Note that there is another Application Note called Plena USB and USB to RS-232 convertor matters in case you encounter problems with this type of connection. In case a PC doesn t have a dedicated RS 232 connection a USB to RS 232 converter can be used. General A firmware update is performed by connecting a RS 232 cable from a PC to a Plena Voice Alarm System unit. For each unit specific firmware is available. Note that all units present in one system must have the same firmware version. This Application Note describes how a firmware update of a system can be done. Introduction It is advised to update a Bosch Plena Voice Alarm System with the latest released firmware. 1 Plena Voice Alarm System Firmware update 1.