Error: connection between multiple registers with the same bus

Hi all.. My project is a simple 8-bit microcontroller with Von Neumann architecture. I realised it using the library maxplus2 in Quartus II Web Edition, so i used the 74XX series. Infact i don't know HDL or VHDL, i used the schematic file in the software. My micro has many registers, one RAM, and a control matrix. The registers and the RAM communicate with the same bus W and i control the communications thanks for three-state buffers and control bits that activate or not them. When i compile the project appear the error: Error (12014): Net "bus_W[5]", which fans out to "74173:inst38|3D", cannot be assigned more than one value

Error (12015): Net is fed by "inst11"

Error (12015): Net is fed by "inst29"

Error (12015): Net is fed by "inst44"

So the software can't know that the communications are regulated and that only two registers can communicate at one time thanks to the control matrix. For example RAM communicate with the accumulator at one time and with the instruction register in a different time. But for the software the communications are simultaneous. What can i do??

I'm sorry for my terrible english.