Bryan L. Programmable controllers. Theory and implementation
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Figure 9-82. Logic matrix functional block.
Figure 9-83. Logic matrix function block used to mask out bits.
During its operation, a logic matrix function block has three possible
outputs. It energizes the top output when the control line is enabled, it
energizes the middle output once the operation is done, and it energizes the
lower output if an error occurs. As an example, let’s examine the logic
function block shown in Figure 9-84, which has a length of 8 and an AND
logic function. When the control input enables the block, the logic function
will AND the contents of registers 1000 through 1007 with the contents of
registers 1100 through 1107, placing the result of the operation in registers
2000 through 2007. Each register typically holds 16 bits of data. So, in this
case, the function block will AND the 128 bits in registers 1000–1007 with
the 128 bits in registers 1100–1107 and store the result (128 bits) in another
matrix (registers 2000 through 2007).
Some controllers have only two operand registers (e.g., R1000 and R1001).
When these controllers perform a logic operation, they store the result in one
of the operand registers, erasing the operand data previously stored in that
Reg 1000
Reg 1100
Reg 2000
Length 01
Logic Function
(AND, OR, NOT)
10010
171615141312111076543210
1000110100101001
Register (Reg 1000)
0000111100000000
Mask (Reg 1100)
0000110100000000
Result after (Reg 2000)
logical AND
AND
Only bits passed
(Others are masked out)
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Figure 9-84. Logic matrix function block example.
DATA CONVERSIONS
Data conversion instructions change the contents of a given register from one
format to another. Typical data conversion instructions include BCD-to-
binary, binary-to-BCD, absolute, complement, and inversion.
A BCD-to-binary (BCD-BIN) data conversion instruction (see Figure 9-85)
converts BCD input data from field devices, such as thumbwheel switches,
into binary format. This conversion allows the input data to be used in math
operations. Conversely, a binary-to-BCD (BIN-BCD) instruction converts
data from the PLC into BCD format, so that field devices that operate in BCD
(e.g., seven-segment LED indicators) can use it (see Figure 9-86).
The operation of a data conversion block is basically the same regardless of
whether it is performing a BCD-BIN or a BIN-BCD conversion. When the
control input is enabled, the block converts the contents of the first register
(either BCD or BIN) into binary or BCD, depending on the conversion
register. A data transfer of that register’s contents to another register(s) prior
to executing the logic matrix block can prevent loss of operand data when
using this type of block function.
Reg 1000
Reg 1100
Reg 2000
Length 08
AND
10010
Reg 1000 Holds data to be masked
Reg 1100 Holds mask
Reg 2000 Holds results
Holding
Register
1000
1001
1002
1003
1004
1005
1006
1007
Mask
Register
1100
1101
1102
1103
1104
1105
1106
1107
Result
Register
2000
2001
2002
2003
2004
2005
2006
2007
Logic
AND
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instruction. It then places the result of the conversion in the second register
and energizes the block output when the instruction is finished. Some PLCs
allow multiple registers to be converted at the same time by specifying a
length in the instruction (see Figure 9-87).
Figure 9-85. BCD-to-binary data conversion.
Figure 9-86. Binary-to-BCD data conversion.
0101100001110110
Reg 1000
0001011011110100
Reg 1200
5 8 7
Reg 1000
Reg 1200
BCD BIN
10010
BCD number
is input into
register
6
Reg 1000 Holds BCD value
Reg 1200 Holds binary value after conversion
(5876 binary)
(5876
BCD)
Reg 1000
(5876
binary)
0101100001110110
Reg 1200
(5876
BCD)
Reg 1000
Reg 1200
BIN BCD
10010
Reg 1000 Holds binary value
Reg 1200 Holds BCD value after conversion
BCD number is transferred
to seven-segment LEDs via
block transfer or other instruction
0001011011110100
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Figure 9-88. Absolute/complement/invert functional block.
Figure 9-87. Multiple-register binary-to-BCD conversion.
Absolute, complement, and invert operations usually occur within a single
register. In other words, the operation stores the result in the register location
that the operand occupied. Figure 9-88 shows a typical absolute/comple-
ment/invert block, which operates as follows:
• An absolute (ABS) functional block computes the absolute value
(always positive) of the operand register’s contents. Thus, if register
1000 contains the value –5876, the result of the block instruction will
be +5876. This value will be stored in register 1000.
•A complement (COMPL) functional block changes the sign of the
operand register’s contents. For example, if register 1000 contains the
value +5876, the result of the complement instruction will be –5876.
Similarly, if register 1000 held the value –7654, the result of the
complement would be +7654.
