Beckhoff i/o systems: digital inputs for 2-wire and 3-wire sensors according to en 61131-2

Keywords
Application Note DK9222-0909-0008
I/O Systems
3-wire sensorsEN 61131-2digital inputinput charakteristicsFieldbus BoxEtherCAT BoxBus TerminalEtherCAT Terminal Beckhoff I/O systems: Digital inputs for 2-wire and 3-wire
sensors according to EN 61131-2
This application example explains why the characteristic of the input is decisive in the selection of digital
inputs for different sensors. The EN 61131-2 standard for current sinking digital inputs defines three types
that differ in current consumption and logic level and are only compatible to one another to a limited
extent. The basic principles and limit values contained in the standard will be presented in addition to an
overview of EN 61131-2 products from Beckhoff. The overview can also be found at www.beckhoff.com/
EN61131-2.

Fig. 1, left: Voltage supply and signal on one wire; right: separate wires for voltage and signal
Difference between 2-wire and 3-wire sensors
There is a basic conflict of goals in the recording of digital input signals in the I/O field: as opposed to 3-wire sensors, 2-wire
sensors reduce the amount of wiring and the use of materials; however, the current required to drive the sensor causes
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Application Note DK9222-1009-0008
I/O Systems
constant power loss with corresponding heat losses at the I/O modules. 3-wire sensors have an external voltage supply; the digital inputs for this type of sensor have low power consumption. 2-wire sensors conduct the signal and the voltage supply via the same wire. Therefore, in order to maintain the function of the sensor, a permanent minimum current, the so-called quiescent or residual current, must flow via the wires and the digital input. The residual current must be lower than the switching threshold of the following coil or switching element while at the same time ensuring the reliable operation of the sensor.
The sensors and the digital inputs of the controller are chosen in accordance with the EN 61131-2:2003 standard, to which the manufacturers of operating equipment and peripheral devices of controllers are bound. The standard describes the operating equipment requirements and tests for the PLC in order to ensure the compatibility of peripheral devices and controllers. Depending on the threshold value (see fig. 2 for an overview) for switching from high to low, the digital inputs are suitable either for 2-wire or 3-wire sensors, since the residual current also flows via the digital input of the controller.
Why are there three types?
The three types (1 – 3) distinguish the development steps in sensor technology. Type 1 is suitable for electromagnetic switching
devices, such as relay contacts, push buttons, switches, etc. and dates from the time when mainly mechanical contacts were
used and semiconductors were not so widespread. Type 1 is therefore only suitable to a very limited extent or not at all for
the use of 2-wire sensors, for which a high quiescent current is required. For this reason, digital inputs for 3-wire and 2-wire
sensors are separated, being represented by type 1 and type 2 in the standard.
In comparison with the present state of the art, the power consumption of semiconductor-based circuits in earlier 2-wire sensors was several times higher. Therefore, according to EN 61131-2, a maximum current consumption of 30 mA is possible by the sensor; this value is adapted to the standard for 2-wire proximity limit switches, IEC 60947-5-2. If this range is fully exploited, then type 2 digital inputs are not technically implementable as multi-channel input modules: a current consumption of 30 mA per channel would result in a current consumption of 240 mA to 480 mA or even 960 mA per module with the present typical channel density of 8, 16 or even 32 channels. Even in the case of low voltage 24 V DC, the power consumption of 16 digital input channels would be around 11,520 mW. Beckhoff offers type 2 inputs in the packaging densities 2, 4, 8 and also 16; the inputs exhibit a typical characteristic in accordance with fig. 3 and a current consumption of at least 6 mA. In principle, type 2 digital inputs can be used as type 1 or type 3, but the power consumption is then unnecessarily high.
