ALTGX rx_signaldetect[0]

Hi Experts,

I have been playing with my 'Cyclone IV GX Transceiver starter kit' and am using the ALTGX megafunction in Basic mode. I use a slow clock rate 600mbs. I use a fibre optic module as the physical media but I don't really think this is relevent. Any way I loop back from tx to rx. I am sending 8 bit data with test pattern that includes all binary data codes. I notice that signal rx_signaldetect[0] goes false for certain binary data patterns (patterns are hex 73, DC, DD, and F2). Any idea why?

Also I notice if I just unplug my fibre so I only have noise then the rx is just rubbish. rx_signaldetect is solid active, mostly I get rx_ctrldetect active and pattern 9C but data and other control codes as well. How do I detect a bad link?

Thanks

Dave