Page 347 - Programmable Logic Controllers, Fifth Edition - Mobile version

1756-M08SE
ControlLogix
System
SERCOS interface module
SERCOS Sensors

Receiver Transmitter

AC Induction Motor MP-Series
Brushless
Servo Motors
Fiber optic cables Kinetix 6000
Multi-Axis
Servo Drives

Figure 14-46 Smart belt packaging operation.
Source: Image Courtesy of Rockwell Automation, Inc.

HART • Sine waves of these two frequencies are superim-
HART is an open master-slave communication proto- posed onto the analog signal cables to give simulta-
col developed to communicate with smart field devices. neous analog and digital communications.
Smart field devices contain more information than the • The DC and low-frequency current signals are modu-
traditional 4 –20 mA signal. In addition, they carry out lated by the 1200 and 2200 Hz frequencies; a tech-
some functions that are/were originally programmed nique known as frequency-shift keying or FSK.
within the PLC. HART Protocol allows the simultaneous • As the average value of the FSK signal is always zero
communication of the continuous 4 –20 mA as well as a there is no effect on the 4 –20 mA analogue signal.
second digital communication path resting on top of the • A minimum loop impedance of 230 ohms is re-
analog signal, but not interfering. This allows access to quired for communication.
the data of the field device and/or exact machine-readable
product description of the field devices including their
data and functions. More instrumentation devices are 14.7 Supervisory Control
available with the HART protocol than with any other and Data Acquisition (SCADA)
digital communications technology. In some applications, in addition to its normal control
The HART digital signal is superimposed onto the functions, the PLC is responsible for collecting data,
standard 4–20 mA signal, as illustrated in Figure 14-47. performing the necessary processing, and structuring the
• The digital signal is made up of two frequencies, data for generating reports. As an example, you could
1.2 and 2.2 kHz, representing bits 1 and 0 have a PLC count parts and automatically send the data to
respectively. a spreadsheet on your desktop computer.

HART Smart HART
digital signal instrument
20 mA HART-enabled
I/O
Analog 4 to 20 mA

signal
1 1 0 1 0 0 1 Analog 1.2 and 2.2 kHz
4 mA
HART digital data
Time
Figure 14-47 HART communication protocol.

328 Chapter 14 Process Control, Network Systems, and SCADA

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