Connecting LiFePO4 batteries with canbus to 2 x Islands:

Source:
https://diysolarforum.com/threads/sunny-island-and-eg4-powerpro-help.72311/

The Powerpro, SI master, and slave are connected via the ComSync (CANbus) as follows:

Powerpro BMS CAN port -->Master ComSyncIn
Master ComSyncOut-->Slave ComSyncIn
Slave ComSynOut has RJ45 terminator plug

The CANbus on the SI side is terminated at the Slave ComSyncOut. There is no terminator on the Powerpro side since there is not an extra CANbus port on the Poweredge.

There are separate RS485 ports on the Powerpro for connecting additional batteries. But this does not appear to be the CANbus.

Looking at the SMA CAN specs...

The RS485 and CAN both communicate via the ComSync bus. Even though the SI is only using the CANbus for the BMS communication, it might also need the RS485 bus terminated at the battery side (see below). Will give that a try if nothing else works.

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So now this is starting to make some sense. Based on bus specs of the battery and the Sunny Island (see my previous post), the ComSync port on the Sunny Island contains two separate buses: CAN (pins 4/5) and Sync (pins 3/6). The CAN bus is used for BMS communication. The Sync bus isn't used for CAN, but is used for master-slave communication.

Normally the Sunny Island master and slave each have a terminator plugged into the shared RJ45 port for the two buses (see picture below). But in order to connect the battery communication cable into the Sunny Island master, the terminator has to be removed. The termination that used to occur in the master now has to occur in the battery.

On the battery side only the CAN bus is terminated (internally). The Sync bus is not terminated at the battery end. (Pins 3/6 on the battery bus are used for the emergency stop function.)

The Sunny Island slave still expects the Sync bus to be terminated "upstream". The actual error on the Sunny Island is "Slave does not receive a Syncpuls". It goes into Standby mode when it is not terminated.

So I'll need to terminate the Sync bus on the battery side/upstream from the master. Which should be as easy as wiring a 120 ohm resistor between the wires for pins 3/6 in the Cat5 cable coming from the Sunny Island to the BMS???

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... and that solved the problem!!!

The terminator for the CAN bus is built into the battery. But the terminator for the Sync bus needs to be added between pins 3 and 6 of the Cat 5 cable (see below).

Now I just need to get out the solder and heat shrink