CODEWITHSUNDEEP
can-bus
ayman
ayman

Reputation: 49

CAN bus sending data from two masters with equal balance

I have two master nodes connected to the same CAN bus, both send data to my PC.

first master ID = 0xFFA1 second master ID = 0xFFA2

Since the first master ID is lower than the second it takes control of the bus more than the second master. And this causes some delay in the data.

Is there a way to make load balancing between two nodes so that each node send an almost equal amount of messages.

I tried making the first node send data while switching between two IDs 0xFFA1 and 0xFFB2, and the second node sends data with ID 0xFFB1. And it didn't help.

Upvotes: 0

Views: 993

Answers (1)

Lundin
Lundin

Reputation: 215305

There is no such thing as "masters" in CAN, nor in higher layer protocols like CANopen for that matter (a "master" in CANopen is just a supervisor node). Who gets to send what is defined by the CAN identifiers - CAN is primarily focusing on data, not nodes. What matters is what is sent, rather than who is sending/receiving, since every message is broadcasted.

It sounds as if you have 2 nodes that wildly spam the bus with identifier 0xFFA1 and 0xFFA2 messages, as fast as they are able, leading to 100% bus load. Then the node sending 0xFFA2 will "starve". Sending data "as fast as you are able" is never the correct way to use CAN.

Instead you need to define a higher layer protocol that dictates real-time characteristics. In control systems, this is most commonly done by having nodes send data at fixed intervals, such as once per 10ms or 100ms. This alone should fix your starvation problem.

If you want to prevent nodes from sending at the same time, then you could provide a means for them to synchronize. A trick used in CANopen and other protocols, is to have one node send out a "sync" message at given fixed time intervals. After reading this sync message, all nodes should act within x ms from receiving it.

Upvotes: 2

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