The MB_COMM_LOAD block only needs to execute one scan cycle. The project attached only executes the block on the first scan cycle of the PLC. You can use the Positive edge trigger bit.

Note also that the ERROR output only stays on for one scan for the MB_SLAVE so you have to move it to another memory location to view the error.

I am attaching an example project for a slave. It has error capturing logic also.

Hope it helps!