PROTOCOL
The EVO/ECM-Modbus RTU communicates RS485 using baud rates from 1200 to 38400 bauds, no
parity and 1 stop bit.
The EVO/ECM-Modbus RTU responds to three Modbus master commands to address data
registers 1 through 26. Registers 1 through 16 and 19 through 26 are directly addressed.
Registers 17 and 18 can only be accessed using the service address 255.
Each Modbus register is 16 bits long. These bits are organized as two bytes (8 bits). Each byte
contains two nibbles (4 bits). A Modbus register can hold one piece of information, or multiple
pieces of information. A piece of information can use both bytes, or multiple information can be
stored in each byte, nibble, or bit.
Register 1, Register 14 and two additional bits are also organized as 34 coil registers. Each coil is a
bit that can be accessed individually. The two additional bits are for setting the controller to write
defaults to dynamic registers and to reset the controller.
The chart below describes the data stored in each register.
Command 03 reads multiple registers
Command 16 writes multiple registers
Command 06 writes to a single register
Command 01 read multiple coil registers
Command 05 write a single coil register
REGISTER USAGE
Dynamic Registers
Registers 1 through 5 contain dynamic operating data. The EVO/ECM-Modbus RTU uses this
information to turn the motors on/off, and to set each motor's airflow. These registers may be
written by the Modbus RTU Master, or set to values stored in the default registers. The defaults
are restored on RTU power up, on RTU reset, or when a communications timeout occurs. The
Modbus master should read these registers on each scan of the trunk.
Read Only Registers
Registers 6 through 9 only respond to a read command from the Modbus Master. These registers
contain each motor's RPM. The Modbus master should read these registers on each scan of the
trunk.
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