# Modbus Addresses

For Modbus protocol, the RTU port and the node number of the targeted equipment determine the net and node fields:

- The RTU Serial Port 1 is reserved for Modbus protocol, and is given the designation of “Modbus Port.”
- For Catalyst Ethernet version NET1 is assigned to the Ethernet Networks
- Modbus allows nodes in the range of 1 to 256.
- The address component is fully compatible with the standard Modbus register reference scheme. This has been extended to include floating point and global data. The table below presents this scheme. In the table, x represents a digit in the range of 0 to 9. Except where noted, the RTU will accept any values for xxxx.
- By factory default, the SNA for 16-bit channels is 1*1*40xxx.

Register Address | Description | Size / Limitation | Catalyst SNA
--- | --- | --- | ---
0xxxx | Coil number xxxx | 1-bit | 1*node*0xxxx
1xxxx | Input number xxxx | 1-bit | 1*node*1xxxx
3xxxx | Input register number xxxx | 16-bit | 1*node*3xxxx
4xxxx | Output register number xxxx | 16-bit | 1*node*4xxxx
5xxxx | Floating Point register | 32-bit | 1*node*5xxxx
Note: The 5x and 4x references share the same data space. That is, an address of the form 5nnnn takes the 32 bits beginning at location 4nnnn and extending through 4nnnn+1. These 32 bits are then interpreted as an IEEE floating point number.

The following list identifies special issues with regard to this addressing scheme:

- When any address is out of range for a particular processor the processor will issue an error diagnostic that will be passed on to the user.
- 1x and 3x references may not be written by the RTU.
- The 5x reference is non-standard, See Note in above table.
- The controller's memory protect switch may prevent writing to a 0x or 4x reference.
- A controller not in run mode may prevent writing or report unexpected values.
- The Modsoft reference data screen can enable and disable specific points. Those operations may impact RTU operations.