The Allen-Bradley 1756-DHRIO is a ControlLogix Data Highway Plus/Remote I/O communication interface module. This module has Two (2) channels, supporting communication rates of 57.6 kilobytes per second, 115.2 kilobytes per second, and 230.4 kilobytes per second, depending on the cable length. It can transfer a message through a maximum of three chassis and four communication networks. With this module, information exchange can be done among devices such as PLCs, SLCs, and ControlLogix controllers, even on different networks.

It supports the following types of communication: Remote I/O, Control and Information Protocol (CIP) Messaging, and Data Highway Plus (DH+) Messaging between devices. It can have up to 32 Logix connections and the remote I/O link can connect to a remote I/O device, as well as other intelligent devices. When connected to a remote I/O network, it can act as an adapter or scanner and transfer analog, digital, block, and specialty data even with the absence of message instructions. Prior to installing the 1756-DHRIO module, you need to program the network type switches for each channel and install and connect a ControlLogix power supply and chassis. You can install as many modules as you can in the same chassis but this will depend on your available power supply. Note that modules can also be installed or removed even when the chassis power is applied but you need to observe safety precautions, particularly that the area is nonhazardous and power is removed, to prevent an electrical arc that can cause excessive wear to the contacts of both the module and the connector. Make sure to discharge potential static because the module is sensitive to electrostatic discharge.

RIO and DH+ protocol uses the same cable type. Recommended cables are Belden 89463 twinaxial, 0.52 mm² (20 AWG) rated at 200 °C (392 °F). Applicable spare connector is Phoenix terminal with part number MVSTBR2.5/3-ST BK AU.

DH+ and Remote I/O network requires the use of a terminating resistor as it follows daisy-chain topology. This resistor is installed at the first and last node of the network. Resistor values depend on the network speed. 82 Ohms is used for network speed of 230.4Kbps. 150 Ohms is used for network speed of 57.6 Kbps or 115.2 Kbps. Other considerations are also considered including the type of devices connected to the network. The terminating resistor is installed across line 1 and line 2 of the cable. The 3rd cable is the shield cable.

To terminate the cable following RIO protocol, Line 1 is connected to the bottom pin or terminal 1, shield cable is terminated to the middle pin and Line 2 is terminated to the last pin. To wire the cable appropriately as DH+ protocol, simply reverse the connection of Line 1 and Line 2.