Monitoring Ethernet II frames in Ubuntu; with and without a local network

Hi,

I'm trying to send raw 'Ethernet II' frames from my ethernet interface (eth0) using Ubuntu, to some embedded electronics (I'm trying to get my head around low level networking). I would like to only use 'Ethernet' (layer 2), and not use 'IP' (layer 3) or any higher protocols, to keep things simple for the electronics.

I have found 2 ways of send raw 'Ethernet II' frames using Ubuntu that seem to work: 1. Using packeth 2. Using python (as per the highest voted stack overflow answer here)

When I plug my Ubuntu's Ethernet interface (eth0) to my home router's network port using a ethernet cable, I can start a capture using Wireshark on eth0, and correctly view the transmitted 'Ethernet II' frames, when using either of the methods given above). But when I disconnect the Ethernet cable and repeat transmitting an 'Ethernet II' frame I see nothing at all in Wireshark. Why???

I am just learning networking, so any help appreciated... I'm having trouble working out what the difference between the 2 scenarios should be at the Ethernet Layer??? Connecting eth0 to a local network provides it with an IP address, but I'm not sure why this should affect eth0's ability to transmit... what am I missing here??? I have turned eth0 on using 'ifconfig eth0 on', but that didn't help... What should be needed theoretically for two Ethernet level devices to talk to each other?

Thanks