Generally speaking, they will share a common name like G1 G2 G3 G4 which will stick out compared to the naming convention of the other devices soldered to the PCB around it. As mentioned earlier however, not all devices will have them clearly marked, and the contacts may not be near each other. You would be surprised how many devices still leave pads or holes like the row below. I hook up all to see if I can work with the device so that's what we are going to do. Some devices won't even let you interface but TX (Transmit) will let you see what the device is dumping to the bus. You also don't need to hook up both TX and RX. To try and figure out what the pins are protocol wise and if they are live. You can probe at this point with something like a Logic Analyzer or even just a multimeter if you want. I use the power of the device itself to provide my voltage. When I can though I only plug in the TX RX and GND. In most cases VCC will be provided by your tty to USB and will measure at 3.3v. In this case the pins attached TX RX GND and VCC. Looks like the holes on this are SPI but we want UART. But the device also has holes if you want practice with that. We can see they were super nice and gave us jumpers to play with. Now because the final product seldom has the jumpers we are really just looking to see if they give us the options for UART the jumper may not exist anymore, but if they used UART for testing (since UART generally has debug and run info piped too it) we can use the holes are pads to connect too since even if the header is removed functionality (the output) is seldom disabled. Now while it's also important to know that UART pins aren't necessarily next to each other for a good % of devices they are. In this case we are looking for signs of UART (3 to 4 pins) next to each other. It's important to first remember, that FCC applications are generally NOT done with final products. If you managed to snag a picture of the box or simple google for a products FCC ID and it transmits a signal we can use the below to find it. Thankfully the first part of any kind of hardware probing is why waste money if we can look first? Good question. We are going to need a few tools to help us.
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