Okay, so let's resolve whether you can connect to the MicroLogix 1200 Series A controller at all.
MicroLogix 1200 controllers have a "Communications Default" button on them, right below the trim pots near the Channel 0 serial port plug. When you press this button and the "DCOMM" LED is lit, the Channel 0 serial port will be set to the default configuration of DF1 Full Duplex at 19200 baud.
When in the DCOMM mode, connect your PC and use the Autoconfig feature in the RSLinx DF1 Full Duplex driver setup. If this works, then you'll be able to clear the controller memory or download to the controller.