Automatically configurable master and slave devices in a master/slave multi-drop serial network are disclosed. In one embodiment, the devices and network utilize the Modbus serial communications protocol over a serial multi-drop network, such as an RS-232 or RS-485 (also referred to as EIA-485) based network. Alternatively, the disclosed embodiments may be used with any master/slave serial multi-drop network. In one embodiment, the master and slave devices include pseudo-slave and pseudo-master auto-configuration operating modes, respectively, whereby slave devices which are introduced to the network may send unsolicited identifying messages to the master device which will determine and assign a unique network address thereto, causing the slave device to configure itself to be responsive to messages addressed therewith and then enter its normal slave operating mode to await such messages. The master device, thereafter, maintains a database of the slave devices and their associated network addresses. In one embodiment, legacy slave devices, which do not support the disclosed pseudo-master auto-configuration mode, are supported by the master device which determines when network addresses are in use by such legacy devices, as will be described, and avoids assigning those addresses to newly introduced auto-configurable slave devices.