I need a feasabilitiy check as this will be my second Arduino project.
This project will use 3 Arduino Ethernets. The first two will be in different rooms and will detect if a relay has been activated. If the relay has been activated they will send a signal over the network to the monitoring station (the third Arduino) which will activate a pizio alarm and/or a flashing light. I would like the people at the monitoring station to be able to push a button to "silence" and/or clear this alarm.
From a programming standpoint, that means the monitoring station needs to be able to accept/interpret/send ethernet and local signals at the same time (client & server). Is that possible? I've only seen examples for client only or server only.
Yes, this is possible, the Ethernet shield (an Arduino Ethernet is more or less the same as an UNO with an Ethernet shield) is capable of having 4 concurrent connections, it doesn't matter if they are incoming or outgoing connections. You probably don't find examples of doing both a the same time but you can merge the client and server examples relatively easily.
But from your description I don't see the need for the monitoring station to be server and client. It just receives the signals from the two sensor stations. What does it have to send? And where to?
I might have it send "clear" commands back to the other two units to clear the source of the input via a relay. I'm guessing that this would all have to be done via uni-cast; so I'd be using the four concurrent connections (in:mon-sens1, out:mon-sens1, in:mon-sens2, out:mon-sens2). Right?
Do I have to keep all connections active or can I do:
Detect Input
Establish Connection
Send Alert
End Connection
I think you have to use unicast as I believe that the Ethernet shield doesn't support multicast traffic.
I don't think you have to keep the connections active but that depends largely on the latency requirements you have. I still don't see why the return connection is necessary as you can send information back when the sensor client connects to the monitoring station. If you keep the connections active you'll have only two connections anyway as an existing connection can be used in both directions.