Modbus station address

Modbus defines valid station addresses in the range 1 to 247. A Modbus slave only responds to telegrams that contain its own address. Address 0 is not a valid station address. It is used for broadcast telegrams to all stations. These are not answered. Addresses 248 to 255 are reserved.

The ModbusRTU library defines further collective addresses. This enables a station to respond to several addresses.

 
TYPE MODBUS_UNITID :
(
    MODBUS_UNITID_BROADCAST         := 0,
    MODBUS_UNITID_ALLVALID     := 256, (* response on address 1..247 *)
    MODBUS_UNITID_ALLBUTBROADCAST := 257, (* response on address 1..255 *)
    MODBUS_UNITID_ALL         := 258 (* response on address 0..255 *)
);
END_TYPE