Glossary
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cial purposes, such as holding the status input from external devices, while
other bits are available for general use in programming.
bit address
The location in memory where a bit of data is stored. A bit address must
specify (sometimes by default) the data area and word that is being ad-
dressed, as well as the number of the bit.
bit designator
An operand that is used to designate the bit or bits of a word to be used by
an instruction.
bit number
A number that indicates the location of a bit within a word. Bit 00 is the right-
most (least-significant) bit; bit 15 is the leftmost (most-significant) bit.
block
Block can refer to one of three aspects of PC operation: a block in the FM
area, a block instruction (program), or a logic block. A block in the FM is the
unit used to transfer data to and from the File Memory Unit and equals 128
words. Refer to
block instruction, block program,
and
logic block
for defini-
tions of these.
block instruction
A special class of instruction used within ladder-diagram programming to al-
low flowchart-like coding, which is often difficult to write with ladder diagrams.
Function codes for block instructions are indicated between pointed paren-
theses <like this>.
block program
A section of program written within a ladder diagram but based on block in-
structions. Block programs can also contain some, but not all, of the lad-
der-diagram instructions.
buffer
A temporary storage space for data in a computerized device.
building-block PC
A PC that is constructed from individual components, or “building blocks.”
With building-block PCs, there is no one Unit that is independently identifi-
able as a PC. The PC is rather a functional assembly of components.
bus bar
The line leading down the left and sometimes right side of a ladder diagram.
Instruction execution proceeds down the bus bar, which is the starting point
for all instruction lines.
call
A process by which instruction execution shifts from the main program to a
subroutine. The subroutine may be called by an instruction or by an interrupt.
carry flag
A flag that is used with arithmetic operations to hold a carry from an addition
or multiplication operation, or to indicate that the result is negative in a sub-
traction operation. The carry flag is also used with certain types of shift oper-
ations.
clock pulse
A pulse available at a certain bit in memory for use in timing operations. Vari-
ous clock pulses are available with different pulse widths.
clock pulse bit
A bit in memory that supplies a pulse that can be used to time operations.
Various clock pulse bits are available with different pulse widths, and there-
fore different frequencies.
common data
Data that is stored in the LR Area of a PC and which is shared by other PCs
in the same the same system. Each PC has a specified section of the LR
Area allocated to it. This allocation is the same in each LR Area of each PC.