1 Introduction
1.1 Motivation and target groups for this user manual
Interference-free transmission of signals plays a central role in the field of MCR technology
(measurement, control, regulation). However, signal transmission is affected by increasingly
electrical environments. This holds especially true for the weak signals emitted by the
measuring sensors.
If the measuring signals are low voltages or electric currents that must be securely transmitted,
carefully conditioned or evaluated, then there is an increase in the electromagnetic and
high-frequency interference they are exposed to. This is due to the following reasons:
– The increasing number of electrically operated components in all performance classes,
especially motors operated via frequency inverters and other actuators
– The increasing miniaturization and packing density of device components
– The growing number of wireless communication and control equipment
– The ever-increasing performance of digital systems working with higher transmission
frequencies
Insufficient attention given to the above disturbance variables, incorrect adjustments or lack
of planning, can all affect interference-free signal transmission.
As a precautionary measure, the control signals sent to the active components of technical
systems are provided in an “electrically more robust” way. In general, however, they are
subjected to the same disturbance variables, implementation and planning risks.
This user manual is an introduction to the technical and practical basics of analog data
transmission that are essential for automation and process control technology. In addition,
this user manual points out the risks to functional safety and draws attention to frequent mistakes
that are made during the planning or installation phase or found when troubleshooting
the systems. This user manual is intended for all interested parties, in particular for trainees
and technicians who want to become familiar with analog data transmission in the field of
automation and process control technology.
1.2 What type of signals are involved?
This user manual focuses on analog electrical voltage and current signals, collectively referred
to as “analog signals”. If signals vary smoothly between a minimum and maximum
value, they are referred to as “analog” or “continuous-valued” signals. The value range is
very large and almost infinitely large regarding the measuring accuracy.
Analog signals, for example, are generated using a sensor that records states and state
changes of physical variables and converts them into an electrical signal. Typically, the following
variables are measured in system and process technology:
– Temperature
– Pressure
– Fill level
– Flow rate
– Vibration
– Deformation with regard to load measurement

– Humidity
– Gas concentration
– Electrophysical variables such as voltage, current, field strength, etc.
Electrical conductors are used to transmit analog signals from the signal source to the destination
device. Using a sensor signal, various options are available for the destination device:
– A display device (e.g., fill level indicator in vehicles)
– A control system (e.g., for temperature control of a heating circuit)
– A signal converter (e.g., amplifier for a microphone signal)
A measuring transducer can be connected downstream of the sensor in order to convert the
analog measuring signal into a so-called standard signal, thus enabling further signal processing
via additional standardized, electrical modules. The measuring transducer may already
be integrated in the sensor housing.
Compared to continuous-valued, analog signals, the situation is quite different when binary
signals are considered. They can have only two possible values used to signalize the states
“ON” and “OFF” or “1” and “0” respectively. Binary signals are often equated with “digital”
signals. This is due to the fact that digital signals are usually binary coded. Signals that can
take on a limited number of values in steps, and that are referred to as “discrete-valued” signals
are to be classified between the analog and binary signals.
Continuous-valued (analog) and discrete-valued signals can be measured continuously by
means of sampling and quantization, in this way becoming digital signals. In general, digital
signals are binary coded and further processed in digital computer systems. It is also typical
to reconvert digital signals into analog signals. The devices used for these conversion processes
are referred to as A/D converters and D/A converters.
Conversions between “analog” and “digital” can be performed either by installing converters
specially provided for this purpose or in a hidden way within processing components. The
digitized signal type offers advantages with regard to transmission, storage, lossless copying
and automatic correctability of signals. However, the conversion has some disadvantages:
– Device costs increase.
– Time response may be too slow if there is any need for rapid reactions.
– There are system-related errors (e.g., the resolution may be insufficient for specific applications).
In the field of MCR technology, analog signals are often only evaluated binarily for a control
system. This is the case, for example, when monitoring a temperature that should initiate
countermeasures when exceeding a limit value. The currently measured temperature, for
example, can only be used for comparison purposes to determine whether the temperature
will exceed or fall below the limit value.
The transmission of analog signals in the area of telecommunications or the transmission of
analog useful signals through modulation of a significantly higher-frequency carrier signal is
outside the scope of this user manual.

2 Basics
2.1 Signal conditioning in MCR technology
2.1.1 Measuring signals
In the field of MCR technology, an analog measuring signal generally passes through the
following stations:
1. A sensor reacts to a physical variable and converts it to an electrically evaluable signal.
Either the sensor generates a voltage in the circuit or changes the circuit to which it is
connected and that is supplied by a source of current or it changes the voltage drop
along the electrical circuit fed with constant current. Sensors converting physical variables
to electrical variables for measurement reasons are often referred to as measuring
transducers or transmitters.
Sensors are typically used to measure the following physical variables:
– Temperature
– Pressure
– Substance concentrations
– Frequency (e.g., speed, flow rate)
– Electromagnetic and electrical properties (e.g., light, high-energy radiation, conductivity)
2. In general, the sensor is connected to an interface module used for signal conditioning.
It is an electronic module which can have one or several of the following functions:
– Electrical amplification, filtering and standardization of the measuring signal
– Electrical isolation of the measuring circuit from the device output circuit
– Electrical power supply of the sensor, if required
– The sensor and the interface module can be installed together in one housing. A
device integrated in this way is sometimes referred to as transmitter.
3. The conditioned measuring signal is transmitted to a device or system which evaluates
and further processes the measuring information. This can either be a display device or
a control system with a very simple or highly complex structure. Depending on the characteristics,
the following designations are commonly used for control systems:
– PLC (Programmable Logic Controller)
– DDC (Direct Digital Control)
– DCS (Distributed Control System/process control system)
In simple MCR systems, it is possible to combine interface blocks and the control system
in one device. The sensor may also be added, if required.
4. In industrial control systems, information is usually transmitted using communication
bus systems. These systems enable a variety of information to be transmitted using
only a limited number of electrical cable connections. An analog sensor signal needs to
be conditioned for its transmission on a bus system. Conditioning takes place in an interface
module and generally covers the following points:
– Digitization of the analog signal
– Signal integration into the bus access protocol (including addressing)
5. Transmission on the bus to the control system. In more extensive bus systems, several
subsections can be used, if required. These are provided with repeater modules to
compensate signal losses.

The electrosensory acquisition, conditioning and evaluation of status data referring to the
environment or an industrial system are considered to be the fundamental and core areas
in the field of MCR technology. Figure 2-1 provides a schematic view of these three areas:
– The signal acquisition in the “field”, as the monitored area to be controlled is called.
– Signal “conditioning” by means of electronic components for amplification, conversion
and protection from interferences on the signal path.
– Analog and/or digital signal processing in an evaluation and control unit.
Figure 2-1 Analog signal from the sensor to the control unit
Analog IN/OUT
Digital IN
Temperature
Frequency
Potentiometer