I like my ISY994i and recently upgraded it to 5.3.4 so that I could use a UD mobile app on my new Samsung galaxy.  All has been working fine.  However, I need to replace a couple I/O Lincs that have failed.  Since they're not available anymore, I looked into a z-wave option and bought a Zen17 univeral relay.  Of course, I can't link it to my ISY.  Based on all that I've read, I need to add a z-wave board or other piece of hardware before I'll be able to control these new universal relays with my ISY.  I've done some reading, but it's very confusing with all the z-wave series modules and compatibility issues.  Ultimately, I just want to replace a couple I/O lincs that have failed.  One I have connected to a photocell and use to control various exterior lights, and the other I use to manage my garage door.

If someone could advise me on the best option, short of spending hundreds on a new controller, I'd really appreciate it.

Get a Z-Wave board from UD and put that your 994i and use that to communicate with your Z-Wave stuff.  In light of the Insteon debacle I upgraded a bunch if my stuff to Z-Wave with the new series Z-Wave board added to my 994i and now I have both Insteon & Z-Wave stuff from multiple manufactures all working well on my 994i.

issue may be 5.3.4 needs the Zwave 500 board and it still shows out of stock.  IDK if they will come back in stock, have been looking for months