Binary
Register
1000
1001
1002
1003
1004
1005
1006
1007
BCD
Register
1100
1101
1102
1103
1104
1105
1106
1107
Length = 8
8 registers are converted after execution
Reg 1000
ABS
COMPL
INV
10010
Absolute
• Makes number positive
• Before execution Reg 1000 = –5,876
• After execution Reg 1000 = +5,876
Complement
• Changes sign of value stored in register
• Before execution Reg 1000 = +5,876 or
Reg 1000 = –7,654
• After execution Reg 1000 = –5,876 or
Reg 1000 = +7,654
Inversion
• Inverts every bit in a register
• Before execution Reg 1000 = 0000 1111 0000 1111
• After execution Reg 1000 = 1111 0000 1111 0000
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SET CONSTANT PARAMETERS
Sometimes a constant value, which will be used later in the program for
comparisons or set points must be stored in a register. For this reason, some
PLCs provide a set constant parameters (SET) block instruction, which
allows a fixed value to be assigned to a register. When a set constant
parameters block is enabled (see Figure 9-89), it sets the referenced register
(register 1000) equal to the value specified (in BCD, binary, etc.) and turns
ON the output when the operation is completed. This instruction is very useful
when resetting storage or I/O registers to zero during their initialization.
Figure 9-89. Set constant parameters functional block.
INCREMENT
An increment (INCR) instruction (see Figure 9-90) increases the contents of
a register by one. This instruction is useful, for example, when keeping track
of events or the number of executions of a routine. An increment block may
also be used with a counter that has a large preset count to keep track of how
many times the maximum count has taken place.
• An invert (INV) functional block inverts all of the bits in the operand
register. If the binary number in register 1000 is 0000 1111 0000
1111, the number will be 1111 0000 1111 0000 after the instruction,
and the block output will be ON when the instruction is finished.
Figure 9-90. Increment functional block.
Reg 1000
=
3456
SET
After Execution Reg 1000 = 3,456
10010
Done/Enable
Reg 1000
INCR
Before Execution Reg 1000 = 123
After Execution Reg 1000 = 124
10010
Done/
Enable
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Figure 9-92. Right-rotate execution.
SHIFT AND ROTATE
A shift (SHIFT) instruction moves the bits in a register to the right or to the
left. Figure 9-91 illustrates the execution of a right-shift instruction. A left-
shift instruction is identical, except that the bit is moved in the opposite
direction (shifting out the most significant bit). Shift blocks use bit-in and bit-
out variables to specify the location of the bit whose value will be shifted—
a bit-in variable is the value to be added to a register, while a bit-out variable
is the value to be deleted from a register. These bits can be real I/O locations
that can be used to input or output data through the shift operation.
Figure 9-91. Right-shift execution.
A rotate (ROT) instruction, like a shift instruction, shifts data to the right or
left; but instead of losing the shift-out bit, this bit becomes the shift-in bit at
the other end of the register (rotated bit). Figure 9-92 illustrates the operation
of a right-rotate instruction.
10101010101010101
Register XMSB LSB
Shift-in Bit
Before
Shift (Right)
1010101010101010
Register XMSB LSB
Shift-out Bit
After
Shift (Right)
1
1010101010101010
Register XMSB LSB
Rotate
bit-out
Rotate
bit-in
Before
Rotate (Right)
1010101010101010
Register XMSB LSB
After
Rotate (Right)
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Figure 9-93 illustrates functional blocks for both the shift and rotate func-
tions. The control input enables the blocks’ operation. Some block instruc-
tions have right and left lines to determine the shift or rotate direction; others
may indicate shift-right (SHFR) or shift-left (SHFL) and rotate-right (ROTR)
or rotate-left (ROTL) in the instruction. Also, a shift/rotate block may have
several variables available inside it, depending on the PLC model. In general,
Figure 9-93. (a) Shift and (b) rotate functional blocks.
Figure 9-94. Example of a right-shift instruction.
the first register stores the data to be shifted or rotated. If a length is specified,
the first register is the starting location. For example (see Figure 9-94), if the
length is 3 in a right-shift instruction, then the block operation will encompass
48 bit shifts (16 bits/register × 3 registers = 48 bits), starting from register 1000
and ending at register 1002. The number of bits indicates the number of bit
shifts or bit rotates that take place simultaneously when the control input goes
from OFF to ON. Shift and rotate instructions are very useful in applications
where a PLC must track the status of inputs along a path of travel (e.g.,
overhead conveyors in a parts-painting process).
