Hello,
There are two options to communicate with an AB PLC 5/20 using DH + protocol.
One of them is using the AB_PLC5 Driver that comes with lookout. This driver works with 1784-KT,KTX,PCMK and with SS-5136-SD cards (Note the 1784-PKTX is not supported by this driver).
Another option is to get from Allen Bradley the RS Linx OPC Server (it comes with the RS Linx OEM or Professional version) and use the Lookout OPC Client Object to communicate to this OPC Server and let RS Linx handle the communication with the hardware.
I hope this information helps.