I have to look deep in memory.
The default address is 192.168.2.147 if I recall correctly - on eth0 (HMC0)
The second interface might still be "unused" at default DHCP - 192.168.3.147
Note: their default is to use DHCP, so if you have a DHCP server you could attach both ports and watch what address the DHCP server assigns.
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USB PC to serial? What speeds did you try? I think it needs to be 19200, 8bits,, no parity, 1 stop bit.
For the IP address with Power7 (and maybe Power6) follow:
IBM
Although this link also gives the 169.254.2.147 as default address for Power5 and Power6.
Found some old embedded texts: the POWER5 defaults MAY be 192.168.2.147 or 192.168.3.147
And they may be still using DHCP to connect. In that case, get your HMC (virtual) setup to be a DHCP server and provide a direct line between HMC and Power system.. The HMC should "discover" - by assigning it - the IP address of the POWER system.
Good luck with the password for account admin.