I am trying to use PDG to complete a vellum sim, store the sim results on disk using a Vellum IO v2 node (hou 19.5),
THEN export out an alembic of a further down stream alembic ROP

QUESTION: In order to get the full completed sim what node in PDG do I need to use to activate the "save to disk" function of the Vellum IO SOP node and when it's done then the work item is done then generate an alembic from the cached results.

I am trying a ROP Geometry Output, but it doesn't seem to actually activate the Vellum IO v2 node.

It's been a while since I've done something like this but I think I pointed a rop fetch to the rop geo output node inside a file cache (or inside the filecache inside the vellum io node). Then point another rop fetch at the alembic rop.

I'll give that a shot.

As mentioned previously, you can use a ROP Fetch TOP node to fetch the render node which is hidden inside of the Vellum IO -> filecache -> render. In TOPS you can either leave the ROP Fetch on 'Evaluate using ROP Node configuration', which will leave you with a single work item. In another ROP Fetch under it, you can then point to your alembic ROP and save it out (it will start only after your Vellum work item has completed). If you want to get per-frame control in TOPS you will have to set the 'Evaluate using' on a frame range setting. In that case you can use a 'wait for all' node, to wait for all work items to complete and then fire off the Alembic saving. Hope you can get it to work Cheers!

I got it to work eventually. Here is the setup for those curious: