We've talked and discussed everything and anything Z390 over the past few months now. Basically, the two chipsets are really similar but would bring support for 8-core Coffee lake processors. Word out now however is that Intel might be simply replacing Z270 with Z390.
Initially, we all expected Z390 to exists next to Z370 and if we can believe a few newly leaked roadmaps, Intel is planning to phase out the Z370 chipset this quarter with Z390. Earlier on, we already posted a news item pretty much indicating this new rumor, The Z370 to get rebranded as Z390. Of course, take the news with a grain of salt.
But in essence, the Z390 chipset would remain the same as the Z370 chipset, manufacturers who want to offer the support that the chipset originally promised, will have to revert and do this in a more traditional way: with external controllers such as those from Asmedia, reported benchlife.
Z390 has been discussed a lot, it should be a chipset intended for 8-core Coffee Lake CPUs, but these are not expected to launch anytime soon. Z390 will get an LGA1151 socket using the traditional DMI 3.0 chipset-bus (which basically is an x4 PCIe link lane up/downlink. Similar to the Z370 chipset, it'll have 24 PCI-Express gen 3.0 lanes. Also similar is storage at six SATA 6 Gbps ports with AHCI and RAID support; and up to three 32 Gbps M.2/U.2 connectors. LAN remains the same as well. 1 GbE, Intel recommends their Wireless-AC 9560 card for the motherboard manufacturers to pair this chipset with for 802.11ac and Bluetooth 5.
Look and compare below, the two official chipset block diagrams.
Above Intel Z390 as leaked earlier at Intel
Above (for comparison) Z370 - the differences really one are USB 3.1 gen2 and Intel Wireless-AC (CNVi)