Specifications

4 Maxim Industrial Solutions
Programmable logic controllers (PLCs)
Overview
makes system communications easy.
Expanding on this concept is a house-
hold forced-air heater with a remote
thermostat. Here the communication
paths are just a few meters and a
voltage control is typically utilized.
Think now beyond a small, relatively
simple process-control system.
What controls and configuration are
necessary in a factory?
The resistance of long wires, EMI,
and RFI make voltage-mode control
impractical. Instead, a current loop
is a simple, but elegant solution.
In this design wire resistance is
removed from the equation because
Kirchhoff’s law tells us that the
current anywhere in the loop is
equal to all other points in the loop.
Because the loop impedance and
bandwidth are low (a few hundred
ohms and < 100Hz), EMI and RFI
spurious pickup issues are minimized.
A PLC system is useful for properly
controlling such a factory system.
Current communication
for PLCs
Current-control loops evolved from
early twentieth-century teletype
impact printers, first as 0–60mA
loops and later as 0–20mA loops.
Advances in PLC systems added
4–20mA loops.
A 4–20mA loop has several advan-
tages. Older discrete component
designs required careful design
calculations; circuitry was compara-
tively large compared to today’s
integrated 4–20mA ICs. Maxim has
introduced several 20mA devices,
including the MAX15500 and
MAX5661, which greatly simplify the
design of a 4–20mA PLC system.
Any measured current-flow level
indicates some information. In
practice, the 4–20mA current loops
operate from a 0mA to 24mA current
range. However, the electrical current
ranges from 0mA to 4mA and 20mA
to 24mA are used for diagnostics
and system calibration. Since current
levels below 4mA and above 20mA
are used for diagnostics, one might
conclude that readings between
0mA and 4mA could indicate a
broken wire in the system. Similarly,
a current level between 20mA and
24mA could indicate a potential
short circuit in the system.
An enhancement for 4–20mA
communications is the highway-
addressable remote transducer
(HART™ system) which is backward
compatible with 4–20mA instru-
mentation. A HART system allows
two-way communications with
smart, microprocessor-based, intelli-
gent field devices. The HART protocol
allows additional digital information
to be carried on the same pair of wires
with the 4–20mA analog current signal
for process-control applications.
PLCs can be described by separating
them into several functional groups.
Many PLC manufacturers will organize
these functions into individual
AC
PLUG
RELAY
FAN
FAN
THERMOSTAT
HEATER
ELEMENT
ROOM
THERMOSTAT
A household electric heater serves as a simple example of process control.
CONTROL ROOM INDUSTRIAL-SIZED HEATER
ONE KILOMETER OF WIRE
Longer-range factory communications.
ANALOG
INPUT
CONTROLLER/
SECURE
MICROCONTROLLER
DIGITAL I/O
= MAXIM SOLUTION
ANALOG
OUTPUT
SENSORS
ACTUATORS
OTHER
PLC
UNITS
OTHER
PLC
UNITS
ISOLATED POWER-SUPPLY FUNCTIONS
DIGITAL I/O
FIELDBUS
Simplified PLC block diagram. For a list of Maxim's recommended PLC solutions, please go to: www.maxim-ic.com/plc.