Rockwell Automation 1404-M4_M5_M6_M8 Powermonitor 3000 User Manual, Firmware rev. 3.0 or LATER - Manual (Page 95)

Publication 1404-UM001F-EN-P - November 2009 95
Communication Chapter 4
For function code 16, if the data length is larger or less than the
element number of the data table accessed, error code 4 occurs. It
means the data length for function code 16 should be strictly the same
as the size of the accessed data table.
If the data written to the power monitor by using function code 16 is
outside of the legal range as shown in Appendix A, error code 5
occurs.
For function code 03, 04, and 16, if any undefined starting address is
sent to the power monitor, exception code 2 is returned and error
code 6 occurs. If the starting addresses other than the first Modbus
address of the data tables are sent to the slave with function code 16,
this error code also occurs.
For function codes 03 and 04, the starting address may be any address
within the data table. However, for floating point data tables, one
element occupies two Modbus addresses. Therefore, only odd
Modbus address are allowed when accessing floating point data table.
If the starting address is even, error code 7 occurs.
The Controller Command table is the only one table that has write
only attribute. If you try to use function code 03 to read this table,
error code 8 occurs and a 02 exception response packet is returned.
Auto-sense Protocol Selection
The primary purpose for auto-sense is to permit configuration by
using RSPower or RSPowerPlus software on a point-to-point RS-485
connection by disabling the Modbus master station and enabling a
DF-1 connection with RSLinx software. The port switches back to the
Modbus protocol when it detects incoming Modbus data packets.
Simultaneous use of Modbus and DF-1 master stations on the same
network is not permitted or supported.
When auto-sense is selected, when a port configured as Modbus
detects incoming DF-1 data packets, it automatically switches to the
applicable DF-1 protocol at the same baud rate and other
communication parameters. The port may return a communication
error to the first non-selected packet and then switch protocols. The
initiator should be set up to retry communication if it receives an
error.