Are you using 32bit modules as that is a way to get more IO, If you are already using 32 bit IO, then you are near the limit of the IO that the 870 can control it can only control 296, so 8x32=256+14(onboard)= 270 this only leaves 26 points left, you could use the DNet module to get some more IO, or if you use another Micro you should be able to message the signals but I would not use that method for time critical IO.