I also posted this in the NI forum here: http://forums.ni.com/ni/board/message?boar...thread.id=35223 Trying to communicate with a master device using the Modbus library 1.2 and LV 8.5. We've matched all the serial parameters I think (9600, 8 data, parity None, 2 stop, no flow). But when I run the Serial Slave example, we see nothing in the holding registers. The master is using function 16 (write mult. registers). Should Read All Holding Registers work then? Master is writing to register address 0x01 if it matters. We've confirmed correct data is being sent and that the serial port (PXI-8430) is working. I changed the serial settings in MAX as well, but are the advanced ones there critical? Anything else to check? Is there another VI I can use? Thanks.