Page 262 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1 Input SW Ladder logic program L1 Input SW Ladder logic program
NEG TOD
SW NEGATE SW TO BCD
Source A N7:52 Source N7:23
101 10
Destination N7:53 Destination O:20
–101 16

Figure 11-17 SLC 500 NEG (negate) instruction. Figure 11-19 SLC 500 TOD (convert to BCD) instruction.

• The positive value 101 stored at the source address • The source displays the value 10, which is the
N7:52 is negated to –101 and stored in destination correct decimal value; however, the destination
address N7:53. displays the value 16.
• Positive numbers will be stored in straight binary • The processor interprets all bit patterns as binary;
format, and negative numbers will be stored as 2’s therefore the value 16 is the binary interpretation of
complement. the BCD bit pattern.
The program of Figure 11-18 is an example of the clear • The bit pattern for 10 BCD is the same as the bit
(CLR) instruction. The operation of the logic rung can be pattern for 16 binary.
summarized as follows: The convert from BCD (FRD) instruction is used to

• When input switch SW is closed the CLR instruc- convert binary-coded decimal (BCD) values to integer val-
tion is executed. ues. This instruction could be used to convert data from a
• Upon execution it sets all bits of a word to zero. BCD external source, such as a BCD thumbwheel switch,
• In this example it changes the value of all bits stored to the binary format in which the processor operates. The
in the destination address N7:22 to 0. program of Figure 11-20 is an example of the FRD instruc-
tion. The operation of the logic rung can be summarized as
The convert to BCD (TOD) instruction is used to follows:
convert 16-bit integers into binary-coded decimal
(BCD) values. This instruction could be used when • When input switch SW is closed the FDR instruc-
transferring data from the processor (which stores data tion is executed.
in binary format) to an external device, such as an LED • The BCD bit pattern stored at the source address
display, that functions in BCD format. The program of I:30 is converted into a binary bit pattern of the same
Figure 11-19 is an example of the TOD instruction. The decimal value at the destination address, N7:24.
operation of the logic rung can be summarized as follows: At times it is necessary to make conversions to ana-

• When input switch SW is closed the TOD instruc- log input and output values to ensure correct interpreta-
tion is executed. tion and processing. As a result, data must be scaled, or
• The binary bit pattern at the source address N7:23 is resized, before it can be used by a PLC control algorithm
converted into a BCD bit pattern of the same deci- or output to a field device. The SLC 500 Scale data (SCL)
mal value at the destination address O:20. and Scale with Parameters (SCP) instructions are used to

Input Ladder logic program
Input Ladder logic program L1 SW
FRD
L1 SW FROM BCD
CLR SW Source I:30
SW CLEAR 16
Destination N7:22
0000000000000000 Destination N7:24
10

Figure 11-20 SLC 500 FRD (convert from BCD)
Figure 11-18 SLC 500 CLR (clear) instruction. instruction.

Math Instructions Chapter 11 243

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