The PLC-K521 series of Discrete Input Expansion Modules are offered with either 8 or 16 inputs. These modules are attached to the PLC-K5 CPUs to expand beyond the onboard inputs. The PLC-K5 CPU series can handle up to 136 I/O points with a maximum of 6 expansion modules. If a user were to select the PLC-K508-40AT, equipped with 40 I/O points, and an additional 6 PLC-K521-16DX units, the user would achieve the maximum 136 I/O points within the constraint of 6 expansion modules. The PLC-K521-__DX modules come with the option to either sink or source the inputs; each input is rated for 3.5mA. All modules are opto-isolated so units are not damaged.