CAN Instruction Return Value

CAN Instruction Return Value

I've picked up project with the following line of code:

IF (CAN(-1,8,tsdo,rsdo,$2010,2,520) = -1) THEN

Occasionally, for some Axes, the instruction is NOT evaluated as -1. Trying to find out what is returned exactly. If this is some kind of read status, it may be helpful to know the details of the return value so I can better diagnose the issue. 

Note, this is a Read CANOPEN Object. No return value is listed in the help or manual for this type of operation. 

Thank you,

Jet