Texas Instruments | How To Simplify Isolated 24-V PLC Digital Input Module Designs (Rev. B) | Application notes | Texas Instruments How To Simplify Isolated 24-V PLC Digital Input Module Designs (Rev. B) Application notes

Texas Instruments How To Simplify Isolated 24-V PLC Digital Input Module Designs (Rev. B) Application notes
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How To Simplify Isolated 24-V PLC Digital Input Module
Designs
Anant Kamath, Systems Engineer, Isolation Products, Interface Group
Digital Input Modules
Digital Input (DI) modules are used in Programmable
Logic Controllers (PLCs) and Motor Drives to receive
24-V digital inputs from field sensors and switches.
Isolation is used to manage ground potential
differences. The input signal is interpreted as a logic
high or low using a voltage comparator with hysteresis.
Some form of current limit is also implemented to
avoid excess current draw from the 24-V inputs; power
dissipation is a critical concern in high-density compact
multi-channel designs.
The IEC 61131-2 standard for programmable
controllers specifies three different types of digital
input receivers: Types 1, 2 and 3. Ideal
implementations for Type 3 should draw as close to 2
mA as possible in the ON-state, and have voltage
transition thresholds between 5 V and 11 V.
5V
Schmitt
Trigger
Buffer
Optocoupler
R2
24 V
Sensor/
Switch
(a)
5V
24 V
+
Sensor/
Switch
Schmitt
Trigger
Buffer
Host
Controller
Discrete Current Limit
(~12 components)
±
(b)
Copyright © 2017, Texas Instruments Incorporated
Figure 1. Traditional DI modules a) Basic current
limit b) More accurate but complex current limit
Common Digital Input Implementations
Figure 1 shows the two most common
implementations of Digital Input receivers in use today.
In the first case shown in Figure 1 (a), the voltage
thresholds are set by resistors R1 and R2, and R2
serves as a basic current limit. Such an
implementation for Type 3 inputs can easily draw up to
12 mA at 32-V input. The second implementation
shown in Figure 1 (b) uses several discrete
SLLA370B – August 2017 – Revised March 2018
Submit Documentation Feedback
Field
PLC Digital Input Module
ISO1211
RTHR
24 V
Sensor/
Switch
RSENSE
SENSE
VCC1
IN
OUT
FGND
Host
Controller
GND1
Copyright © 2017, Texas Instruments Incorporated
Figure 2. One channel of a Digital Input Module
with ISO1211
New Solutions for Digital Input Modules
Host
Controller
R1
Optocoupler
components (9 to 15) to implement a better current
limit and controlled voltage thresholds. In this case, for
Type 3, the current draw can be as high as 6 mA
across temperature depending on the design. In both
cases, the current limit is much higher than the ideal
current limit of 2 mA for type 3 inputs. A Schmitt trigger
buffer is usually needed after the optcoupler to provide
hysteresis for noise immunity.
Texas Instruments' ISO1211 and ISO1212 devices are
isolated digital input receivers with integrated current
limit, compliant to IEC 61131-2 Type 1, 2 and 3
characteristics. The ISO1211 is suitable for channel-tochannel isolation and the ISO1212 is suitable for multichannel designs. Figure 2 shows the implementation
of one channel of a Digital Input module with ISO1211.
The resistor RSENSE controls the current limit, and the
resistor RTHR, the voltage transition thresholds. The
recommended values of RSENSE and RTHR and
application circuits for multi-channel and other
scenarios are provided in the ISO121x product
datasheet. The ISO121x simplifies system design by
integrating an accurate current limit, voltage
comparator with hysteresis, reverse polarity protection
and isolation, while not needing field-side power. For
Type 3 characteristics, ISO121x limits the input current
draw to < 2.5 mA, a factor of 5x lower than traditional
approaches.
How To Simplify Isolated 24-V PLC Digital Input Module Designs Anant Kamath,
Copyright © 2017–2018, Texas Instruments Incorporated
Systems Engineer, Isolation Products, Interface Group
1
www.ti.com
Ambient Temp = 25 °C
8-ch Competing Solution with Basic Current Limit
8-ch with ISO1212
Figure 3. Board temperature comparison:
Traditional solution vs ISO1212
Compared to traditional approaches using
optocouplers, designing with ISO121x has the
following advantages:
1. Lower Power Dissipation: The precise current
limit in the ISO121x can reduce the current drawn
from digital inputs by up-to a factor of 5 reducing
power dissipation and board temperature. As
shown in Figure 3, a traditional solution can heat
up to 84°C during a room temperature test, where
as ISO121x based solution is much cooler at 45°C.
2. Smaller boards and modules : ISO121x based
solutions reduce component count leading to
smaller boards. Lower power dissipation also
allows packing more channels in a smaller module.
3. Simplified System Design : With ISO121x, the
IEC 61131-2 input characteristics, current limit and
isolation are guaranteed by the datasheet. No
additional Schmitt trigger buffer is needed. This
simplifies system design.
4. High Speed Operation : ISO121x offers 4 Mbps
data rate and 150 ns latency - allowing much faster
interfaces than general purpose optocouplers.
Multi-channel single-chip solutions
Figure 4 compares the layout of a design with four
ISO1212 devices with a competing 8-channel singlechip solution. The Y-dimension of the board is limited
by the placement of the input screw
terminals/connectors. However, the X-dimension is
higher for the 8-channel single-chip solution. This is
because of additional space taken due to routing
congestion close to IC. On the other hand, since
ISO1212 has only two channels there is more flexibility
in placing the IC closer to the input terminals resulting
in much simpler routing, leading to a smaller solution.
2
Routing Congestion
Near the IC may increase
board area
8-ch with 4x ISO1212
(ISO1212 EVM)
Figure 4. Layout of ISO1212 based design vs
competing 8-channel single-chip solution
Other advantages of ISO121x based solution over
some multi-channel serializer devices include:
1. No need for field side power supply: This saves
cost on connectors/terminal for the 24-V field side
supply as well as the corresponding surge
protection.
2. Channel independence: Damage to one channel
on the field side (say, due to short circuit) does not
impact any of the other channels.
3. Higher speeds: Serialization in multi-channel
devices limits speeds to <20 kHz, while ISO121x
devices can support up-to 2 MHz clocking.
Design for Surge, EFT and ESD Immunity
The ISO121x devices have been designed for Surge,
EFT and ESD immunity according to IEC 61000-4-x
standards. Please refer to the application section of
the product datasheet for design, and layout guidelines
to achieve the best transient immunity.
Conclusion
The ISO1212 and ISO1211 devices bring a modern
solution to Digital Input Modules, integrating IEC
61131-2 input characteristics, voltage comparator with
hysteresis, a precise current limit and galvanic
isolation in a small package. Modules designed with
ISO121x have lower power dissipation, allow higher
channel density, are compact and simple to design
compared to traditional solutions.
Table 1. Alternative Device Recommendations
Device
Optimized Parameters
Performance Trade-Off
SN65HVS880
8-channel digital input
serializer
Non-isolated, 3.6mA current
limit, Needs field side supply
SN65HVS885
8-channel digital input
serializer
Non-isolated, 3.6mA current
limit, Needs isolated DC-DC
How To Simplify Isolated 24-V PLC Digital Input Module Designs Anant Kamath,
Systems Engineer, Isolation Products, Interface Group
Competing single-chip 8-ch-solution
(Source: ISO1I811T EVM from Infineon)
SLLA370B – August 2017 – Revised March 2018
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