The programmable controller 1785-L40B is a central part of every control system based on the Allen Bradley PLC-5 series. The 1785-L40B connects local, extended, and remote I/O modules on various communication interfaces into one control system. Four built-in ports are available on the 1785-L40B controller. Ports are marked as channel 1A, channel 1B, channel 2A, and channel 2B. All ports can function as remote I/O scanners, remote I/O adapters, and DH+ communication interfaces. DH+ communication can do remote programming and monitoring of the 1785-L40B and can create a peer-to-peer communication network between the various PLC-5 and other controllers. Remote I/O connection is used for real-time data exchange between the controller and I/O, operator interface, and other devices.

All four channels that exist on the 1785-L40B can function as remote I/O scanners or remote I/O adapters. Remote I/O scanner mode is used to read and write I/O information between a controller and remote I/O device. In this mode, the 1785-L40B is a supervisory controller for other controllers that are in adapter mode and remote I/O adapter modules. When any of the four channels, 1A, 1B, 2A, or 2B are in remote I/O adapter mode, the controller monitors and controls its controller-resident local I/O while communicating with the supervisory controller via a remote link. On the 1785-L40B Data Highway Plus (DH+), a link can be used on all four existing channels. DH+ communication can be used for data transfers to other PLC-5 controllers or other higher-level components of a control system. The DH+ link can program multiple PLC-5 controllers. Maximum cable length for DH+ link depends on the transmission rate. It is crucial that all devices on a DH+ link are configured to communicate at the same transmission rate.