Help with header connect between custom board and Nimbelink Skywire modem (using XBee footprint)

[rimersara]

Hey everyone,

I am using a custom board based on some previous work of some colleagues, and am having difficulty modifying the cellular communication setup for my specific modem. I am looking for some guidance on developing a breakout/shield for interfacing between my board and my modem, in which my board has a 6-pin serial communication setup and the modem has the XBee footprint. Ideally, I would like to make this as easy and/quick as possible (e.g. solutions where I do not need to actually print a new board, and can build off of already-available shields with only adjustments being some soldering). I thank you all in advance for your assistance, and can of course provide any other clarifying details!

Here is the rest of the message and further details…

Originally, the board was built with a 6-pin serial output that was then routed through a custom XBee breakout to the modem. I am still using that same setup, but my issue is that I believe the XBee breakout is not working for my specific modem, and I was hoping for some feedback from people here for recommendations on the easiest and/or quickest way moving forward with it – specifically (and hopefully) using some of the Sparkfun XBee shields.

So, the quick details are as follows:

- My modem: [[Nimbelink 4G LTE CAT 4 Skywire Embedded Modem with a Sierra Wireless HL7588 module ](https://nimbelink.com/products/4g-lte-cat-4/)

- [[A PDF of specs for hardware setup for this modem (and other Skywire modems) ](https://nimbelink.com/Documentation/Skywire/1001594_Skywire_Hardware_Developers_Guide.pdf)

- Original modem the board was designed to be used with: [[Nimbelink 4G LTE CAT 4 Skywire Embedded Modem with a Sierra Wireless HL7588 module ](https://nimbelink.com/products/3g-global-hspa/)

[/list]

I’ve attached to this message a few schematics of the breakout boards that I have been referring to for my design, as well as a rough design of my own breakout board. Unfortunately, I am not an expert in circuits nor Eagle, but hope that I have basic enough knowledge to have provided the details such that someone might be able to assist me with what I am still not understanding.

So, here is what I am still having trouble with:

- My board has a 6-pin breakout connector for serial communication with my Skywire modem. The 6-pin breakout basically corresponds to 6 of the pins on the modem, namely,

- VCC - Input - Supply voltage - Skywire Pin 1

- DOUT - Output - Serial out - Skywire Pin 2

- DIN - Input - Serial in - Skywire Pin 3

- GND - Skywire Pin 4 (also have tied the ground to Skywire pins 9, 10, 11, 15, 16 as instructed by Skywire guide)

- VGPIO - Output - Modem status output - Skywire Pin 13

- PWR_ON - Input - Modem on signal - Skywire Pin 20

- My board runs off of a 3.7V, 2A Li-ion battery, and is able to supply the 6-pin connector with up to 5V

- The board uses a UART serial device to decode/encode the cellular communication

- Although there is some impedance, thus I know I am losing some current in the board, I am still measuring ~5V on the output of the PWR_ON and VCC breakout pins (although, my measured voltage at PWR_ON board pin is always greater than the VCC board pin).

- Right now, I believe that the major issue is that the way my current XBee breakout board is designed, I am not getting enough power to the PWR_ON Skywire pin

Thus, I am trying to redesign an XBee breakout board to see if I can fix the issue. While ideally, I would like to build off of the [Sparkfun XBee Shield, I do not believe it will supply enough current to my modem (the modem during a cellular connection can use ~600mA or more, and the XBee Shield is designed only to handle 500mA). Therefore, I am working on designing my own. But thought I would ask here for help if anyone has any better ideas. (Or can maybe confirm that one option is to take off the voltage regulator on the Shield).

Attached to this message are a few more references, namely:

- The current XBee Breakout board that is not working for me - 

- My initial new design for the breakout board, that is based on Skywire references for how it should be designed - 

Additionally, three other Skywire schematic designs for shields between their modems and other boards–but that have more features than are needed than mine–are uploaded and can be accessed at the following links (I had difficulty attaching them to this message):

- A Raspberry Pi shield, [[example here](https://drive.google.com/file/d/1ggAJORl6bBWugzVwHI5bwAOCVXo4J19x/view?usp=sharing).

- The Skywire development kit with Arduino headers [[example here](https://drive.google.com/file/d/1AOBJj9C2mQKkXdKTE4OFFgTcKSHilkup/view?usp=sharing).

- A BeagleBoard shield [[example here](https://drive.google.com/file/d/1LSKAcB56t3RKf1ZQy6CG3n001MfW5cPn/view?usp=sharing).

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And just as a final note, via Skywire’s instructions, these are just the design criteria I am needing to follow with the shield for my modem:

- A 0.1 μF capacitor should be placed nearest to the VCC pin, followed by a 100 μF capacitor.

- Pin 20 is implemented and must be driven with an open collector output from the host system or with a discrete open collector transistor. Voltage is < 1.8V when implemented.

Thank you again.

Sincerely,

SR](https://drive.google.com/file/d/1LSKAcB56t3RKf1ZQy6CG3n001MfW5cPn/view?usp=sharing)](https://drive.google.com/file/d/1AOBJj9C2mQKkXdKTE4OFFgTcKSHilkup/view?usp=sharing)](https://drive.google.com/file/d/1ggAJORl6bBWugzVwHI5bwAOCVXo4J19x/view?usp=sharing)](https://www.sparkfun.com/products/12847)](https://nimbelink.com/products/3g-global-hspa/)](https://nimbelink.com/Documentation/Skywire/1001594_Skywire_Hardware_Developers_Guide.pdf)](https://nimbelink.com/products/4g-lte-cat-4/)