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[Thelogman]

I am trying to build a basic drowning trap. I have two sets of floodgates, one set of them is holding back an ocean on either side, while the other side seals in the unlucky victims.

The problem is, I want the one set to go from open to closed and the other set to go from closed to open when the pressure plate is triggered.

Is this possible with pressure plate logic? If so, how? Am I better off just attaching the floodgates to levers and pulling them and hoping my dwarves are fast enough?

[Sphalerite]

You can't do this directly.  When a pressure plate is stepped on, it will send an 'open' signal to whatever it's connected to.  This means it will cause both floodgates to open.

What you can do is remove one of the floodgates and replace it with a single-tile raising drawbridge.  This will act as a floodgate with inverse action to the conventional floodgate.  When triggered by the pressure plate, it will raise, blocking the passage, at the same time that the floodgates open and let the water in.

Keep in mind that floodgates and bridges both have a 100 step delay time between when the pressure plate is triggered and the device activates.

[Thelogman]

Oh, okay, this makes perfect sense, thanks!

Just one other question, how much exactly is 100 "steps"? How quickly do Goblins move? Do they move about a speed of 1 square per "step"? If anyone has an idea, this would be really helpful so I know how BIG my drowning room/maze needs to be.

[Sphalerite]

In my experience, a very fast goblin might move 30 tiles in the time it takes a bridge to activate.  Most will be slower.

One trick you can use to keep them from getting out of your drowning chamber is to have the pressure plate immediately followed by a single-tile channeled hole (without a ramp in it), covered with a floor hatch, linked to the pressure plate.  Pressure plates have zero delay on opening, so when the goblin steps on the pressure plate the hatch will instantly open and stop him from moving any further.  I have a drowning trap in my current fortress that works on this principle.