I agree this is very specific, but I thought perhaps someone could help. As I look at the '09 datasheets, I can't tell when the data lines become valid on a write cycle. I ask because I have created a few projects that place themselves between the CPU and the CPU socket (CPLD based). To play nice with a potentially shared address and databus, my code honors the BA and TSC lines on the IC.? If either line is high, as noted in the datasheet, I tristate the address and data lines. Otherwise, the address lines are valid during the entire E cycle. As for data, it depends on the state of the R/W line. If R/W is high, the external databus is connected to the CPU databus during the entire cycle if TSC and BA are both low. If R/W is low, I initially set the external data bus according to this assign statement: assign data_ext = (!r_w & e& !(tsc | ba) ? data_09 : 8'bz); The '09 boots, but certain devices fail to operate.? If I change the assign to: assign data_ext = (!r_w & (e | q) & !(tsc | ba) ? data_09 : 8'bz); or assign data_ext = (!r_w& !(tsc | ba) ? data_09 : 8'bz); The devices begin working. Either of the modified equations works, but I don't know if can safely place data on the external databus during the entire cycle, like the address lines, or if I need to be off the bus for some small portion of the cycle.? I was hoping the datasheets could help, but I am missing the key portion of the timing diagrams. I hope someone can enlighten me.? I have the system working, I just want to make it as good as possible. Jim -- Jim Brain brain at jbrain.com www.jbrain.com