Automatic mode switching unit for a serial communications data system

An automatic switching unit for use in combination with a backup processor and a series communications system such as for a retail store operation, and where the series communications system includes a controller having multiple serial ports, each port being connected in a series loop with a plurality of data terminals for registering sales transactions. The switching unit includes a monitor for monitoring each of the series loops to detect the malfunction of any port, and also includes a switching network. When the monitor detects a malfunction of any port, the switching network automatically connects the backup processor and all the data terminals in those loops associated with serial ports detected to be malfunctioning in a single series loop, and automatically disconnects the malfunctioning ports from the single series loop. In this way the backup processor is automatically substituted for the malfunctioning ports, and all the data terminals that had been connected to one or more series loops with the malfunctioning ports are automatically connected in a single series loop with the backup processor.