Specifications

SIMATIC Instructions
9-92
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
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, which
when detected 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
n
) before switching to auto control. The PID 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
) = process variable (PV
n
)
 Sets old process variable (PV
n-1
) = process variable (PV
n
)
 Sets bias (MX) = output value (M
n
)
The default state of the PID history bits is “set” and that state is established at
CPU 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 will
not be 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 CPU.