mechanical complexity to the overall design. Some customers break out a ground point near the switch’s power input for this.

I appreciate this isn’t a totally definitive answer, but hopefully it helps explain the thought methodology here.