Image Register Memory; Control Relay Memory; Special Memory; Temporary Memory - Siemens SIMATIC 505 User Manual

RLL instruction used in the program requires one or more 16-bit words of
L-Memory.

Image Register Memory

A block of memory within the controller is reserved for maintaining the
status of discrete inputs/outputs. This memory type is called the discrete
image register. A word image register holds the values of word
inputs/outputs.

Control Relay Memory

A block of memory within the controller is reserved for control relays.
Control relays are single-bit internal memory locations and do not
represent actual hardwired devices.

Special Memory

A block of memory within the controller may be allocated for loops,
analog alarms, and Special Function programs. This memory type is
called Special Memory (S-Memory). All loop and analog alarm
parameters are stored in S-Memory when you program the loop or analog
alarm. Likewise, when you create a Special Function program or
subroutine, the program is stored in S-Memory.

Temporary Memory

A block of memory within the controller is temporarily reserved during
run time whenever a Special Function program is run. One block is
allocated for each SF program that is being run. This memory type is 16
words in length and is called Temporary Memory (T-Memory) since it is
not saved when the program has completed running. The controller writes
data related to the Special Function program to the first 7 words. You can
read this data and/or write over it if you choose. You can use all 16 words
just as you would use Variable Memory, except no data is saved when the
program has completed.

Variable Memory

A block of memory within the controller may be allocated for user
operations. This memory type is called Variable Memory (V-Memory).
For example, you can do a math operation and store the result in V-
Memory. You can enter values directly into V-Memory with a
programming unit.

Constant Memory

A block of memory within the controller may be allocated for constants
(unchanging data). This memory type is called Constant Memory (K-
Memory). You can use a programming unit to load a table of data into K-
Memory and read the table during run time whenever you need the data
for an operation.

Status Word Memory

A block of memory within the controller is allocated for storing status
information relating to controller operations. This information is stored in
one or more status words: STW1, STW2, etc. These status words can be
used in the RLL program to signal and/or correct alarm conditions.
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