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Just out of curiosity, if all you want to do is push data from one board to another why use something as complex as Ethernet? There are all kinds of SERDES chips out there that could do the job much more simply.
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A fair enough question. I actually looked at a LOT of SERDES chips before settling on Ethernet PHYs. We needed a galvanically isolated interface to isolate the host processor from the test set. That meant either capacitively or magnetically coupling everything going between the two boxes. The current system uses an array of iCoupler devices to isolate a bunch of high-speed SPI buses. (one for each ADC/DAC) This has has worked fine, but we now want to up the data rate by a factor of 10, as well as move our signal generator into the isolated domain (as opposed to using an analog signal isolator). The signal generator uses a 100Msps 14-bit DAC, which made the current scheme unworkable. So, we opted to put a local FPGA on the board to manage the board, house the signal generator firmware, and packetize the ADC data streams. Serializing the data stream to the host CPU seemed fairly obvious at this point.
I actually found a few very nice SERDES chips, but the I/O count was usually too high. The TBI devices had a minimum of 11 lines in each direction as well as the clock, and needed firmware to do the 8b/10b encoding in addition to clock-crossing/rate change FIFO's. The TI parts that were suitable had two 16-bit interfaces plus control lines, which put the I/O count near 35. Given that we only have 79/81 pins (depending on which part we opt for), that's a pretty high ding to the pin budget. Both the RGMII and dual RMII interface can be managed with 13 lines or less. I even poked around a little on the idea of using a GX part, but that meant using a BGA or QFN part - which for boards that are hand assembled was a no-go. (We do have a BGA machine, but it's getting old and cranky)
I was also worried about using devices that were a bit too specialized, and thus might not be available in the future. This system will need to be serviceable for at least 10 years. I don't mind specifying a lifetime buy, but if I can use commodity parts that are expected to be widely available, I prefer to go that route. Ethernet PHY chips met all those requirements. They use magnetic coupling, the parts are cheap and plentiful and should be around a while, and the interfaces are highly standard.
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