Open center usualy means open on the input side of the valve. IE: pump port to tank port.
But the output ports are both blocked in the center postion. That's what holds a cylinder in position.
If the outputs are also open, then thats a motor spool.
But J_J contradicts this. He defines an open center valve in post #20 as one that, by definition, has a motor spool. His definitions don't allow an open center valve for use to control cylinders in use cases such as a front end loader; such valves as are needed for a loader are, according to J_J's definitions, called tandem valves (even though they are open center as regards the input side).
I thought "open center" referred to the input side only, and then the spool choice was made depending on whether you're controlling a cylinder or a motor. This seems like a logical convention. But some conventions aren't logical and J_J's definitions throw a wrench into the mix. Either he is wrong, we are wrong, or there is some weird twisted black hole of communication going on in this thread.
Foster Manufacturing has some interesting language here:
http://www.fostermfgcorp.com/page/circuits/circuits.htm that would suggest J_J is wrong. Their language seems to imply that "tandem" is simply the type of open center valves designed to be connected in blocks, as most of us are familiar with. And that such tandem valves can have circuit arrangements for the supplied fluid that are either parallel, series, or combo parallel/series, which simply determines the functionality of the block with regards to whether multiple functions or implements can operate at once or not. But the word tandem, by itself, doesn't suggest anything about whether or not the implement-side ports are blocked in neutral or not. It simply denotes that multiple single-spool, open center valves can be physically put together into a block of valves. Whether or not said valves have the implement-side ports blocked in the neutral position is purely a function of the spools that are installed into the valves. A block of tandem, open center valves could have some with motor spools and some with regular spools.
But the glossary of terms I found here:
http://www.acshydraulics.com/glossary.html agrees with J_J. In fact it looks like maybe he took his definitions from this glossary.
And other language I read describes "tandem" as one of the available circuit types along with parallel and series.
xtn