What "extra care" is needed for Externally powered serial devices?

I am just about to start connecting my Daisy chained G2 18v15 Motor Controllers to my Arduino, and I’ve been able to wrap my head around most of the Motor’s manual, but I’ve encountered this in my many readings, and still don’t fully understand what it means or what risks are involved, and haven’t been able to find an answer on the web or with a forum search:

Unpowered devices with a TTL serial port can turn on or partially on, drawing power from the serial line, which means that extra care must be taken when turning power off and on to reset the devices.

My Setup:

8 x G2 18v15 Motor Controllers, split into two Serial groups with 4 on each daisy chain. The daisy chain connects all optional Serial TTL lines (Rx,Tx,Txin,err,rst,gnd), and the controllers are powered by an external 12V Switching Power Supply (aprox 500W PC ATX power supply).

I’m running an Arduino Due for this project, and I intend to also power it with the same external 12Volt supply.

The motor controllers are wired to common ground in the daisy chain, to the Arduino, both of which are mounted on 3D printed plastic and isolated from the frame, which is aluminum, and which isn’t grounded (yet) with the external 12Volt supply. The power supply is connected by a series of 14 AWG wire pairs running in a parallel bundle for about ten feet to the actual frame, and then are only connect directly to the motor controllers currently, and the Arduinos 12 volt barrel plug. I would gladly take advice or links to a good tutorial on grounding.

For now, I’ve only used the Arduino on USB power, and haven’t connected the Motor Controllers serially. However, I started to plug in the ground to the Arduino last night, and I swear I heard something pop, so I stopped. I checked out the Arduino and ran it with USB, and everything seemed to be okay, so I’m not sure if maybe I just heard something else or I actually had a close call. And because I don’t understand this pop-out from the manual, I’m going to refrain from connecting anything else until I figure it out.

Any insight or advice would be greatly appreciated :nerd_face:


Hearing a pop when you make a connection is certainly a reason to be cautious!

The note you mentioned from the user’s guide is referring to sending serial signals to one of the Simple Motor Controllers (SMC) while the SMC is unpowered. Doing so could cause the SMC to be powered (or partially powered) from the serial lines. In general it is not good practice to send logic signals to an unpowered controller. For example, this could happen if the SMC was reset while the Arduino was still sending signals.

If you are powering all of your devices (Arduino and SMCs) from the same power supply, that should take care of your common ground. However, if you are still concerned about your connections, you could post a wiring diagram of your proposed connections, and I would be happy to take a look.

By the way, if you had the SMCs connected when you heard the pop, you might consider testing those separately before putting them in your system, just in case. Also, you might consider only connected one of your SMCs at first then expanding slowly, so if there is a problem it would limit the potential damage. Please note that you should generally not make or remove any connections from a powered system, so to do this you should power everything off before adding another SMC and powering it back up.