PCB Design troubleshooting


Yes, the SN65HVD75 works well.

There are still basic layout issues, but those probably won’t effect the functionality of the board. I just wouldn’t mass produce this until those were cleaned up. As a one-off he should be able to MOD the board to get it to work.


I’m powering the boron over the USB input. In other breadboard tests, this allows for 5v out via the USB pin (thus powering the rs485 chip). Is there another reason to use the 3.3v?

I re-ran the autorouter and the airwire is back. What’s odd about that is, it’s a connection between two points that are not connected on the schematic: the RX end of R8 and the RTS end of R7. I’m assuming that’s some sort of artifact?


fixed that by connecting R1, R7 and R8 to 5v independently, rather than sharing a connection (in the schematic).

Still interested if I need to switch the 5V for the 3.3V.

I’d also be interested in knowing if I there’s someone/where I could ship the latest eagle files to see if it all makes sense.


The Boron’s I/O is not 5 volt tolerant. This means that if you apply 5V to an IO pin it has a high probability that damage will occur. (i.e a dead Boron)

So yea, you need to make the change @AndyW suggested.

Board I/O characteristics:

I/O Tutorial:

For a board like this you should never ever use an Auto-router. Lay the traces out by hand. You will learn a lot more from doing PCB design this way. (You should never use one IMO :slight_smile: )

Just post your files here. That’s what the open community is for.


So, I’m not supplying 5V to the pins, I’m powering the Boron at 5V using the USB cable, and then pulling 5V off of the USB pin to power the IC. that seems to work well with my initial breadboard tests, which just involved hooking up to a prefabbed TTLMAX board. ttl-rs485-serial-converter-board-max485-1285-53-B

I’ll upload my board and schematics ass soon as I can figure out how.


My eagle .brd and .sch can be found here:


Because you are powering the RS485 chip with 5V, the RX pin will be a 5V signal going to your Boron. This is where the violation is :wink:


Notes from your schematic:

  1. Change to a +3V3 device (Boron is 3v3 only)
  2. Add termination resistor on the output of IC
  3. General clean-up to make the schematic more readable
  4. General Note: Look at datasheet of the SN65HVD75 for layout guidelines.

I’ve attached a pic of how might draw this little device :slight_smile:


Rad! I went ahead and got a bunch of through hole parts to try it on a breadboard.


Cool. Let us know how it works.


When I breadboard this is seems like there isn’t enough power (yellow LED comes on but neither of the other two do). When I disconnect the resistors the other LEDs come on. Any idea what could be causing that?


fixed the power issue, but no signal yet! One question I have - does connecting Tx and Rx to the 3.3V net short the data circuit? Removing those taps doesn’t fix the circuit, it doesn’t communicate in either configuration, but It wanted to check.


You don’t want to connect the Tx or Rx pins from your Boron directly to 3v3. Did you get a SN65HVD75? If you are trying to run your SN65HVD3088 part at 3v3 it probably won’t work. It has a 4.5Vmin spec.

Do you know if your code is actually transmitting data? Or whatever device you are connected to is sending data back to the Boron?

Do you have access to an oscilloscope to see if you are actually Tx’ing or Rx’ing any data at the Boron?


The original .sch did not have 3 leds. If you share you current .sch / breadboard layout (photo?) - it would make it easier to advise ?


An oscilliscope is on the wish list!

I did get the SN65HVD75 (the pins are much smaller than the 3088, makes soldering a bear), so I think that should be fine.

the Tx Rx pins aren’t directly to the 3v3, they’re via the 10k resistors (my understanding of the schematic you drew). Is that incorrect?


New SCH and BRD pics are attached. I’d send a photo of the breadboard I put together, but it’s pretty much a jumble of connector wires



Any thoughts on why this might not work? I don’t have to put any firmware on the SN65HVD75 chip or anything, do I?


@Ablock130, did you try using it and it is not working or are you asking for a review of the design? As I see it, the only firmware aspect is the control of the data flow direction on the SN65HVD75 via D7 of the Boron. The SN65HVD75 is a HALF-DUPLEX transceiver meaning it only transmit or receive, not both at the same time.


I have a breadboard all put together based on the schematics above and I can’t get it to chooch for whatever reason. I am controlling data flow via D7 (I think actually based on your libraries), so I don’t think that is the issue- but I guess it could be.


Maybe you should post your code if you can. The experts here could see if your Boron is setup correctly and determine if Tx data would actually come out of your Boron on the desired pin.