Specifications

14 Maxim Industrial Solutions
Programmable logic controllers (PLCs)
Analog output functions
Overview
The analog output remotely controls
activities and functions. It can be
part of a complex loop like a PLC or a
proportional-integral-derivative (PID)
system, or it can perform a simple
function such as turning a light or fan
on and off.
The analog output primarily takes
commands from the microprocessor
and translates them into analog and
digital signals to control motors,
valves, and relays. As an example,
a digital word from the CPU can be
converted to an analog voltage or
current by a DAC and signal-
conditioning circuitry. A proper
signal is tailored for each output with
any needed signal conditioning
provided, including bias, offset, and
gain calibration. Calibration issues
are discussed in the chapter entitled
"Trim, calibrate, and adjust" on page 143.
Producing discrete, selectable,
voltage- (bipolar and unipolar) or
current-output conditioning circuits
can be an intimidating task. This
is especially true as one begins to
understand the necessity of control-
ling full-scale gain variations and
the multiple reset levels for bipolar
and unipolar voltages and different
output-current levels.
Signal protection
The analog output circuitry is
connected to wiring, long and short
in the field or factory, so the output
module must protect the system
from ESD, RFI, and EMI. Voltage
outputs tend to be appropriate for
short-distance transmission wiring;
current outputs are commonly used on
long cables to reduce EMI from sources
like arcing switches and motors.
Signal monitoring
Output signal-monitoring functions,
including detection and reporting
of intermittent wire faults, are
important safety considerations.
Cabling in the field or factories is
subject to movement and vibration
which, in time, will cause wires to
open or short to other conductors.
Equipment and personnel must
remain safe, which necessitates
careful monitoring. As a cable is
failing, there is usually a period
of intermittent operation prior to
complete failure. The intermittent
operation offers an opportunity to
detect the error before complete
failure occurs. As an important part of
preventive maintenance, this failure
detection improves safety and helps
to minimize any plant downtime.
Because EMI, RFI, and power-surge
conditions can be extreme in a
factory, any monitoring must be
reliable and not subject to nuisance
tripping. Error reporting must
be robust. In practice, reporting
is done by establishing minimum
timeout periods for detecting and
reporting errors. A large noise
pulse, for example, can appear like a
Analog output functions
Maxim’s product offerings are found throughout this block diagram of PLC analog-output functions.
For a list of Maxim's recommended PLC solutions, please go to: www.maxim-ic.com/plc.
DAC
DAC
DEMUX
ISOLATION
CONDITIONING CIRCUITRY
ANALOG OUTPUT:
V TO V, OR I TO V
TO ALL
ISOLATED POWER SUPPLY
VOLTAGE
REFERENCE
VOLTAGE
MONITORS
HOT-SWAP
CONTROLLER
THERMAL
MANAGEMENT
VOLTAGE/
CURRENT
TO FIELD
WIRING AND
ANALOG
ACTUATORS
EXCITEMENT, BIAS,
CALIBRATION TO
FIELD WIRING AND
INPUT SENSORS
FROM CPU
MODULE
PRECISION
RESISTORS
RFI/EMI
FILTERS
SWITCHED
C FILTERS
SENSOR SIGNAL
CONDITIONER
ESD/SIGNAL
PROTECTION
DIGITAL
POTENTIOMETER
HART
MODEM
= MAXIM SOLUTION