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Feedback: Secondary (EL5101)

Utilize the EL5101 EtherCAT Terminal to interface with secondary encoders, enhancing measurement precision for various mechanical components by integrating additional position or velocity feedback into the EtherCAT network.


The EL5101 EtherCAT Terminal is an interface for the direct connection of incremental encoders with differential inputs (RS422). A 32/16 bit counter with a quadrature decoder and a 32/16 bit latch for the zero pulse can be read, set or enabled.

Incremental encoders with alarm outputs can be connected at the interface’s status input. Interval measurement with a resolution of up to 100 ns is possible. The gate input allows the counter to be halted. The counter state is taken over with a rising edge at the latch input.

Due to the optional interpolating micro increment function, the EL5101 can supply even more precise axis positions for dynamic axes. In addition, it supports the synchronous reading of the encoder value together with other input data in the EtherCAT system via high-precision EtherCAT distributed clocks (DC).

🔹 Why Use the Encoder Interface (EL5101)?

The EL5101 EtherCAT Terminal is very useful for bringing secondary or auxiliary encoders into the EtherCAT network. Auxiliary encoders can be used to measure the position, or velocity of any linear or rotary mechanism. The position value can be used to close a position/velocity loop or the value can be used as a reference from the RapidCode API.


(1) Auxiliary encoders can used to reference the position or velocity of hydraulic or pneumatic cylinders so the user knows when the cylinder has achieved the desired position or velocity.

(2) Secondary feedback encoders can be used to close a position or velocity loop directly on the load to improve accuracy when backlash and compliance are present in the system. A ball screw actuator is an example of a system that will have some backlash and compliance in the ball nut and screw. A secondary linear encoder could be used to improve the positioning performance by measuring the position of the slide directly.

EL5101 EtherCAT Information

PDO Inputs

Status: Status bits



Value: Encoder position

Latch: Latch position

PDO Outputs

Ctrl: Control bits.



Value: Relevant counter value

Other Useful Terminology

Frequency: Current calculated frequency

Period: Period of the last possible measurement the refresh rate depends on the cycle time and the current frequency.

Window: Number of edges counted during the time window.

🔹 Reading EL5101 PDOs

Our RapidCode API allows us to pull the PDO values by doing some simple calls. See examples below to better understand how to do this.

EXAMPLE 1: Reading PDO Inputs - Value

RapidSetup Network I/O Page

1. Go to RapidSetup “Network I/O” page to look at the Index of the specific PDO Inputs address you want to read.

2. To read “Term 1 (Beckhoff - EL5101).Inputs.Value” you need to call the RapidCode API function NetworkInputValueGet.

See here for more PDO and SDO methods.