At this point you really need an oscilloscope to find out what the MicroLogix is putting on the RS485 line, so you can compare it to what ModScan32 puts on the RS485 line.
I've done many applications with the MicroLogix 1400 and it works really well as a Modbus RTU Master.
If possible, maybe try connecting the MicroLogix 1400 to the USB/RS485 device and test out ModSim32 as a slave device.