____________________________________________________How To Improve Speed and Reliability of Isolated Digital
Inputs in Motor Drives
Anant Kamath, Systems Engineer, Isolation Products, Interface Group

Digital Input Receivers in Motor Drives
Digital Input receivers are used in AC and servo motor
control to interface various 24-V signals to the control
module of the drive. These signals include inputs fromfield sensors and switches, position and speed
feedback encoded as 24-V signals, clock or PWM
inputs for speed control, and emergency stop signals,
such as Safe Torque Off (STO). Isolation is used tomanage ground potential differences.
5VOptocoupler
R224 VSensor/Switchdesigner to independently adjust these parameters.
The ISO121x can reduce power dissipation by a factorof 5x compared to traditional approaches, as shown in
the How To Simplify Isolated 24-V PLC Digital Input
Module Designs TI Tech Note.

RSENSE(sensor / switch /Position Feedback /Safety Device)

Schmitt
Trigger
Buffer

HostController

ISO1211

RTHR24-V Input

SENSE

VCC1INOUT

FGND

HostController

GND1

Copyright © 2017, Texas Instruments Incorporated

Figure 2. Digital Input Module with ISO1211

R1A CMOS based integrated solution such as theISO121x offers several advantages over traditional
optocoupler solutions for motor drive digital inputs.
Figure 1. Traditional implementation of Digital
Input Receivers in Motor Drives
Common Implementations
Figure 1 shows the most common implementation ofDigital Input receivers in motor drive applications in
use today. The voltage thresholds and the receiver
input current are set by the resistors R1 and R2, in
combination with the optocoupler’s input
characteristics. A Schmitt trigger buffer is usuallyneeded after the optocoupler to provide hysteresis fornoise immunity.
New Digital Input Solutions for Motor Drive
Applications

Standard optocouplers have a response time of tens of
micro-seconds, and are not capable of high speed
operation. ISO121x devices offer much higher speed
and response time, and are ideal for high speed digital
inputs such as for position and speed feedback andcontrol signals in motor drives.
Max. Response Time /Propagation Delay

Maximum Clock
Frequency

Optocoupler

20 µs

20 kHzISO121x

140 ns2 MHzDevice

Controlled Thresholds and Noise Immunity

Texas Instruments' ISO1211 and ISO1212 devices are
high-speed isolated digital input receivers withintegrated current limit, voltage comparator withhysteresis and reverse polarity protection. TheISO1211 can be used for channel-to-channel isolationand the ISO1212 is ideal for multichannel designs.
Figure 2 shows the implementation of one channel of aDigital Input receiver with ISO1211. The resistor RSENSE
controls the current limit, and the resistor RTHR, thevoltage transition thresholds. This allows the systems

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Higher Speed and Lower Response Times

Motor drives are noisy systems, and it is imperative todesign interfaces including digital inputs for noise
immunity. Optocoupler input response and current
transfer ratio varies greatly from device to device, andwith temperature. Over a period of time, the LEDs
used in the optocoupler also age, resulting in a further
shift in input and transfer characteristics. Keeping
these factors in mind, digital inputs designed with
optocouplers have a wide variation in input voltage
transition thresholds. To keep the current draw low,
often the system designer is forced to design thedigital input receiver to draw the minimum required
current at a higher input voltage (i.e. the maximum
threshold is pushed higher, beyond the desired

How To Improve Speed and Reliability of Isolated Digital Inputs in Motor
Drives Anant Kamath, Systems Engineer, Isolation Products, Interface Group
Copyright © 2017, Texas Instruments Incorporated1

www.ti.comspecification). In comparison, as shown in Figure 3, anISO121x based solution has controlled thresholds andhysteresis, and sufficient margin to the low and highlevels, to account for noise, variation in the 24-V
supply and voltage drop in the input wiring.

5VOptocoupler+/- 24V

HostController

24 V

24 V

Schmitt Trigger
BufferMax = 18 V
Max = 15 V
Min = 12 V

Min HYST = 1 V
No HYSTISO1212
RTHR

Min = 4 V
0V

0V

CIN

RSENSESENSE1
IN1
FGND1

Optocoupler Solution

VCC1

OUT1

ISO121x
OUT2Figure 3. ISO121x Based Receivers Have Higher
Noise Margin to the High and Low Levels+/- 24V
RSENSESENSE2

HostController

IN2
FGND2

GND1

Current for positive polarity

Safety Monitor
Devce
STO
Switch 1ISO1212

STO1

Current for negative polarity

CIN

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RSENSESENSE1Figure 5. Implementing Bipolar, AC or Sink-Source
Configuration using ISO1212

IN1
FGND1

OUT1
Power Stage
EnableOUT2STO
Switch 224 V

Motor Drive

RTHR

RTHR
STO2

CIN

RSENSE

Bipolar, AC or Sink-Source Operation

SENSE2
IN2
FGND2

GND1

STO_COM

Copyright © 2017, Texas Instruments Incorporated

Figure 4. Safe Torque Off (STO) Signals
Implemented with ISO1212
Increased Reliability
The ISO121x devices, like other digital isolation
products from Texas Instruments are manufactured
using a well-controlled CMOS semiconductor
manufacturing process. These devices have a very
low Failure in Time (FIT) rate of <0.3 per 1 billion
hours of operation. In comparison, optocouplers havea FIT rate of 10 to 30 per 1 billion hours of operation,orders of magnitude worse. The ISO121x devices also
do not age with time, unlike optocouplers.
DeviceFIT rate

Aging

Optocoupler

20Significant

ISO121x

0.3

Negligible

Critical signals such as Safe Torque Off (STO) that are
active-low, and hence operate at a continous highlevel in normal operation can cause accelerated aging
in optocouplers. Safe Torque Off (STO) when
implemented with ISO121x as shown in Figure 4
benefits from a lower FIT rate, and negligible impact
from aging.

Digital inputs for motor drives are sometimes
implemented in bipolar or sink-source configuration. To
implement bipolar operation, usually a full-bridge
rectifier using four diodes is used ahead of anoptocoupler. The ISO1212 device can be used tocreate an input module that can be used for bipolar,
AC or sink-source operation, as shown in Figure 5,
resulting in a compact solution, while maintaining thebenefits of current limit, and precise voltage
thresholds.
Conclusion
The ISO1212 and ISO1211 devices bring a modern
solution for Digital Input receivers in motor drive
applications offering higher speed, well controlled
voltage thresholds, better noise immunity andincreased reliability and longevity compared to
traditional solutions with optocouplers.
Table 1. Alternative Device Recommendations
DeviceOptimized Parameters

Performance Trade-Off

ISO1211

Single-channel isolated
digital input receiver

For channel to channel
isolation

ISO1212Dual-channel isolated
digital input receiver

For group isolated inputs.
Lower cost per channel

8-channel digital input
serializer

Non-isolated, Lower Speed,Needs field side supply

SN65HVS880

Table 2. Adjacent Tech Notes
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2How To Improve Speed and Reliability of Isolated Digital Inputs in Motor
Drives Anant Kamath, Systems Engineer, Isolation Products, Interface Group
Copyright © 2017, Texas Instruments Incorporated

How To Simplify Isolated 24-V PLC Digital
Input Module Designs

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