Allen Bradley (AB) Mirologix 1500 with Catalog number 1764-LSP processor or 1764-LRP processor is a low cost and versatile platform to control a lot of small industrial application. Its channel 0 can be set as DF1 full duplex, DF1 half duplex slave, Modbus RTU master, Modbus RTU slave and ASCII mode. If you can not get online with the controller, it is easier to reset it back to DF1 full duplex than to figure it out.
Reset Channel 0 communication to default
If you remove the cover of 1764-processor as shown in the picture below, you can see a push button on the unit. You need to push and hold the button for 5 seconds. Then channel 0 is reset to DF1 full duplex.