Hi and thanks for making and supporting such a cool product. It’s a very generous contribution to the community.
I’m a colleague of yzhang’s (he posted about this setup in the software forum) and I’m trying to help him get back up and running while we wait on a replacement “blue PCB” from Labmaker. Sadly, I don’t think that the cape is the source of the issue. In the interest of speed, I’ve purchased an entire replacement setup (and most of the components to assemble 4 more reactors but that’s another story.) Shipping to the US is very slow so I’m hoping that you can point me in the right direction to get this back up and running ASAP.

I’ve done some troubleshooting with an oscilloscope, logic analyzer and ohmmeter and wanted to share what I’ve found so far.
1) We currently only have one bioreactor, so nothing to compare it to.

2)The power supply I received with the bioreactor was DC 7V 1.5A, not 12V 3A
I haven’t read all of the datasheets yet but the power requirements for the TPS54427 6V regulators came to mind:
“Power Supply Recommendations
The TPS54427 is designed to operate from input supply voltage in the range of 4.5 V to 18 V. Buck converters
require the input voltage to be higher than the output voltage. in this case the maximum recommended operating
duty cycle is 65%. Using that criteria, the minimum recommended input voltage is Vo/0.65.” So 7 volts puts it at 85% duty cycle, probably OK since there’s a thermal overload cutoff.

3)Our problem exists on all ports (T0-T7.)

4)Our problem exists whether the bioreactor is connected to power or not.

5)After hearing that Labmaker was sending a replacement “blue PCB,” I removed the multiplexer with hot air and carefully soldered on a replacement PCA9548A. That didn’t make any difference.

6)I’ve used a logic analyzer and an oscilloscope with 10X probes to make sure that probe capacitance isn’t an issue. Rise times seem fine.

7)I haven’t connected the pump for any of this.

8)I’ve watched the signals with both ‘bash cb.sh’ and various i2cget/set/detect/dump commands to emulate the script. The results are the same: writing to the multiplexer when the channel has been set to a port with a reactor makes a mess of the SDA and SCL lines until it’s changed to a different channel.

9)I2C signals (as measured on the BeagleBone pins) are fine with the bioreactor control cable disconnected.
Zoomed out:

Zoomed in:

10)I2C signals (as measured on the BeagleBone pins) go OK until they hit a multiplexer channel that is connected to the bioreactor. At this point, both the SDA and SCL lines will get stuck around 1.7V.
Zoomed way out (note that the signal goes from the usual 0-3.3v, then gets stuck at 1.7V for a while, then goes down to almost, but not quite zero):

Zoomed out:

The moment everything goes wrong:

11)Once the multiplexer is connected to a channel with the bioreactor, any attempt to read/write to the multiplexer will fail e.g. i2cset -y 2 0x74 0x00 0x01 Error: Write failed, as long as it is connected to a channel containing a bioreactor. If the cable is removed, it will recover, e.g. i2cdump -y 2 0x74 will work. If a different channel is selected and the cable is reconnected, then everything is fine again.

12)The temperature sensor at 0x1b is not affected until the multiplexer is connected to a channel with the bioreactor. After that, it displays the same symptoms e.g. i2cset -y 2 0x1b 0x00
Error: Write failed.

13)On the “Laser Measure” board, the resistance between SDA & GND (and maybe SCL & GND? I can’t remember) is ~300 ohms. I measured it multiple times, with multiple ohmmeters, in both directions. That seems very wrong. SDA & SCL to 3.3V are something like 10k ohm IIRC.

14)Disconnecting the 5 pin cable that connects the “Side Board” to the “Laser Measure” board, prevents the 1.7V lockup issue from occurring. I haven’t been brave (foolish?) enough to attempt powering the bioreactor with this cable disconnected.

15)Yzhang reported unusual OD measurements leading up to the failure.

All of this, particularly points 13-15, point towards some failure on the “Laser Measure” board.

If you’ve got any tips on how to narrow it down to a specific component, which I would then replace, I would appreciate it very much. It can’t be worse than the TSSOP-24 multiplexer that I swapped for no reason ;P

Thanks for your time, and sorry about the giant wall of text.