Modicon State RAM Layout

The Modicon compact 984 says:

Each 0x or 1x value implemented in user logic is represented by one bit in a word in
state RAM, by a bit in a word in the history table, and by a bit in a word in the
DISABLE table. In other words, for every discrete word in the state RAM table there
is one corresponding word in the history table and one corresponding word in the
DISABLE table.Counter input states for the previous scan are represented on page
F in an up-counter/down-counter history table. Each counter register is represented
by a single bit in a word in the table; a value of 1 indicates that the top input was ON​

in the last scan, and a value of 0 indicates that the top input was OFF in the last scan.

Does anyone know the specifics of how the SRAM history/disable tables are set up? It says they start from the end of the SRAM and work forward.

We are using Proworx to program the PLC. And I think it is allowing us to have the 0x,1x,3x,4x registers overlap with the enable/history registers. And I am trying to determine where the conflict is.

In Proworx32 go to Configuration/Controller Properties to see what the register ranges have been set up as.
For example the one I am looking at has 0xxxx 1536, 1xxxx 0512, 3xxxx 0048, 4xxxx 1872. If you try to use addresses outside of these preselected ranges you will have trouble.

Brian.

Our address limits are:

0xxxx - 1600 (Memory Addresses 1 through 100)
1xxxx - 256 (Memory Addresses 101 through 116)
3xxxx - 99 (Memory Addresses 129 through 227)
4xxxx - 1800 (Memory Addresses 241 through 2040)

So this uses 2040 registers(Since the 3x,4x must start on a 16 word boundary). But then I think there is a history word and a disable word for each of the 0xxxx and 1xxxx starting from the back of the 2k state ram and moving forward.

I am trying to figure out which history/disable words are conflicting with the 4xxxx registers that are used in the overlap range.

Is it:

Disable 0xxx1
.
.
Disable 0xxxN
Disable 1xxx1
.
.
Disable 1xxxN
History 0xxx1
.
.
History 0xxxN
History 1xxx1
.
.
History 1xxxN

OR is it:

Disable 0xxxN
.
.
Disable 0xxx1
Disable 1xxxN
.
.
Disable 1xxx1
History 0xxxN
.
.
History 0xxx1
History 1xxxN
.
.
History 1xxx1

I am not the programmer of the PLC, I only have a PDF of the ladder logic and a screen print of the register usage.

The History/Disable words do not show in the register usage.

Any help is appreciated.

Why do you think the State/history/disable etc bits are accessible to you as the plc programmer?

I have never seen them. The book only mentions them for information. Do not worry where they are as you cannot access them. Modicon does not let you screw up things like other plcs.