Page 243 - Programmable Logic Controllers, Fifth Edition - Mobile version

• When the weight reaches a preset value, the flow is
cut off.
• While the vessel fills, the PLC performs a compari-
son between the vessel’s current weight and a pre-
determined constant programmed in the processor.
• If the programmer uses only the equal instruction,
BCD
problems may result. Thumbwheel input module
switch
• As the vessel fills, the comparison for equality will (TWS) Bit
be false. At the instant the vessel weight reaches the Decimal BCD address
desired preset value of the equal instruction, the in- 1 0
struction becomes true and the flow is stopped. 0 1
1s units 5 1 2
• However, should the supply system leak additional 0 3
material into the vessel, the total weight of the ma-
terial could rise above the preset value, causing the 0
instruction to go false and the vessel to overfill. 1 4
5
10s units 6 1
• The simplest solution to this problem is to program 0 6
7
the comparison instruction as a greater than or equal
to instruction. This way, any excess material enter- 1
ing the vessel will not affect the filling operation. 1 8
100s units 7 1 9
• It may be necessary, however, to include addi- 0 10
tional programming to indicate a serious overfill 11
condition.
0
0 12
1000s units 0 0 13
10.5 Numerical Data I/O Interfaces 0 14
15
The expanding data manipulation processing capabilities
of PLCs led to the development of I/O interfaces known L1
as numerical data I/O interfaces. In general, numerical L2
data I/O interfaces can be divided into two groups: those
that provide interface to multibit digital devices and those Figure 10-38 BCD input interface module connected to a
that provide interface to analog devices. thumbwheel switch.
The multibit digital devices are like the discrete I/O Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com.
because processed signals are discrete (on/off). The dif- The seven-segment LED display board, shown in Fig-
ference is that, with the discrete I/O, only a single bit is ure 10-39, is a typical Binary Coded Decimal (BCD) output
required to read an input or control an output. Multibit device. It displays a decimal number that corresponds to the
interfaces allow a group of bits to be input or output as a BCD value it receives at its input. Conversion of the four bi-
unit. They can be used to accommodate devices that re- nary bits to a single decimal digit on the display is performed
quire BCD inputs or outputs. by the LED display device. The BCD output module is used
The thumbwheel switches (TWS), shown in Fig- to output data from a specific register or word location in
ure 10-38, are typical BCD input devices. Each one of the memory. This type of output module enables a PLC to oper-
four switches provides four binary digits at its output that ate devices that require BCD coded signals.
correspond to the decimal number selected on the switch. Figure 10-40 shows a PLC program that uses a BCD
The conversion from a single decimal digit to four binary input interface module connected to a thumbwheel switch
digits is performed by the TWS device. The BCD input and a BCD output interface module connected to an LED
module allows the processor to accept the 4-bit digital display board. The program is designed so that the LEDs
codes and input their data into specific register or word display the setting of the thumbwheel switch. Both the
locations in memory to be used by the control program. MOV and EQU instructions form part of the program. The
Data manipulation instructions can be used to access the operation of the program can be summarized as follows:
data from the input module allowing a person to change
set points, timer, or counter presets externally without • The LED display board monitors the decimal set-
modifying the control program. ting of the thumbwheel switch.

224 Chapter 10 Data Manipulation Instructions

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