Page 46 - Programmable Logic Controllers, Fifth Edition Mobile 2

Sourcing output
module Sinking output
module
0 Sinking 0 Sourcing
1 field device 1 field device
2 2
3 NEG (–) 3 POS (+)
4 Power 4 Power
5 supply 5 supply
6 POS (+) 6 NEG (–)
7 7
(+) (–)
Common Current Common Current

Figure 2-20 Sinking and sourcing outputs.

• Input and output points that are sinking or sourc- module accepts the digital value from the processor and
ing only can conduct current in only one direction. converts it back to an analog signal that drives the field
Therefore, it is possible to connect the external sup- tank level meter.
ply and field device to the I/O point with current Analog input modules normally have multiple input
trying to flow in the wrong direction, and the circuit channels that allow 4, 8, or 16 devices to be interface to
will not operate. the PLC. The two basic types of analog input modules are
voltage sensing and current sensing. Input modules have
2.3 Analog I/O Modules user-selectable dip switch settings to choose whether each
input will be a current or voltage input. Analog sensors
Earlier PLCs were limited to discrete or digital I/O in- measure a varying physical quantity over a specific range
terfaces, which allowed only on/off-type devices to be and generate a corresponding voltage or current signal.
connected. This limitation meant that the PLC could have Common physical quantities measured by a PLC analog
only partial control of many process applications. Today, module include temperature, speed, level, flow, weight,
however, a complete range of both discrete and analog pressure, and position. For example, a sensor may measure
interfaces are available that will allow controllers to be temperature over a range of 0 to 500°C, and output a corre-
applied to practically any type of control process. sponding voltage signal that varies between 0 and 50 mV.
Discrete devices are inputs and outputs that have only Figure 2-22 illustrates an example of a voltage sensing
two states: on and off. In comparison, analog devices input analog module used to measure temperature. The con-
represent physical quantities that can have an infinite nection diagram applies to an Allen-Bradley MicroLogic
number of values. Typical analog inputs and outputs vary 4-channel analog thermocouple input module. A varying DC
from 0 to 20 mA, 4 to 20 mA, or 0 to 10 V. Figure 2-21 voltage in the low millivolt range, proportional to the tem-
illustrates how PLC analog input and output modules perature being monitored, is produced by the thermocouple.
are used in measuring and displaying the level of fluid This voltage is amplified and digitized by the analog input
in a tank. The analog input interface module contains the module and then sent to the processor on command from a
circuitry necessary to accept an analog voltage or cur- program instruction. Because of the low voltage level of the
rent signal from the level transmitter field device. This input signal, a twisted shielded pair cable is used in wiring
input is converted from an analog to a digital value for the circuit to reduce unwanted electrical noise signals that
use by the processor. The circuitry of the analog output can be induced in the conductors from other wiring. When
using an ungrounded thermocouple, the shield must be con-
nected to ground at the module end. To obtain accurate read-
PLC Level ings from each of the channels, the temperature between the
Level indicator
transmitter Analog Analog thermocouple wire and the input channel must be compen-
input Processor output sated for. A cold junction compensating (CJC) thermistor is
module module
integrated in the terminal block for this purpose.
The transition of an analog signal to digital values is
accomplished by an analog-to-digital (A/D) converter,
the main element of the analog input module. Analog
Figure 2-21 Analog input and output to a PLC. voltage input modules are available in two types: unipolar

PLC Hardware Components Chapter 2 27

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