A MicroLogix 1000 with it's channel 0 port set to DH485 should see DF1 full duplex coming into the port and auto switch from DH485 to DF1 full duplex. However it sometimes doesn't take right away, needs to be power cycled with DF1 poling for it right after it finishes booting up or won't work when using a USB to serial adapter. My guess would be the last one I.E. it doesn't recognize the data packets coming from the USB to serial adapter (I believe it has to do with packet timing).
Try setting your DF1 driver up for "SLC-CH0/Micro/PanelView", full duplex, 19200 baud, No parity, 1 stop bit and CRC error checking. Don't use the auto-config and select OK. First question, does it take I.E. does RSWho show "RUNNING"? Next question, select the "+" sign next to the "AB_DF1...", does a MicroLogix 1000 show up below "00, Workstation..." and if not, what if anything does show up below? If a MicroLogix does show up, without a red X then you are talking to the MicroLogix 1000. If there is a Red X through it (and it is labeled "MicroLogix 1000...") then it can see it but it isn't making a full duplex connection to it. If that is the case let it run for a few minutes and see if it eventually comes up.
If none of this works, you may have a bad PLC or your USB adapter isn't compatible with RSLinx (the likely scenario).