Thanks Richard for your quick responses.

Unfortunately I did not manage to get ethergate to work with catalyst and had to sub-hire a Catalyst CIB (which worked fine).

However I did manage to run "tcpdump udp" on the PowerMac:
it showed a steady stream of packets to port 6454 (artnet) and none to 333 (proprietry procotol).
Unfortunately tcpdump (unlike ethereal) does not understand artnet and all I saw was strings of (non-zero) hex data.

I hope in the next weeks to get enough time (hah !) to install ethereal for MacOS and see what traffic is really there.

The thing is that bringing up the information window ('w') and stepping through to the page showing DMX details listed the ethergate as having universe 0, and it was sensitive cables disconnecting (so it wasn't historical "memory" of the ethergate).

thanks again.
- JonT