ICP DAS USA I-7531-FD User Manual page 8

Note 1: In normal situation, the CAN_GND does not need to be wired, but
the CAN_GND's voltage level of different CAN devices are not
equal in some case. When in this situation, it maybe cause some
problems and reduce the stability of the CAN bus system. The
user can connect CAN_GND of these CAN devices to let the
voltage level be same and eliminate this situation.
Note 2: Electronic circuits are susceptible to electrostatic discharge
(ESD) and will get worse when encountering continental
climates, so F.G. provides a path for ESD to lead to the earth
ground, thereby enhancing the module's ability to protect against
static electricity. If the user wants to use F.G., both pin 7 and pin
14 need to be connected to the earth ground, because the two
pins are not connected inside the module.
I-7531-FD CAN Repeater User Manual (ver. 1.0, 2021/10/21) ------8