last updated by Alex Olivier (original author) on 4/24/2012


PNI Corp's FieldForce TCM IMU is a high-end compass module designed to provide "reliable, pinpoint accurate pitch, roll, and compass heading". This module allows accurate and robust navigation in areas where GPS is unavailable (such as inside buildings, underwater, or underground). The PNI website advertises below 1 degree accuracy in field conditions.

PNI provides a very detailed manual for using the TCM module, which can be found on the sensor's webpage (after signing up with PNI). The sensor report summarizes much of the manual in addition to providing my perspective on using the sensor.


At a steep price of $996.00 (even more if you get the cable or the complete evaluation board), the FieldForce TCM is clearly not designed for the hobbyist's budget. However, that doesn't mean you can't obtain a FieldForce TCM for a student or hobbyist project. After contacting PNI Corp by emailing, I learned that they have a formal educational outreach program. If you agree to write a sensor report (much like this one) and give them some details about your completed project, they will happily send you one of their sensors!

One recommendation, be sure to fully investigate your options before requesting a sensor. You may find (as I did) that you will need to request helpful accessories, development boards, and cables besides your main sensor. PNI also offers two types of FieldForce TCM module - the XB and the MB. The XB module uses RS232 Serial logic, while the MB module uses TTY Serial logic. I recommend the MB, as you will be able to communicate with it using an FTDI chip. With the XB module, you will most likely have to use a Serial converter chip (more on this later).

Electrical Characteristics

This sensor features an on-board processor as well as accelerometers and compass modules. All parts are produced by PNI.

Supply Voltage3.6 to 5v DC
Average Current Draw (8 Hz sample rate)16 mA typical
Average Current Draw (max sample rate)20 mA typical
Peak Current Draw120 mA peak, 60 mA average over 2ms
Sleep Mode Current Draw0.3 mA typical

Pin Descriptions

Wiring Diagram

With the FieldForce TCM, there are only four important connections to make: 5v, GND, RX, and TX. On the XB module you will need to connect three pins to GND instead of just one.

Pin NumberTCM XB 9-Pin ConnectorTCM XB Evaluation Cable Pigtail ColorTCM MB 4-Pin Connector
2GNDGray+5 VDC
4NCOrangeUART Rx
7UART TxYellow
8UART RxBlue
9+5 VDCRed

Pin 1 is shown in the following technical drawing.

Additional Parts Needed to Use It

If you are using the FieldForce TCM-XB module, you'll most likely want a MAX232 serial converter chip, capacitors, and an FTDI cable.

If you are using the FieldForce TCM-MB module, you will need an FTDI cable.

Physically Connecting to the Sensor

Because the FieldForce TCM has an on-board microcontroller, we don't necessarily have to connect our sensor to an Arduino. We can connect our sensor directly to our computer or to any microcontroller. In fact, for testing purposes, I recommend connecting the sensor to a computer before involving any additional microcontrollers. To connect your computer to a FieldForce TCM sensor (either the XB or the MB module), I would very strongly recommend using PNI's Evaluation Cable (can be found on FieldForce webpage). This cable will make it much easier to make the connection to the delicate Molex connector on the TCM's board. The cable seems to connect your sensor directly to USB.

If you do not obtain an evaluation cable, you will have to manually connect to the sensor's Molex connector to access the important pins.

On the FieldForce TCM-XB model, there is a very small 9-pin Molex male connector (Part no. 53780-0970) that mates with a female Molex connector (Part no. 5116-0900). You can attach 30-32 gauge wire to ultra-small crimp connectors (Part no. 0507538041 or 0506418041), then insert the wire with crimp connectors attached into the female Molex connector. For a tutorial on using crimp connectors, see this tutorial.

Unfortunately because the crimp connectors, 30-gauge wire, and the female Molex housing itself are all so delicate, they can be infuriating to use successfully. I had difficulty with the thin wire snapping frequently. As a last result, you could solder directly to the pins, as I did. Thesesolder connections can be easily removed using solder wick.

Connecting to a Computer or a Microcontroller

The FieldForce TCM-XB module uses RS232 Serial to communicate, while the FieldForce TCM-MB module uses TTY Serial to communicate. When we think about "serial", we are usually referring to the TTY Serial. Depending on the type of communication, we will want to connect to the FieldForce module in a different way.

RS232 Serial vs. TTY Serial

A lot of times when we say that sensors communicate through "serial", we mean that they use 0v-5v TTY logic levels. We can communicate with these sensors with our computer using an FTDI cable (available at Sparkfun or AdaFruit) or via our Arduino using Serial and the Arduino's RX and TX pins. However, RS-232 Serial (which actually communicates through the serial port of a computer) uses completely different logic levels to communicate. To communicate using RS232 Serial using a computer, we must either have a serial port on our computer, or use a USB-to-Serial converter cable. To communicate using RS232 Serial using a microcontroller, we must use a serial converter chip (like the MAX232).

 RS232 SerialTTY Serial
Logical LOW-3 to -15v0v
Logical HIGH3 to 15v5v

Connecting to the FieldForce-TCM MB Module

If you have obtained the TCM MB module, you can use an FTDI cable to talk to your chip. The wiring diagram for this setup is straightforward (as seen in the diagram below).

Connecting to the FieldForce-TCM XB Module

If you have obtained the TCM XB module (like I did), you will need to use a serial-level converter chip so that your computer or Arduino can both talk and listen to the logic levels of the sensor. I used the Max232 chip, available from SparkFun, DigiKey, or your other favorite electronics distributor (AKA Eric Rosenthal).

