Official firmware: http://www.tvix.co.kr/eng/support/M41_5100firmware.aspx
Changed firmware: OpenTViX Forum Post or get it here: OpenTViX Downloads - TViX HD M-4100 and M-5100. This is basically the official tvix_M-x100_1.4.52.fwp with telnetd, sshd, httpd, ntpclient in addition. It will autosync time to GMT+1 (and 1 hour auto summer time) at every boot. Password for the root user is
There is also a discussion thread about newer TviX Beta firmware: http://www.mpcclub.com/forum/showthread.php?t=19531 (The latest seams to be v 1.7.15).