10101010101010100
1010101010101010
1010101010101010
Shift Length =
# of Bits =
Shift in Bit =
3 (48 bits)
1
0
Reg 1000
Reg 1001
Reg 1002
1
Reg 1000
Length 16
# of Bits 1
ROT
10010
11
12
Control
Right
Left
Reg 1000
Bit In 200
Bit Out 300
Length 16
# of Bits 1
Shift
10010
11
12
(b)(a)
Control
Right
Left
Bit in or out can be a real I/O address
or a bit in a register
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EXAMPLE 9-10
A PLC application controls a batching process where the reading of
a temperature input (Batch Temp) is critical to the process. The
process’s temperature transducer is connected to a four-channel, 0–
10 VDC analog input module with a 12-bit resolution. The remaining
four bits of each channel are used as status indicators for the module
(see Figure 9-96). Illustrate how to test for a fault in this analog input
interface’s critical temperature measurement.
S
OLUTION
By testing bit 17 of register 1000 (which is the destination of the critical
temperature reading channel) for an OFF condition, we can determine
if the channel has failed. Figure 9-97 shows how an XBOFF instruc-
tion accomplishes this test. If bit 17 is OFF, a fault has occurred; if it is
ON, the channel is OK. The instruction that drives the logic of this
EXAMINE BIT
An examine bit (XB) functional block examines the status (ON or OFF) of a
single point, or bit, in a memory location. This type of instruction is used
when “flags” are set during a PLC program and then later tested and
compared. A flag is a bit that is specially marked for later examination. In an
examine bit ON (XBON) instruction, the block examines the bit position
specified in the register for an ON condition. It then energizes the output if the
status of the bit is ON. Conversely, in an examine bit OFF (XBOFF)
instruction, the instruction energizes the output if the specified bit is OFF.
Figure 9-95 illustrates an examine bit block.
Figure 9-95. Examine bit functional block.
Reg 1000
Bit 10
XBON/XBOFF
Examines bit 10 of register 1000 for an
ON (XBON) or an OFF (XBOFF) status.
10010
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Figure 9-96. Analog input interface for a batching application.
Figure 9-97. A fault has occurred if register 1000 bit 17 is 0 (OFF).
instruction is a contact that is closed by the program when the reading
of the analog signal takes place. For the instruction to be operational
at all times, even when no reading is taking place, the instruction block
enable line must be programmed directly to the left power rail without
any contact instructions.
171615141312111076543210
12-bit analog data
in BCD format
Channel OK (ON)
Overflow (ON)
Underflow (ON)
Sign (ON = (–), OFF = (+))
1A
1B
2A
2B
3A
3B
4A
4B
Channel 0: To Register 1000
Channel 1: To Register 1001
Channel 2: To Register 1002
Channel 3: Not Used
Batch Temp
Cooler Temp
Boiler Temp
Analog Input
No Connection
Reg 1000
Reg 1000
Bit 17
XBOFF
100
Analog
Module
Enabled
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9-12 DATA TRANSFER INSTRUCTIONS
Data transfer instructions move, or transfer, numerical data within a PLC,
either in single register units or in blocks (a group of registers). These
instructions can move data to or from any location in the memory data table,
with the exception of user-restricted areas. Typical uses of data transfer
instructions are the movement of constant and/or preset values to counters and
timers, the reading of analog inputs and multibit input modules, and the
transferring of data to output modules.
As with other instructions, data transfer instructions may be in either ladder
or functional block format, although block format is most common. The
ladder format functions used to transfer data are the get (GET) and put (PUT)
instructions (see Figure 9-98), which are generally used with PLCs that
provide basic ladder format implementation of arithmetic and data compari-
son instructions. A GET data transfer instruction accesses data from a certain
register, whereas a PUT instruction stores data in a specified register.
Figure 9-98. GET and PUT instructions used in the ladder format.
The functional block group of data transfer instructions forms perhaps the
most useful set of functions available in enhanced PLCs, after the basic relay
instructions. The names of the data transfer instructions may differ depending
on the controller, yet they implement the same transfer functions. Table 9-9
shows the different instructions available with data transfer operations.
MOVE
A move (MOV) functional block instruction transfers information from one
location to another, with the destination location being a single bit or register.
Figure 9-99 shows move bit (MOVB) and move register (MOVR) functional
blocks. Some PLCs also offer move byte instructions.
Reg 1000
10
ADD
PUT
GET
Reg 1001 Reg 2000
GET
+
Reg 2000
11
GET
Reg 3000
If contact 10 closes, the contents of registers 1000 and 1001
are added and stored in register 2000. If contact 11 closes, the
contents of register 2000 are transferred (stored) into register
3000. The contents of register 2000 are not altered.