Specifications

13PLCs
loops, where signaling is analog using
any current within the loop range.
Practical loop lengths can be up to
tens of kilometers. The only caveat
is that the resistance of the loop
should not cause the transmitter to
run out of voltage while working
to maintain the proper current.
For many sensors, the current provides
all the operating power needed.
Any measured current-ow level
indicates information just as an analog
voltage can indicate information. In
practice, the 4–20mA current loops
operate from a 0 to 24mA current range.
However, the current ranges from 0 to
4mA and 20mA to 24mA are used for
diagnostics and system calibration.
Readings between 0 and 4mA could,
for example, indicate a broken wire in
the system, and similarly, a current level
between 20mA to 24mA could indicate
a potential short circuit in the system.
An enhancement for 4–20mA
communications is the Highway
Addressable Remote Transducer (HART
®
)
protocol, which is backward compatible
with 4–20mA instrumentation. HART
allows bidirectional, half-duplex
communications with microprocessor-
based, intelligent eld devices.
The HART protocol allows digital
information to be carried on the same
pair of wires with the 4–20mA loop.
Maxim has introduced several
devices, including the MAX15500,
MAX5661, and DS8500, which greatly
simplify the design of 4–20mA
loop interfaces in PLC systems.
Signal Protection
The analog output circuitry is
connected to wiring, long and short
in the eld 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 eld
or factories is subject to movement
and vibration which, in time, can
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.
During the intermittent phase, a
product with output conditioning
capability, such as Maxim’s MAX15500 or
MAX5661, can detect the failure before
it is completed. 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 so, in practice,
reporting is done by establishing
minimum timeout periods for detecting
and reporting errors. A large noise
pulse can look like a momentary cable
interruption, but a mechanical cable
interruption tends to last longer than a
noise pulse. This noise interruption can
occur when a large motor is turned on or
o and capacitive or inductive coupling
occurs between its cabling and other
cables in close proximity. Consequently,
waiting for a short time (a fraction of
a second) allows the fault detector
to distinguish between a real cable
intermittent fault and a noise pulse.
Extra safety is provided if more
conditions than just cable health are
monitored. Cable drivers under normal
conditions operate within dened
temperature rise limits, but shorts on
long, higher resistance cables may
still allow the driver to generate the
voltage needed, thus avoiding a voltage
fault detection, but at the expense of
higher power dissipation. Thus, output
driver chip-temperature detection is
needed. Compounding this problem
is the wide range of temperatures
over which industrial equipment
must operate. Ambient temperature
sensing and temperature rise of output
drivers are both often needed.
The eld or factory can be spread over
several acres, so monitoring power-
supply voltage drops or brownout is
also important for system reliability.
Output drivers must have enough
headroom to fully enhance their internal
transistors to avoid excessive power
dissipation even with normal loads.
Managing an Output
Fault
If an output fault occurs, errors must be
latched and presented to a hardware
interrupt pin. This gives the system
microprocessor time to react to short
duration cable outages. By denition,
intermittent cable faults will be
asynchronous and many will occur while
the processor is busy. An interrupt is
generated so the processor can then poll
the output device registers for the exact
condition and respond accordingly.
The output to the eld or factory needs
to be protected against common wiring
errors and shorts. Some faults cannot be
tolerated, such as a direct lightning hit.
However, the outputs should withstand
reasonable fault voltages. The most
common errors are shorts to ground
or to the 24V power supply, and these
errors should be tolerated without
the need to replace components.
Managing System
Functions
Some sensors require excitation to
function, and an output module supplies
such signals. Typical examples are an
AC signal for capacitive and variable
reluctance sensors or a DC signal for
a simple LED in a backlit switch.
The analog output can also provide
other system-management functions
such as monitoring the local isolated
power supply, board temperature, or
calibration.