Do these FPGA's use the 3.3V and 5V rails?
I would be a little worried about the quality of the regulators for those voltages on a pcie powered riser.
Is the DDR4 required to mine CryptoNight7, or does it just improve the hashrate?
The DDR4 is not required for CNv1 (Not to be negative nancy, It's cryptonight version 1 released under monero version 7 -- I saw some people trying to get this clarified elsewhere, thought I'd try to help).
Currently the DDR4 is not used under any designs.
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Regarding risers, we're going to do some testing on a few variations of the board. I may just end up with a bulk buy and shipping the boards with a riser included. I won't know how we will proceed with this until after sales close.