Satellite Transceiver Breakout - Swarm M138 - Hookup Guide

This Tutorial is Retired!

This tutorial has been retired; the satellite service for this product will be terminated at the end of December 2024.

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Contributors: PaulZC
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Breakout Pins

The table below describes the function of each of the Satellite Transceiver Breakout - Swarm M138 breakout pins:

Pictured are the breakout pins

Pin NameFunctionDescriptionNotes
CH340 RXIInputCH340 USB Interface: Receive Data In.Logic level is 3.3V.
TXOOutputSwarm Serial (UART) Interface: Transmit Data Out.Logic level is 3.3V. By default this pin is linked to CH340 RXI. Open the jumper to isolate.
CH340 TXOOutputCH340 USB Interface: Transmit Data Out.Logic level is 3.3V.
RXIInputSwarm Serial (UART) Interface: Receive Data In.Logic level is 3.3V. By default this pin is linked to CH340 TXO. Open the jumper to isolate.
TX/RXOutputSwarm TX / RX pin.Logic level is 3.3V. High during TX, low during RX.
GPIO1I/O (Configurable)Swarm GPIO1 pin.Logic level is 3.3V. Can be configured in many different ways. See below for more details.
VINPowerPower input.3.0V Min. 5.0V Max.
GNDPowerPower ground / 0V.

Power can be provided via the USB connector or the VIN pin, or both. The modem will draw power from whichever voltage is higher. The on-board "ideal diode" power mux circuit allows both to be connected simultaneously.

The peak current draw during transmit depends on the supply voltage. Please see Current Draw below for more details.

By default:

  • Modem TXO is connected to CH340 RXI
  • Modem RXI is connected to CH340 TXO

You will need to open the jumper links on the back of the board to use the TXO and RXI pins directly.

Pictured are the T X O and R X I jumpers

You may find it useful to read this tutorial first:

How to Work with Jumper Pads and PCB Traces

Handling PCB jumper pads and traces is an essential skill. Learn how to cut a PCB trace, add a solder jumper between pads to reroute connections, and repair a trace with the green wire method if a trace is damaged.

GPIO1

GPIO1 is a multi-function input/output pin. It can be configured into different modes via the GP command:

ModeDescription
0Analog, pin is internally disconnected and not used (default)
1Analog ADC, pin can be read to measure input voltage (0-3.3V)
2Input, pin can be read as a general purpose digital input (High or Low)
3Input, low-to-high transition exits sleep mode
4Input, high-to-low transition exits sleep mode
5Output (Open Drain), set low as a general purpose digital output
6Output (Open Drain), set high as a general purpose digital output
7Output (Open Drain), low indicates unread messages pending for user
8Output (Open Drain), high indicates unread messages pending for user
9Output (Open Drain), low indicates unsent messages pending for transmit
10Output (Open Drain), high indicates unsent messages pending for transmit
11Output (Open Drain), low indicates unread or unsent messages
12Output (Open Drain), high indicates unread or unsent messages
13Output (Open Drain), low indicates sleep mode is active. Otherwise output is high
14Output (Open Drain), high indicates sleep mode is active. Otherwise output is low

We've included both pull-up and pull-down resistors for GPIO1, configurable via a dual split pad jumper. By default, GPIO1 is pulled up to 3.3V so that the open drain output modes generate the correct logic level output. You can remove the pull-up by changing the jumper:

Pictured is the schematic for the G P I O 1 pin jumper

GPIO1 can sink a maximum of 8mA.

TX/RX

TX/RX is a push-pull output which is: high (3.3V) when the modem is transmitting; low (0V) when the modem is receiving or idle.