I was trying to get a handle on the multitude of serial connections and power connections and whatnots for FOMOphobia.
As usual, I made a drawing. Diagrammatically, each module would look like this. Mostly serial communication from the RPi to the two displays with the GPIO pins controlling a relay that triggers the bell.
The logic voltage converters maybe need a word of explanation: In old serial communications, voltage could be anywhere from 5 to 15 volts DC. In the Raspberry Pi, the voltage on the serial port is 3.3 volts DC. I will either need to build or buy a little device based on the MAX3232 chip to convert serial voltage to and from the RPi’s 3.3VDC to an acceptible TTL voltage.
An additional RPi monitors the server for data for the printer.
Simple as that.