I'm not confident with networking settings in bitcoin.conf but try to use these settings.
Connect to this node exclusively (use instead of
addnode):
Maybe it's disconnecting because the old version is misbehaving, try any of these:
Also, make sure that bitcoind isn't running in the background while editing your config file.
Thanks for that working a treat now.
This sounds like XY problem, do you have any particular reason to stay use Bitcoin Core 0.12.0 which is outdated? If there aren't any, i suggest you to upgrade your Bitcoin Core.
AFAIK nc50lc suggestion should fix the problem, but if it's not, then try to share your debug.log file
I think it is important that Bitcoin maintains its backward compatability and i want to see that older nodes can always stay connected to the network.