FYI: this is what I use for CAN bus testing:

DB9 wired with all three -L lines together, and all three -H lines together, then a 120R termination resistor across CAN1. I also have external 12V power wired to a socket, but that is optional.

With this in place, a TX and CAN1 produces RX on CAN2 and CAN3, etc.

Regards, Mark