Siemens SIMATIC S7-200 System Manual page 165

Modes
There is no built-in mode control for S7-200 PID loops. The PID calculation is performed only
when power flows to the PID box. Therefore, "automatic" or "auto" mode exists when the PID
calculation is performed cyclically. "Manual" mode exists when the PID calculation is not
performed.
The PID instruction has a power-flow history bit, similar to a counter instruction. The instruction
uses this history bit to detect a 0-to-1 power-flow transition. When the power-flow transition is
detected, it will cause the instruction to perform a series of actions to provide a bumpless change
from manual control to auto control. In order for change to auto mode control to be bumpless, the
value of the output as set by the manual control must be supplied as an input to the PID
instruction (written to the loop table entry for M
instruction performs the following actions to values in the loop table to ensure a bumpless change
from manual to auto control when a 0-to-1 power-flow transition is detected:
Sets setpoint (SP
-
n
Sets old process variable (PV
-
Sets bias (MX) = output value (M
-
The default state of the PID history bits is "set" and that state is established at startup and on
every STOP-to-RUN mode transition of the controller. If power flows to the PID box the first time
that it is executed after entering RUN mode, then no power-flow transition is detected and the
bumpless mode change actions are not performed.
Alarm Checking and Special Operations
The PID instruction is a simple but powerful instruction that performs the PID calculation. If other
processing is required such as alarm checking or special calculations on loop variables, these
must be implemented using the basic instructions supported by the S7-200.
Error Conditions
When it is time to compile, the CPU will generate a compile error (range error) and the compilation
will fail if the loop table start address or PID loop number operands specified in the instruction are
out of range.
Certain loop table input values are not range checked by the PID instruction. You must take care
to ensure that the process variable and setpoint (as well as the bias and previous process
variable if used as inputs) are real numbers between 0.0 and 1.0.
If any error is encountered while performing the mathematical operations of the PID calculation,
then SM1.1 (overflow or illegal value) is set and execution of the PID instruction is terminated.
(Update of the output values in the loop table could be incomplete, so you should disregard these
values and correct the input value causing the mathematical error before the next execution of the
loop's PID instruction.)
) before switching to auto control. The PID
n
) = process variable (PV
n
) = process variable (PV
n--1
)
n
S7-200 Instruction Set
)
)
n
Chapter 6
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