Unit Power feeds

The Unit power feed to the NX IO system is to supply power for the internal circuits of the NX units, such as the internal circuitry in each slice. This needs to be a clean (low noise) power source. The Unit power is supplied via an NX-ECC20x or NX-EIC20x network coupler via terminals UV and UG (24VDC and 0VDC respectively). 

On NX series CPUs such as the NX102 and NX1P2, the Unit Power feed to the NX bus is via the CPU power supply.

IO Power Feeds

The IO power feed is via terminals IOV and IOG (24VDC and 0VDC respectively). Note that NX series CPUs such as the NX102 and NX1P2 do not connect to the IO power bus of the NX IO system. Therefore the first module in the NX bus should be the NX-PF0630 or NX-PF0730. These units feed in the IO power to the devices which use this (such as Digital inputs and Digital outputs). 

Why are these power feeds separated?

The IO systems have galvanic isolation between the IO side and the system side. This is to prevent electrical noise from the field IO system from feeding back to and affecting the electronics of the NX units. The design intention is to use two separate power supplies for these two circuits. 

In a well designed control system, a " clean" power supply is used in to feed power to all loads with CPUs, such as PLCs, HMIs, Remote IO systems, Vision systems etc. A "dirty" power supply is used for all loads which will be subjected to noise, such as inputs and outputs, outputs which switch contactors and relays, VSDs etc. Bear in mind that all the wires to the inputs around your machine are effectively antennas, picking up and channeling electrical noise to your control system. 

It is tempting to use one larger 24VDC power supply and connect to both the UV / UG and IOV / IOG terminals in order to save cost. However by doing this, the galvanic barrier which was implemented to avoid electrical noise from bridging from the IO side to the internal circuitry is effectively bypassed. 

While it is possible to not experience problems with such a single power supply setup, under higher noise environments which lead to abnormal behavious such as communication faults, this is normally the first step in remediation of the control system. 

The below diagram shows how these two power supplies are used, note that both the clean and dirty feed the NX couplers, dirty for IO and clean for Unit power.