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Joined: 03 January 2018
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Posted: 24 April 2018 at 7:43am | IP Logged Copy link to clipboard   Quote Chopperp

My CMM used diodes to isolate the USB 5V & the regulator 5V which meant that the 5V supply was really about 4.6V as implied above. I had an RF TXRX module that was playing up intermittently. I eventually cut the diode from the USB & bridged the diode from the regulator. Module is now much happier on the real 5V from the regulator.
I agree that one or the other selection idea with jumpers is the better way to go.
As an aside, I recently connected a TTL to USB adaptor to the CMM. Later on, I had need to power down the CMM. It didn't until I realised it was still receiving power via the new adaptor. Removing it's power lead fixed that.
My 2c worth.


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Joined: 08 November 2016
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Posted: 24 April 2018 at 11:18am | IP Logged Copy link to clipboard   Quote Boppa

I always cut the ends off usb leads that cease to function, and I am amazed that they dont actually catch fire in some cases. The thinest I have seen the actual copper inside was literally like a hair, it was almost invisible to the naked eye, yet it came from a tablet that was supplied with a 2.5A 5vdc usb plugpack for charging through the usb cable
It was close to a meter long, pushing (well trying to) 2.5A to the tablet, through wires that can barely be seen...

Replacing it with a different one cut the charging time almost in half, so I suspect that that usb cable was swallowing most of the power
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Posted: 24 April 2018 at 1:39pm | IP Logged Copy link to clipboard   Quote miyek.ard

if you plug a mouse or keyboard into that USB port on your PC, does that mouse or keyboard work?
Yes, the USB port works fine.

As shown it can happen by accident. It is the reason why i always use a 3 pin header with a single jumper.
Which jumper are you referring to? I only has one kind of jumper (Vbus)

When you say "I changed the CON8 power connection to CON1", are you connecting ONLY 5V and GND to CON1?
Tx and Rx also connected. Refer image below.

how are you connecting the Micromite to the PC in this configuration?
Initially, I use CON8 as power and I fit JP1. Then, I changed to CON1 as power and CON8 for transferring data. The problem is I forgot to remove JP1 for my second connection!

I can confirm the issues with some of the USB cables.
I checked the USB cable with other MM+ and it works fine.
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Joined: 02 October 2012
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Posted: 24 April 2018 at 2:12pm | IP Logged Copy link to clipboard   Quote panky


From your answer above, I understand you are using a serial to USB module on CON1 and using it to supply power to the Micromite? Is this correct?

If this is correct, can you tell us what voltage is being supplied by the serial to USB module?

Have you confirmed that the USB port on your PC and the cable you use to connect to the Micromite are both good and work correctly with some other device?

From your descriptions above, it sounds like you may have damaged either the Rx or Tx drivers on the serial to USB module or the Console Rx or Tx on the Micromite.

As a further test, can you connect 5volts and ground to CON1 and ensure NO connection to the Tx and Rx pins, then with JP1 in, connect a known good USB cable to a known good USB port on your PC. Does this work? Does the PC recognise the USB connection to the Micromite?

Just as an observation, if you had 5v connected on CON1 and jumper J1 in and 5v coming from the USB CON8, this should not cause any major damage with both voltages being the same, so something else must be causing problems. Also, as the Console and USB inputs to the Micromite use seperate pins, it is unlikely that both would be damaged.

Let us know how you are going with your tests.


... all the Maximites, almost all of the MicromMites, all the MM Extremes, all the ArmMites, and loving it!
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Posted: 24 April 2018 at 2:35pm | IP Logged Copy link to clipboard   Quote Azure

Which jumper are you referring to? I only has one kind of jumper (Vbus)

He was referring to another way to do it, not this design.

Edit: Similar questions to Panky, we must have started typing at the same time.

So I think you are saying:
Tested PC USB port, it still works OK (I'm still not convinced that it is OK);
Tested USB cable, it works with other MM's;
MM+ that had JP1 left in does not work properly.

For the MM+ that does not work properly anymore:
It used to work before JP1 was accidentally left in and USB and ext power applied;
It does not work with JP1 connected and just USB power/Tx/Rx connected (no ext power supply).

Possibly the USB power side is damaged and it cannot provide enough power (compared to the other devide you plugged in to test it). Try removing the LCD and connecting the backpack to the PC with the LCD removed, it is still able to communicate with the PC and draw much less power. Also try using a different port like I suggested earlier.

Do you have a multimeter or even better an oscilloscope?

If you do check the input and output to/from the 3.3V regulator. You can check if there is any noise/ripple as well if you have an oscilloscope or just if the voltage is stable if you only have a multimeter.

One last question, what are you connected between the PC and MM+ as your USB/Serial converter, is it working OK on your other MM's.

Let us know your findings so we can try and help you find what has gone wrong.

Edited by Azure on 24 April 2018 at 3:24pm
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