Digital input serializers (DIS), the heart of digital input modules, must operate in harsh industrial environments. Voltage and current transients caused by electrostatic discharge, switching of inductive loads, or lightning strikes, will corrupt the data flow between field inputs and device output unless effective measures are taken to diminish transient impact.
This article provides a short overview on the system level operation of a digital input serializer, introduces the three types of transients commonly experienced in industrial applications, and suggests a protection circuitry that ensures reliable data communication between the field inputs and the serial output of a DIS device.
System Operation and Transient Immunity
A digital input serializer is designed for industrial applications using 12V and 24V nominal voltage rails (Figure 1). The device provides eight parallel inputs that connect to a wide variety of sensor switches. Information on the switches' On-Off status is level-shifted and filtered by a signal conditioning stage whose eight parallel outputs are latched into a serializer. Under the control of a system controller the serializer content is clocked out serially. A digital isolator provides an isolated interface for the control and data lines between the serializer on the field-side, and the system controller of a PLC- or PC-based system on the control-side.
Ensuring reliable data communication in a noisy environment requires that the energy of electrical transients occurring at the field inputs is absorbed before it can unintentionally change the logic input states within the device. Preventive measures in form of transient immunity tests are therefore applied, indicating whether a digital input module is able to reliably transmit field input data to the PLC despite transient occurrences.
Figure 1. Basic system level diagram.
Transient Immunity Tests
The following transient immunity tests in Figure 2 are part of the IEC61000-4 family of electromagnetic compatibility tests, specified by the International Electrotechnical Commission (IEC).
The test for Electrical Fast Transients (EFT) or Burst Immunity, IEC61000-4-4, often represents the most important test as it simulates everyday switching transients caused by the interruption of inductive loads, relay contact bounce, etc. The test is performed on supply, data, and earth ports. A burst is defined as the sequence of pulses of limited duration. In this case, a burst generator produces a sequence of test pulses with a decay time (down to 50 percent of the peak value), of less than 100ns. The typical duration of a burst is 15ms at a repetition rate of 5kHz. The burst period, the time from one burst start to the next, is 300ms. Significant for the test pulse are its short rise time, the high repetition rate, and its low energy content.
While the Surge Immunity Test, IEC61000-4-5, is the most severe transient immunity test in points of current and duration, its application is often limited to long data and supply lines (L > 30m). This test simulates switching transients caused by lightning strikes (direct strike or induced voltages and currents due to an indirect strike), or the switching of power systems including load changes and short circuits. A surge generator's output waveforms are specified for open- and short-circuit conditions. The ratio of the open-circuit peak-voltage to the peak short-circuit current is the generator output impedance. Characteristic for this test are the high current, due to low generator impedance, and the long pulse duration (approximately 1000-times longer than for ESD and Burst tests), indicating a high-energy pulse.
The Electrostatic Discharge (ESD) Immunity Test, IEC61000-4-2, simulates the electrostatic discharge of an operator directly onto an adjacent electronic component. Electrostatic charge usually develops in low relative humidity, and on low-conductivity carpets or vinyl garments. To simulate a contact discharge event, an ESD generator applies an ESD pulse to the equipment under test. Characteristic for this test are the short rise time and the short pulse duration of less than 100ns, indicating a low-energy, static pulse. ESD immunity tests have low priority as their potential occurrence is limited to the handling, installation and maintenance work of input modules, during which operators are advised to wear ESD protective clothes as well as to intentionally discharge themselves prior to any direct contact with the module.
Figure 2. Examples of a Burst- (top), a Surge- (middle), and an ESD pulse (bottom).
Note: currents and voltages are normalized. For detailed values and test levels, refer to the actual standard.