Specifications

Basic Concepts for Programming an S7-200 CPU
4-34
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
Forcing Specific Values
The S7-200 CPU allows you to force any or all of the I/O points (I and Q bits). In
addition, you can also force up to 16 internal memory values (V or M) or analog I/O
values (AI or AQ). V memory or M memory values can be forced in bytes, words,
or double words. Analog values are forced as words only, on even-numbered byte
boundaries (such as AIW6 or AQW14). All forced values are stored in the
permanent EEPROM memory of the CPU.
Because the forced data might be changed during the scan cycle (either by the
program, by the I/O update cycle, or by the communications-processing cycle), the
CPU reapplies the forced values at various times in the scan cycle. Figure 4-19
shows the scan cycle, highlighting when the CPU updates the forced variables.
The Force function overrides an immediate-read or immediate-write instruction.
The Force function also overrides an output that was configured to go to a
specified value on transition to STOP mode: if the CPU goes to STOP mode, the
output reflects the forced value and not the configured value.
As shown in Figure 4-20, you can use the Status Chart to force values. To force a
new value, enter the value in the New Value column of the Status Chart, then
press the Force button on the toolbar. To force an existing value, highlight the
value in the Current Value column, then press the Force button.
Read the inputs
Force values are applied to the inputs
as they are read.
Execute the program
Force values are applied to all
immediate I/O accesses.
Force values are applied for up to
16 memory values after the
program has been executed.
Process any communication requests
Force values are applied to all read/write communication accesses.
Execute the CPU self-test
diagnostics
One Scan Cycle
Write the outputs
Force values are applied to the
outputs as they are written.
Figure 4-19 Scan Cycle of the S7-200 CPU