OK, I get errors when I try to use -bitbngr on the command line.
% ../git/mame/mame64 rainbow -hard1 venix-st251.chd -window -comm null_modem -bitbngr socket.127.0.0.1:1234
Error: unknown option: -bitbngr
%
So what am I missing?
-- edit --
the option is now called -bitbanger