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Application Note DK9222-1009-0008
I/O Systems
suitable for
Level 0 „LOW“
Transition
Level 1 „HIGH“
3-wire sensors, mechanical switching contacts 2-/3-wire sensors, reduced power consumption Fig. 2 Logic level for current sinking digital inputs (24 V DC) from Beckhoff, harmonised according to EN 61131-2
With the increasing decentralisation and miniaturisation in control technology, an increase in the packaging density of digital inputs was necessary in order to gain installation space in the control cabinet and on the plant. Since the quiescent current is also no longer necessary according to present day requirements, type 3 was defined in the standard. Type 3 digital inputs have lower power consumption than type 2, especially at higher voltages, but nevertheless supply the required quiescent current for driving the sensor. Type 3 inputs have lower heat losses and allow a higher channel density per module; in addition to that, they can be used as type 2 and type 1 inputs.
Operating ranges for current sinking digital inputs
The following graph shows the limits and operating ranges for current sinking digital input circuits, as implemented at Beckhoff
in accordance with EN 61131-2. The operating range consists of the “On range”, the “Transition range” and the “Off range”. It
is necessary that both Vmin and Imin be exceeded in order to leave the “Off range” and that Imin be exceeded before Vmin in order
to enter the “On range”. All input V/I characteristic curves must remain within these limit conditions. The range below 0 V is a
valid part of the “Off range” for DC inputs only.

Fig. 3 Characteristic curves of the three types of input (24 V DC) from Beckhoff
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Application Note DK9222-1009-0008
I/O Systems
Usage capability
The development of heat inside the control cabinet is of great importance in control cabinet construction; therefore type 2
inputs with a high current consumption are to be dispensed with. This aspect is particularly important in air conditioned
control cabinets in order to take the heat losses from the input modules into account in the heat balance when designing the
air conditioning unit. Since the types are compatible with one another, the selection can also be simplified by only installing
type 3 inputs. It is particularly advantageous to use type 3 digital inputs, in which the connected 2-wire sensors are supplied
via a current limit in the high/on state. These inputs are therefore suitable for sensors with a high quiescent current, while still
offering reduced power consumption. The constant flow of current means that the power loss does not increase quadratically,
but linearly. The digital inputs with type 3 characteristic offered from Beckhoff work according to the principle described here.
The inputs are available as Bus Terminals, EtherCAT Terminals, Fieldbus Box or EtherCAT Box and can be integrated in any
existing control environment and in any communication bus via the corresponding couplers. The Fieldbus Box and EtherCAT
Box families are IP 67 modules and can be placed directly on the machine without terminal boxes.
Overview of the digital input modules from Beckhoff
The following products comply exclusively with the type 1 specification
EtherCAT Terminals
EL1034 | 4-channel, potential-free, 24 V DC, IP 20
EL1134 | 4-channel, 48 V DC, IP 20
EL1262 | 2-channel, oversampling, 24 V DC, IP 20
EL1512 | 2-channel, 16 bit counter, 24 V DC, IP 20
The following products comply exclusively with the type 2 specification
Bus Terminals
KL1302 | 2-channel, filter 3.0 ms, 24 V DC, IP 20
KL1312 | 2-channel, filter 0.2 ms, 24 V DC, IP 20
KL1304 | 4-channel, filter 3.0 ms, 24 V DC, IP 20
KL1314 | 4-channel, 2 x 0 V DC, 2 x 24 V DC, filter 0.2 ms, 24 V DC, IP 20
KL1434 | 4-channel, 4 x 24 V DC, filter 0.2 ms, 24 V DC, IP 20
Fieldbus Box
IE100x | 8-channel, filter 3.0 ms, 24 V DC, IP 65
IE101x | 8-channel, filter 0.2 ms, 24 V DC, IP 65
