Cutler-Hammer D320 PLC, User Manual

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Chapter 5: CPU Operation and Memory
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4. Program analysis
Executes the program from its first step to its final step and stores the internal/external output
in the working RAM.
5. Peripheral device signal processing
Stores data from communications module or peripheral device in the internal memory.
The following illustration shows the difference between the relay board and PLC sequence processing.
The relay carries out all sequences simultaneously while the PLC processes sequentially throughout
the program.
Introduction to Registers
The D320 PLC has a series of registers for storing data. Different registers store different types of data.
1. R (Relay) register (Can be bit, word or double word)
Indicates the internal memory address which is directly linked with the real-world external
input/output module. The address and number of R registers used by the I/O module is
determined by the type of module and its location on the I/O backplane.
2. M (Memory) register (Can be bit, word, or double word)
An internal bit memory address which supports relay logic operations. Can also be used as a
word or double-word variable for general calculations and programs. M Registers are non-
retentive—when the power of the PLC is Off or the CPU has stopped, the register value is
reset to 0.
3. W (Word) register (Can be word or double word)
Used for general calculations, data storage, and recipe values. Values are preserved after the
power is turned off, but can be cleared by program downloads or special command words.
4. K (Keep) register (Can be bit, word, or double word)
Same usage as M registers. The K Registers are retentive—the value is preserved when the
power is turned off.
5. F (Flag) register (Only process bit)
These bit registers provide special application specific functions to the programmer of the
PLC. They are also used as diagnostic and system control bits, providing Run/Stop control of
the PLC and other system conditions.
LS1 LS2
X1
X1
T1
T1
Y1
X1
TIM CH = 0
V = 100
M0.0
(OUT)
R15.0
(OUT)
R0.1 R0.2
M0.0
M0.0
TC0
Processing of relay sequence
(parallel process)
Processing of PLC program
(serial process)