Chapter 5: CPU Operation and Memory 33
Introduction to Registers
The D50 PLC has a series of registers for storing data. Different registers store different types of data.
1. R (Relay) register (Can be bit, byte, or 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 are predefined for the controller
and expansion module I/O.
2. M (Memory) register (Can be bit, byte, or word)
An internal bit memory address which supports relay logic operations. Can also be used as a
byte or 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 byte or word)
Used for general calculations, data storage, and recipe values. Values are cleared after the
power is turned off, or by new program download.
4. K (Keep) register (Can be bit, byte, or 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 (Can only be 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.
Each type of register is used for a variety of purposes. The register used will be determined by the type
of function being performed.
1. When a calculation or input value exceeds 255 ($FF), use double mode instructions which can
store and calculate values up to 65,535 in the K, M, R, and W registers..
2. When a value needs to be stored even through a loss of system power, use the K area. The K
area is preserved unless specifically erased. The W area is erased by program downloads or
loss of power.
3. For bit operations, such as setting, resetting, shifting, or rotating use the M, K, or R registers.
You cannot perform bit operations on W registers.
4. The Set Value of timers and counters is stored in a special area of the W registers, W2048 to
W2303. These values can also be addressed using register type SV. The Set Values are then
referenced as SV000 to SV255.
5. The Present Value of timers and counters is stored above the Set Values in the W registers,
from W2304 to W2559. These values can also be addressed using the PV designation, PV000
to PV255. The Present Values for channels 0 to 16 are maintained in the Stop state. It is also
retentive—the value is maintained through loss of power.
Internal/External Address Designation
• The memory address designation types are R, L, M, K, F, W, SV, PV, SR, and TC.
Types F and TC can only be used to designate bits.
Types W, SV, PV, and SR can only be used to designate words.
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