multiple drivers due to the non-tri-state driver

I'm further confused again.. I now have a problem in my Top level module:

// Shared buses
wire  [15 : 0 ] dataBus;
// Registers to be switched onto the bus
logic [15 : 0 ] dataRAM;

//bit[ 15:0 ] dataVal;
logic[ 15:0 ] dataVal;  // logic or bit? Made no difference
..
assign dataBus    = ( chipSelectRAM ) ? dataRAM    : 16'bZZZZZZZZZZZZZZZZ;
..
always_ff @( posedge clk  )
..
  dataVal       = dataBus;   // Error here

Error (13076): The node "dataVal" has multiple drivers due to the non-tri-state driver "SDRAMController8Bit:sdramController|ioData[8]"

dataBus is connected to sdramController's ioData wire

dataRAM was being used for outgoing data.

So I modified the code further, renaming dataRAM to dataOut and adding a dataIn and now it compiles:

logic [15 : 0 ] dataOut;
logic [15 : 0 ] dataIn;

assign dataBus    = ( chipSelectRAM ) ? dataOut    : 16'bZZZZZZZZZZZZZZZZ;      // Do I need to include iSDRAMWriteRequest here??
assign dataIn     = ( chipSelectRAM ) ? dataBus    : 16'bZZZZZZZZZZZZZZZZ;      // Do I need to include ~iSDRAMWriteRequest here??

Question: If using dataBus for incoming and outgoing data, do I always need to provide a separate register for incoming and outgoing as I have done above?