I have a hardware project at work which is an FPGA with embedded MAC (a Xilinx v4fx) hooked to an 1000Base-SX SFP and then hooked up to a standard NIC at the remote end. The host PC is running linux.
The problem I see is that any NICs i use kind of go a bit crazy, a netgear GA620 NIC reports the link as alternating between up and down at a rate high rate of knots, and 2 syskonnect NICS just say there is no link at all.
The thing is on the FPGA I can see the data from the NICS, idle packets, autonegotiation packets, even data packets. I can see every packet they send. However, they only see about 10% of the packets I send.
My question is how sensitive are standard nics to jitter and related clocking issues with the incoming gigbit stream? My thoughts are my reference 125Mhz TX clock is not stable enough and the clock recoverly on the NIC is not working correctly. Does that sound semi sensible?
What confuses me is that if I loop the fibre back to the SFP then I register every packet I send without any issue, running at full line speed. But of course, in that situation the RX and TX reference clocks are the same, and so i am guessing any issues would be massively reduced.
Does anyone have any thoughts? Or better yet, an idea of how I can test the setup without blowing our entire budget, after all we're just a university research group and this project has limited funds..