Use the table below to find the current requirements of the devices 

using backplane power. Those devices that are not included in the 

backplane calculations are included in the example’s user-side calculations.

Selecting a User-Side Power Supply

You must provide a power supply that meets your system 

requirements. The following devices require user-side power:

• SLC Servo Module

• Encoders

• I/O modules

• Estop circuitry

• Fast inputs and outputs

You must select a power supply that meets the specifications of a NEC 

class 2 power supply. The power supply must have +5V, ±15V 

capacity, and +24V capacity for fast I/O and Estop circuitry.

Example of the Calculations for User-Side Current Requirements 

In this example, the system includes:

• One seven-slot modular rack

• One 1747-L541 CPU module

• One 1746-IB8 DC input module with eight inputs @ +24V

• One 1746-OV8 DC output module with eight outputs @ +24V

• An SLC Servo Module system that contains:

• Two SLC Servo Modules

• Two termination panels

• Two Allen-Bradley 845H encoders

• Six fast inputs

• Two fast outputs

Using Fast Inputs and Outputs

The fast I/O (FIN1through FIN3, and FOUT1) are 24V DC compatible 

and are used with a user-side +24V power supply. Review potential

24V DC I/O devices for compatibility with the electrical specifications 

as shown in the table below.