I tried on a different Windows 11 laptop and on a Windows 10 I managed to find. Fresh install of the updated FTDI driver, also of Arduino 2.3.7, same symptoms. I’m fully stumped.
A few of the solder joints look a bit iffy; can you test the for shorts/continuity between pins?
The continuity and short tests are all over the place. Some pairs even give different results depending on whether the red or black tester is used. In the same breath, both kits I soldered together give the exact same results in all of the pairs testing. It seems weird that if the variance denotes errors, that both kits error out in precisely the same ways.
Edit: in yet another puzzle, I hooked up to the current board in use on our exhibit floor, and the upload was successful. Which means all the settings are correct in Arduino, and it’s strictly the board that’s the issue somehow. I’m going to take the connector off that current board and swap it for the others that aren’t working. Maybe that’s the lynch pin.
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Regarding my breakthrough last night, I cannot determine what I did with constructing these kits that makes them inaccessible to Arduino uploads. I tried taking the pins out of the current board and putting them into one of the boards–no change, and in fact the board’s lcd screen won’t light up like it did before. The second kit’s screen lights up, but the transfer error is still the same.
I can’t think of what else it’d be at this point besides the pins that where the FTDI basic attaches.
I finally desoldered the LCD board in order to look at the soldering points for all the pieces. Allegedly the connector is causing the issue, yet everything looks solid.
At this point, I’m tempted to say these are both duds 
I just have no idea.
Most of them look good, I’d just add some flux and then add more solder to the pins that don’t have a ‘hershey’s kiss’ style mound (80-90% look fine) and re-test
Also: is the FTDI in 5v or 3.3v mode?
I only found out today I could switch modes on it when you mentioned it. I’ve not altered it since receiving it. It worked with the current LCD serial kit installed on our floor, so I would think it’s in the proper mode. Unless these new kits require 3.3v? Is there a way to determine that?
You can share a photo of its front and I can tell from that
I wonder if possibility a batch of 328s got put in these kits that don’t have a bootloader installed on them. That would explain why they work but can’t be reprogrammed.
The front doesn’t have any indicator of what mode it’s in, as far as the product page describes. The back would be where I’d install a jumper to switch modes:
Edit: to the earlier comment about resetting the board, the product page states “The DTR pin allows an Arduino target to auto-reset when a new Sketch is downloaded. This is a really nice feature to have and allows a sketch to be downloaded without having to hit the reset button. This board will auto reset any Arduino board that has the reset pin brought out to a 6-pin connector.”
I don’t understand the difference between working and being reprogrammed, in how you use them there. If they can’t be reprogrammed, doesn’t that mean they don’t work?
I had mis-remembered, the pads are on the back like you said…but anyhow, you have a 5v version and it should be outputting 5v (you can always test w/ a DMM to make sure it is in fact delivering the full 5v…let me know if it isn’t!)
Yea, if they don’t have a bootloader we’d need to send replacement units because they won’t interface as expected
Can you try a simple loop-back test with the FTDI by itself?
Apologies for all the run-around, but this is an odd duck!
I tried a loop back test with the FTDI by itself and it worked fine. Again, this same FTDI functioned properly when I tried uploading the code to the current board I’m trying to replace. It was just the newly soldered ones I’m getting this error on.
We happened to have yet another board lying around. Soldered that one up, was able to upload the code the first try without issue *shrugs* All I can say is that these two were duds somehow.
Working = they function as serial LCD’s and display the text you send them.
Reprogrammable = you are able to reload them with your own custom code.
Sparkfun flashes the program that makes these function as a serial enable LCD when they make the kit. There’s an important piece of code that goes along with the program that makes them function called a ‘bootloader.’
The bootloader’s job is to listen for a reset signal from the Arduino software when you request to load your new code onto the chip. When it sees a reset, the bootloader looks for a specific set of instructions getting the chip ready to receive a new program. Once it sees those instructions, it begins downloading your new code into the chip and then starts to execute that code.
It’s possible to flash the program to the chip but forget to flash the bootloader. If that happens, the chip will continue running whatever code is already on the chip but lacks a bootloader that allows you to upload new code to the chip.
In your case, it sounds like this may have happened. It’s fixable with the right tools and software but I think it may be more complex than your current abilities. (It can get pretty complex, especially if you’re a beginner.)
I’d hang on to the boards that won’t allow you to upload code, they should still function as serial LCDs just fine and be completely usable as LCDs. Later on when you have more skills and experience you might tackle adding a bootloader to them if you feel adventurous. 
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