about EPM240 high impedance output

Without knowing the channel assignments, the states driven to the connected MAX II pins and the external wiring, the oscilloscope waveforms are effectively meaningless. So I can only comment the code. the 1'bz assignments as such can be expected to work unconditionally. But you don't show the complete code, so we can't know, if they are effective at all.

As an additional comment, the 3.3V IO of a MAX II isn't suited for usual RS485 levels, unless you can be sure, that all peers are using 3.3V only and the bus don't extend beyond the board boundary. Otherwise damage of the MAX II by overvoltages must be feared.