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DS100MB203: How to set EQ and DEM settings for different length of passive direct attach cables.

Part Number: DS100MB203

Tool/software:

Goodday

I have a design that uses the DS100MB203SQ/NOPB as a DEMUX/MUX for 10GBASE-R traffic.

0714.UMAR-10G02.pdf

In summary (see the schematic page attached) the left side goes the host FPGA (S_IN/Out) while the right side goes to the SFP/PHY side (D_OUT/IN). U31 and U32 (HD3SS3212RKSTQ1) is a passive MUX that allows us to select between the SFP ports or a 10G PHY.

S_OUTA/B is always set to FANOUT mode, while we use SEL0/1 to put the mux in end-point mode (S_IN0/1A) or loopback mode (S_IN0/1B).

For this question we are in SFP mode only.

What I have found is that when using Direct attach cables (1m Passive), the data goes though with no loss when connecting the SFP ports directly to another FPGA based system (in end point and loopback). However when I connect two of these DS100MB203SQ designs together (I have multiple circuits on the same board) there is some loss.
If I change the EQ_D1/0 straps to 01 (FR4 10 inch 4–mil trace) this seems to have the best results. But if I go higher or lower on the EQ table the results are worse. The FR4 10 inch 4–mil trace does not seem to match what I would expect for a 1m Direct attach cable.

My question is, what is the best way to work out what EQ/DEM settings to use for different lengths of direct attach cable, or is it just trial and error?

Or is there something in the design that I am missing?

Thank you