Alternate Encoder Mode

The SPARK MAX can be configured to run in Alternate Encoder Mode, which reconfigures the Data Port on the top of the controller to accept an alternative quadrature encoder, separate from the default encoder inputs shared between the front Encoder Port and the default quadrature encoder Data Port pins. Analog input is not affected by Alternate Encoder Mode.

This feature is designed for use in low-RPM mechanisms such as drivetrains, arms, and other manipulators. For high RPM applications it is recommended to use the built-in motor sensor for brushless motors or the default encoder inputs for brushed motors.

Alternate Encoder Specifications

Parameter

Specification

Encoder Output Voltage Level

3.3V or 5.0V

Encoder Type Supported

Quadrature†

Maximum Counts per Second

165000

Index pulses are not currently supported

Before connecting a sensor with 5V output, the SPARK MAX must first be configured into Alternate Encoder Mode and its configuration must be saved on the SPARK MAX. This can be done through the REV Hardware Client or the software APIs.

Maximum RPM with Common Quadrature Encoders

Encoder

Counts per Revolution

Max RPM

REV Through Bore Encoder

8192

1200

CTRE SRX Mag Encoder

4096

2400

Greyhill 63R256

1024

9600

When configured for Alternate Encoder Mode, a quadrature encoder connected to the reconfigured Data Port pins can be used as a feedback device by the SPARK MAX. Please note, the limit switch inputs cannot be used at the same time as an alternate encoder. The limit switch pins are repurposed for the alternate encoder and are thus disabled. Please see Connecting an Alternate Encoder for for more information.

Connecting an Alternate Encoder

Connecting an alternate encoder will likely require a custom wiring harness to connect the necessary encoder power, ground, and signals to the reconfigured Data Port. When configured in Alternate Encoder Mode, the Data Port has the following pinout:

Data Port Pinout in Alternate Encoder Mode

Connector Pin

Pin Type

Pin Function

1

Power

+3.3V

2

Power

+5V

3

Analog

Analog Input

4

Digital

Alternate Encoder Index

5

Digital

Encoder B

6

Digital

Alternate Encoder A

7

Digital

Encoder A

8

Digital

Alternate Encoder B

9

Digital

Encoder C / Index

10

Ground

Ground

The Alternate Encoder Index pin is reserved but not currently supported

Use an Alternate Encoder Adapter (REV-11-1881) to connect a REV Through Bore Encoder directly to the SPARK MAX Data Port. This adapter has a JST PH 6-pin connector that is compatible with the Through Bore Encoder's pinout and a selection switch to change the signal that is connect to pin 4 of the data port.

Another option is the SPARK MAX Data Port Breakout Board. This board can be used to wire an alternate encoder to the Data Port. The following table describes which pads on the breakout should be used for which signals coming from the alternate encoder.

Alternate Encoder Pin-mapping for SPARK MAX Data Port Breakout Board

Breakout Board Pad Label

Alternate Encoder Function

Limit - F

Index†

P6 (P5 in older batches)

A

Limit - R

B

3.3V or 5.0V

Encoder Power

GND

Encoder Ground

The Alternate Encoder Index pin is reserved but not currently supported

Configuring and Using the Alternate Encoder Mode

Below you will find the steps required to set up and use the Alternate Encoder Mode on the SPARK MAX, starting with configuration through either the SPARK MAX Client or the SPARK MAX APIs.

Configuration Using the SPARK MAX Client

Using the SPARK MAX Client, navigate to the Advanced Tab and scroll to the Alternate Encoder parameter section. Enable the alternate encoder by setting the kDataPortConfig parameter to '1'. You can also set the other Alternate Encoder parameters at this time.

Configuring Using the SPARK MAX APIs

If using the SPARK MAX APIs, the Alternate Encoder is automatically configured when the Alternate Encoder object is instantiated. An Alternate Encoder is created the same as a CANEncoder, either by directly using the constructor or calling GetAlternateEncoder() on a previously constructed CANSparkMax.

static constexpr int kCanId = 1;
static constexpr auto kMotorType = rev::CANSparkMax::MotorType::kBrushless;
static constexpr auto kAltEncType = rev::CANEncoder::AlternateEncoderType::kQuadrature;
static constexpr int kCPR = 8192;
// initialize SPARK MAX with CAN ID
rev::CANSparkMax m_motor{kCanID, kMotorType};
/**
* An alternate encoder object is constructed using the GetAlternateEncoder()
* method on an existing CANSparkMax object. If using a REV Through Bore
* Encoder, the type should be set to quadrature and the counts per
* revolution set to 8192
*/
rev::CANEncoder m_alternateEncoder = m_motor.GetAlternateEncoder(kAltEncType, kCPR);

Currently, quadrature is the only available type of configuration for an alternate encoder. This is differentiated from the other types of encoder configurations available for an encoder connected through the front facing Encoder Port on the SPARK MAX.

Configuration Conflicts

Since the alternate encoder inputs and the default digital inputs are shared on the Data Port, the user cannot use both the alternate encoder and a digital inputs in code. Therefore, a std::invalid_argument (C++), IllegalArgumentException (Java), or an Error on the Error Out terminal (LabVIEW) will be thrown if a user tries to construct both types objects in code simultaneously.

Closed-Loop Control

The alternate encoder can be used with the different closed-loop control modes available on the SPARK MAX. The feedback device used by a CANPIDController must be set to use the alternate encoder through SetFeedbackDevice().

/**
* By default, the PID controller will use the Hall sensor from a NEO or NEO 550 for
* its feedback device. Instead, we can set the feedback device to the alternate
* encoder object
*/
m_pidController.SetFeedbackDevice(m_alternateEncoder);

Initial Bring-up

Unlike the built-in sensor on the NEO Brushless motors, the 'phase' of the alternate encoder is unknown to the SPARK MAX. Before enabling any closed-loop control, it is critical that the phase is configured correctly. To verify:

  1. Configure and connect the sensor as a quadrature alternate encoder, but do not run a closed-loop mode.

  2. Plot the output signal of the motor using GetAppliedOutput() and the output of the encoder using altEncoder.GetVelocity(). Confirm that the sensor is behaving as expected. This can be done on the SmartDashboard: frc::SmartDashboard::PutNumber("Alt Encoder Velocity", m_alternateEncoder.GetVelocity()); frc::SmartDashboard::PutNumber("Applied Output", m_motor.GetAppliedOutput());

  3. Verify that the sign of the sensor is correct relative to the motor direction when driving it forward and backward. If it is not, the sensor must be inverted by calling altEncoder.SetInverted(true).