Specifications

5PLCs
PLC Design Goals
Robustness
PLCs are expected to work awlessly for
years in industrial environments that are
hazardous to the very microelectronic
components that give modern PLCs their
excellent exibility and precision. No
mixed-signal IC company understands
this better than Maxim. Since our
inception, we have led the industry
with exceptional product reliability
and innovative approaches to protect
high-performance electronics from real
environmental dangers, including high
levels of ESD, large transient voltage
swings, and EMI/RFI. Designers have long
embraced Maxim’s products because
they continue to solve dicult analog
and mixed-signal design problems.
Higher Integration
PLCs have from four to hundreds of I/O
channels in a wide variety of form factors,
so size and power can be as important
as system accuracy and reliability.
Maxim leads the industry in integrating
the right features into our ICs, thereby
reducing the overall system footprint
and power demands and making
designs more compact. Maxim has
hundreds of low-power, high-precision
ICs that come in the smallest available
footprints, allowing the system designer
to create precision products that meet
strict space and power requirements.
Communications
The PLC communicates on two paths.
One path is with the process itself, and
the other path is with other PLCs in the
system and with the human machine
interface (HMI), distributed control
system (DCS), or supervisory control
and data acquisition (SCADA) system. If
the PLC is placed so it can communicate
directly with the process machines,
actuators, or sensors, the interface can be
analog, binary (on/o), or digital (1s and
0s). If the interface is analog, the cable
length is limited by noise concerns and
the risk to signal integrity. These analog
interfaces are point-to-point, requiring
signicant amounts of wiring—one
cable for each I/O point. Maxim oers
a complete portfolio of analog signal-
processing solutions for these sensitive
interfaces. For more information on
analog signal-processing content, see
the Analog Input Functions and Analog
Output Functions sections in this chapter.
For PLCs at the lowest level in the factory
(in the “eld”), the communication with
other PLCs as well as these PLCs at higher
levels in the structure occurs through a
digital network called a eldbus. Some
eldbuses must be deterministic (i.e.,
meet strict timing requirements), which
allows them real-time distributed process
control. Other eldbus protocols are not
required to be deterministic. By stringing
multiple PLCs along a single eldbus,
eldbuses signicantly reduce wiring
needs over point-to-point connections
to a higher level PLC in the system. In
addition to having multiple PLCs on a
single eldbus, multiple eldbuses can
be bridged to extend them beyond
their normal hardware limits.
It is not uncommon for the path to the
higher level in the control structure
to be through another PLC. That PLC
commonly communicates to the HMI,
DCS, or SCADA system through the
Ethernet (or Industrial Ethernet, if
determinism is still required at the higher
level). For more information, see the
Fieldbus Functions section in this chapter.
Computation
Aside from performing the
communications functions, PLCs must
process inputs from both sides and
drive outputs in response. Depending
upon the main task of the given
PLC, the processing functions may
be extensive and time critical, thus
demanding sophisticated and fast
CPUs. For more information, see the
CPU Functions section in this chapter.
The architecture of the PLC is modular
and can be separated into distinct
functions. PLCs are commonly divided
into computing modules, I/O modules,
and communications modules. The exact
content of each of these modules will
likely be as diverse as the applications.
I/O modules can cover a broad spectrum
of signal types. These are often dedicated
to a specic application such as a
resistance temperature detector (RTD),
sensor, or thermocouple sensor. In
general, the following capabilities are
needed in a PLC: analog input, analog
output, digital data communications
(e.g., a eldbus), digital I/O, CPU, and
power. We will examine each of these
core functions in this chapter; sensors
and sensor interfaces are discussed in
separate sections within this document.
ANALOG
INPUT
CONTROLLER/
SECURE
MICROCONTROLLER
DIGITAL I/O
= MAXIM SOLUTION
ANALOG
OUTPUT
SENSORS ACTUATORS
OTHER
PLC
UNITS
OTHER
PLC
UNITS
POWER SUPPLY
DIGITAL I/O
FIELDBUS
Simplified PLC block diagram.
www.maxim-ic.com/plc