Introduction

The purpose of this document is to outline the CDH-EPS communication protocol over CAN. It will go through the commands, telemetry, responses, and internal diagnostics that will be sent between the OBC and EPS, as well as the CAN frame structure for each interaction.

All the frames discussed below are data frames.

Requirements

• The protocol must support communication between 2 devices, the onboard computer and power systems microcontroller
• The onboard computer should have control of the communication
• The EPS microcontroller should be able to stream telemetry data over the interface

Protocol Stack

• The physical and data link layers are defined by high-speed CAN2.0A
  • 2.0A was selected over 2.0B since the extended identifiers weren’t required. This reduces the overhead of each message.
• The transport layer of CAN2.0A is also used, but only for retransmission control
• Extension of data link layer
  • Defines the format of the 8-byte data frame payload
  • Defines various message types and associated identifiers
• Device roles:
  • Controller: Device capable of sending commands
  • Peripheral: Device capable of responding to commands. It’s also capable of broadcasting telemetry data

Communication Sequence

Command-Response

Most transactions over the interface follow a command-response format. The controller sends a command to the peripheral and waits for the associated response. If a response isn’t received within $t_{cmd-response}$, the controller may discard the transaction or try sending the command again.

Note: It’s possible for the peripheral to send telemetry data before the command response since telemetry streaming is done asynchronously. However, telemetry has a different identifier, so it’s simple to differentiate between telemetry and command responses.

Telemetry Streaming

The peripheral device is capable of streaming telemetry data outside the typical command-response format. To prevent conflicts, telemetry data uses lower-priority identifiers compared to commands and responses. The built-in arbitration of the CAN bus will take care of the priorities.

The telemetry data can also be used to transmit errors to the onboard computer.