Sorry if this has been covered before, someone can even post a link and I'll read it. My question is will Ultegra Di2 work with the new DA Di2? Can I use Ultegra shifters with DA derailleurs or vice versa?

In short, yes.

Everything that uses E-tube wiring, which is everything except old DA 7970, is compatible with each other. At the moment, that means you could use alfine Di2 shifters with ultegra RD and a DA FD. It's all plug and play. You can adjust the shifter controls (what button does what, how fast the bike will change gears if you hold down the button, etc.) in a computer program.

The Dura-Ace and Ultegra front derailleurs are optimized for 11 and 10 speeds, respectively, but I'd imagine there'd be very little funcional difference if used an Ultegra FD on an otherwise 9070 bike.

The rear derailleurs, at least as far as I've seen on the internet and from Shimano, are specific to the number of speeds they were designed for. So, if you want 11 speed but only want to pay for ultegra di2, don't expect the motor in the RD to work with the spacing of the cassette. That said, you could just put a DA RD on an otherwise ultegra bike and it would work just fine. I don't know if the motor in the DA RD will work with 10 speed or not. Maybe someone else can chime in.

Great, thanks for the help.

No experience but having read a lot of the Di2 topics, my understanding is that with e-tube compatible Di2 components, although rear shifting signals are based on how the firmware for the DA 9070 junction box ('the brain') is programmed, the rear derailleur still determines the number of cogs. For firmware updates, you can use Shimano's internal battery charger (BCR2) or the Shimano PC Connector Interface Device (SM-PCE1PC) as the only thing the BCR2 does not do that the PCE1 does is diagnostics for when you have a malfunction as the PCE1 will test stuff out as wellas provide step-by-step instructions on narrowing down the problem. The PCE1 is offered as an aftermarket accessory but is currently limited in availability but with the ability to update firmware, seems to me if you had Ultegra Di2, all you would have to do is swap out the junction box and rear derailleur then update firmware so that all the electronics are on the new version to make current Ultegra Di2, 11 speed compatible.

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