We are in the process of developing a new set of custom boards with a custom serial protocol. We are prototyping this using FPGA development boards from Altera which can take QSFP modules and other boards that can take SFP+ modules.
We have 2 Mellanox QSFP modules:
mfm4r12c-qdr
mc2210411-sr4
Either QSFP module can be successfully used with a breakout cable to connect to the board with the SFP+ module. However, we are unable to connect the 2 QSFP modules together, using a female-female crossover cable.
Should these 2 modules be able to connect to each-other? What is the difference between them?