Strange signals on GPIO 14 & GPIO 4 when ribbon cable is attached to GPIO

Question

I'm using RP3B. I'm using piscope, as I don't have an oscilloscope available.

• After I boot my RP3B with nothing attached to the GPIO port, if I start pigpiod and piscope, I see all my GPIO with a constant signal level, as expected.
• After I boot my RP3B with a ribbon cable (IDE 40 pins) attached, and connected to a empty protoboard, if I start pigpiod and piscope, I can see some activity in GPIOs 4 & 14 (TXD). They look like clock/data running continously. It is not the same signal in both GPIOs. Unfortunately I don't have a print screen (and I'm cautious of connecting that cable again). But I can come with one if it looks safe.
• If I turn off the RPB3, detach the cable, and boot again, GPIOs return to normality (that is, no activity after boot).

Does anyone known what is happening? Thanks.

^{Setup: RP3B, Ribbon cable detached, Protoboard.}

^{piscope after boot with no cable attach. No data/clock signals.}

Software versions:

pi@raspberry:~ $ cat /proc/device-tree/model 
Raspberry Pi 3 Model B Rev 1.2

pi@raspberry:~ $ uname -a
Linux raspberry 4.14.52-v7+ #1123 SMP Wed Jun 27 17:35:49 BST 2018 armv7l GNU/Linux

pi@raspberry:~ $ pigpiod -V
64

piscope version: 0.4

Answer 1

The most likely explanation is that the GPIO are floating (not pulled-high or low) and that the ribbon cable wires are acting like an aerial. It is quite possible that this will generate GPIO activity.

Try the following test.

pigs pud 4 d pigs pud 14 d

That will set the internal pull-downs for GPIO 4 and 14. That might be enough to stop stray signals.

If it doesn't check the modes of those GPIO with the following command.

pigs mg 4 mg 14

It should report 0 0 indicating both GPIO are in INPUT mode.