What is Physical Addressing vs Functional Addressing in CAN? How do these two addressing schemes differ from each other? Let’s Discuss.
Table of Contents
What are Addressing schemes in a CAN network?
It refers to the methods used to identify and communicate with different ECUs on a CAN network. There are primarily two addressing schemes in CAN bus protocol, i.e., Physical Addressing vs. functional addressing.
Physical Addressing in CAN Bus
It is an addressing scheme that labels CAN bus messages based on their physical address location. In physical addressing, each node on the CAN bus is assigned a unique identifier known as a node address or physical address. When a node (ECU) wants to communicate with another node, it includes the physical address of the destination node in the CAN message header. The receiving node checks the address and determines if the message is intended for it. Physical addressing is straightforward and efficient but requires pre-configuration of addresses for each node.
Example: $7E0 is the Physical Address (CAN ID) of the Engine Control Module (ECM). The response from ECM will be with CAN-ID $7E8. And $7E1 is the Physical address of the Transmission Control Module (TCM). The response from TCM will be $7E9.
If the Tester sends a Diagnostic request with CAN ID $7E1, then since it is a Physical address, only the concerned ECU will respond, i.e., TCM with CAN ID $7E9. There is no response from ECM.
Functional Addressing in CAN Bus
It allows messages to be categorized based on their content or functionality. It will enable addressing a group of ECUs that have something in common, like support of a comprehensive functionality (for example, OBD). Functional addressing is also known as message-based addressing or broadcast addressing. It doesn’t rely on specific node addresses.
When a node wants to send a message, it assigns a particular message ID to it, and all nodes on the bus receive the message. Each node then decides whether the message is relevant to its function based on the message ID. Functional addressing simplifies network management as nodes don’t need pre-assigned addresses, but it may lead to more network traffic since all nodes receive every message.
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The request is made using a Functional Address ($7DF). Both ECM and TCM will now respond with CAN IDs $7E8 and $7E9, respectively.
Note: There is no Functional Response CAN ID.
Physical Addressing vs Functional Addressing in CAN Bus
| Physical Addressing | Functional Addressing |
|---|---|
| Pros: Direct and efficient communication between specific nodes | Cons: Increased network traffic due to broadcasting messages to all nodes. |
| Pros: Clear identification of message recipients, minimizing network traffic. | Cons: Requires robust filtering mechanisms to ensure nodes only process relevant messages. |
| Cons: Requires manual configuration of addresses for each node, which can be cumbersome in large networks. | Pros: Simplifies network configuration as nodes do not require pre-assigned addresses. |
| Cons: Limited scalability as the number of nodes increases. | Pros: Enhances flexibility and scalability, especially in dynamic networks where node addresses may change. |
Conclusion
In summary, physical addressing offers direct communication between specific nodes with minimal overhead but requires manual address configuration and may limit scalability. On the other hand, functional addressing simplifies network configuration and enhances flexibility but increases network traffic and necessitates efficient message filtering mechanisms. The choice between the two addressing schemes depends on network size, complexity, and the desired level of flexibility.