The Max232 chip can convert between RS232 Serial and TTY logic levels in both direction. You will want to wire your module up as follows:

Number of Data Bits8
Start Bits1
Stop Bits1

Notice that several additional capacitors are needed - these capacitors allow the MAX232 chip to act as a voltage doubler, outputting more than 5v even though the chip's power source is only 5v.

Module Tx ---> RS232-to-TTY converter ---> TTY output connects to FTDI Rx FTDI Tx ---> TTY-to-RS232 converter ---> RS232 output connects to TCM XB Rx

TCM Binary Protocol

On top of either an RS232 or TTY Serial protocol, the FieldForce TCM sensors also use an additional "PNI Binary Protocol". You can use the PNI Binary Protocol to send various commands to the sensor, including requesting sensor data, starting a calibration session, or setting various configurations. When communicating with your sensor via a terminal application, the settings should be as follows:

Number of Data Bits8
Start Bits1
Stop Bits1

The structure of a PNI Binary packet is as follows: Attach:packet.png Δ

An example packet for performing a 2D calibration using the kStartCal command:

ByteCountFrame IDCalOptionCalOption (2D Calibration)Checksum
00 090A00 0000 145C F9

An example packet for the kGetModInfo command (which has no payload):

ByteCountFrame IDChecksum
00 0501EF D4
Frame IDCommandDescription
1kGetModInfoQueries the module's type and revision number
2kModInfoRespResponse to kGetModInfo
3kSetDataComponentsSets the data components to be output
4kGetDataQueries the module for data
5kDataRespResponse to kGetData
7kSetConfigSets internal configurations in the module
8kGetConfigQueries the module for the current internal configuration value
9kConfigRespResponse to kGetConfig
10kSaveCommands the module to start user calibration
11kStartCalCommands the module to start user calibration
12kStopCalCommands the module to stop user calibration
13kSetParamSets the FIR filter settings for the magnetometer & accelerometer sensors
14kGetParamQueries for the FIR filter settings for the magnetometer & accelerometer sensors
15kParamRespContains the FIR filter settings for the magnetometer & accelerometer sensors
16kPowerDownUser to completely power-down the module
17kSaveDoneResponse to kSave
18kUserCalSampCountSent from the module after taking a calibration sample point
19kSetConfigDoneResponse to kSetConfig
20kSetParamDoneResponse to kSetParam
21kStartIntervalModeCommands the module to output data at a fixed interval
22kStopIntervalModeCommands the module to stop data output at a fixed interval
23kPowerUpSent after wake up from power down mode
24kSetAcqParamsSets the sensor acquisition paramters
25kGetAcqParamsQueries for the sensor acquisition parameters
26kAcqParamsDoneResponse to kSetAcqParams
27kAcqParamsRespResponse to kGetAcqParams
28kPowerDownDoneResponse to kPowerDown
29kFactoryUserCalClears user magnetometer calibration coefficients
30kFactoryUserCallDoneResponse to kFactoryUserCall
31kTakeUserCalSampleCommands the module to take a sample during user calibration
36kFactoryInclCalClears user accelerometer calibration coefficients
37kFactoryInclCallDoneRespond to kFactoryInclCall
46kSetModeRespResponse to kSetMode
49kSyncReadQueries the module for data in Sync Mode

TCM Studio: A GUI for your Chip

From the PNI website, you can download "TCM Studio" free of charge. TCM Studio is a GUI for testing and using your sensor. It creates messages using the PNI binary protocol so you don't have to! Once you install the GUI, you can ensure that your sensor is working properly. Select the serial port your sensor is connected to, then select 38400 for the baud rate. If all is well, your sensor should connect automatically after you configure these settings. If not, something's not right. If you are using the MB module and connecting via FTDI, ensure your Rx/Tx connections are correct. If you are using the XB module and connecting via FTDI through a MAX232 chip, double check the circuit's connections.

When the sensor connects properly, a digital LED on the GUI will turn green. At this point, you can try out the GUI's features, including the handy "test" mode. The test mode lets you manipulate a digital helicopter's heading by moving your sensor in space. It's an intuitive way to see the sensor's outputted roll, pitch, and yaw data.

With the GUI, you can also calibrate your sensor, log data and graph data in real time, and configure your sensor's settings (including telling the on-board processor how your sensor is mounted). See the sensor handbook for more detailed information.


The PNI website lists several application areas for the FieldForce TCM sensor, including:


UAVs and Drones
Surveillance and security drones
Robotic spy-planes/gliders
First responders
Autonomous search and rescue
Border sentry
Tilt sensing for rollover avoidance (stabilization)
Explosive Ordnance Disposal (EOD)
Offshore rig service
Mine detection and disarmament
Climate mapping
Oceanic research
Stabilization/attitude control


Far Target Location and Designation
Laser Range Finding
Night Vision Systems

Scientific and Ocean Systems

Building Inspection
Pipeline Inspection
Oil Drilling and Downhole applications
Speed Detection Systems
Remote Vehicle Detection
Intelligent Traffic Control Systems
Vehicle Tracking
For hobbyist and artistic purposes, we can think of other applications, such as gesture recognition, head tracking, etc.

Sample Code

I will be working on an Arduino library to use most of the PNI Binary Protocol commands. Check back soon!


Keywords for this sensor include: inertial navigation, heading, compass, magnetometer, accelerometer, AHRS, IMU