IE1502 | 2-channel, 32 bit counter, 24 V DC, IP 65
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Application Note DK9222-1009-0008
I/O Systems
IE230x | 8-channel, 4 inputs, 4 outputs, 0.5 A max, filter 3.0 ms, 24 V DC, IP 65 IE231x | 8-channel, 4 inputs, 4 outputs, 0.5 A max, filter 0.2 ms, 24 V DC, IP 65 IE232x | 8-channel, 4 inputs, 4 outputs, 2 A max, filter 3.0 ms, 24 V DC, IP 65 IE233x | 8-channel, 4 inputs, 4 outputs, 2 A max, filter 0.2 ms, 24 V DC, IP 65 IE240x | 16-channel, 8 inputs, 8 outputs, 0.5 A max, filter 3.0 ms, 24 V DC, IP 65EtherCAT Terminals EL1024 | 4-channel, for 4 sensors, filter 3.0 ms, 24 V DC, IP 20 The following products comply with the type 3 specification
Bus Terminals
KL1402 | 2-channel, filter 3.0 ms, 24 V DC, IP 20
KL1412 | 2-channel, filter 0.2 ms, 24 V DC, IP 20
KL1404 | 4-channel, filter 3.0 ms, 24 V DC, IP 20
KL1414 | 4-channel, filter 0.2 ms, 24 V DC, IP 20
KL1408 | 8-channel, positive switching, filter 3.0 ms, 24 V DC, IP 20
KL1418 | 8-channel, positive switching, filter 0.2 ms, 24 V DC, IP 20
KL1862 | 16-channel, flat-ribbon cable, filter 3.0 ms, 24 V DC, IP 20
KL1872 | 16-channel, flat-ribbon cable, filter 0.2 ms, 24 V DC, IP 20
EtherCAT Terminals
EL100x | 2-/4-/8-channel, filter 3.0 ms, 24 V DC, IP 20
EL101x | 2-/4-/8-channel, filter 10 µs, 24 V DC, IP 20
EL1104 | 4-channel, filter 3.0 ms, 24 V DC, IP 20
EL1114 | 4-channel, filter 10 µs, 24 V DC, IP 20
EL1202 | 2-channel, < 1 µs input delay, similar to EN 61131-2, 24 V DC, IP 20
EL1252 | 2-channel, time stamp, < 1 µs input delay, similar to EN 61131-2, 24 V DC, IP 20
EtherCAT Box
EP1008 | 8-channel, filter 3.0 ms, 24 V DC, IP 65
EP1018 | 8-channel, filter 10 µs, 24 V DC, IP 65
EP1258 | 8-channel, 2 channel time stamp, filter 10 µs, similar to EN 61131-2, 24 V DC, IP 65
EP1816-0008 | 16-channel, D-Sub, 0.5 A max, filter 10 µs, 24 V DC, IP 65
EP2308 | 8-channel, 4 inputs, 4 outputs, 0.5 A max, filter 3.0 ms, 24 V DC, IP 65
EP2316-0008 | 16-channel, D-Sub, 8 inputs, 8 outputs, 0.5 A max, filter 10 µs, 24 V DC, IP 65
EP2318 | 8-channel, 4 inputs, 4 outputs, 0.5 A max, filter 10 µs, 24 V DC, IP 65
EP2338 | 8-channel, I/O configurable, 0.5 A max, filter 10 µs, 24 V DC, IP 65
For application notes see disclaimer on the last page Beckhoff New Automation Technology
Application Note DK9222-1009-0008
I/O Systems
– EN61131-2 compatible products www.beckhoff.com/EN61131-2
– The modular fieldbus system for automation www.beckhoff.com/Busterminal
– EtherCAT www.beckhoff.com/EtherCAT
– The compact IP 67 modules www.beckhoff.com/Fieldbusbox
– EtherCAT extends its reach into the IP 67 world www.beckhoff.com/EtherCAT-Box
This publication contains statements about the suitability of our products for certain areas of application. These statements are based on typical features of our products. The examp-les shown in this publication are for demonstration purposes only. The information provided herein should not be regarded as specific operation characteristics. It is incumbent on the customer to check and decide whether a product is suit-able for use in a particular application. We do not give any warranty that the source code which is made available with this publication is complete or accurate. This publication may be changed at any time with-out prior notice. No liability is assumed for errors and/or omissions. Our products are described in detail in our data sheets and documentations. Product-specific warnings and cautions must be observed. For the latest version of our data sheets and documentations please visit our website (www.beckhoff.com).
Beckhoff Automation GmbH, October 2009The reproduction, distribution and utilisation of this document as well as the communication of its contents to others without express authorisation is prohibited. Offenders will be held liable for the payment of damages. All rights reserved in the event of the grant of a patent, utility model or design.
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