Allen-Bradley MicroLogix 1200 Reference Manual page 252

19-10 Process Control Instruction
Publication 1762-RM001C-EN-P
Time Mode (TM)
Tuning Parameter Address
Descriptions
TM - Time Mode PD10:0.TM
The time mode bit specifies when the PID is in timed mode (1) or STI
mode (0). This bit can be set or cleared by instructions in your ladder
program.
When set for timed mode, the PID updates the CV at the rate specified in
the loop update parameter (PD10:0.LUT).
When set for STI mode, the PID updates the CV every time the PID
instruction is scanned in the control program. When you select STI,
program the PID instruction in the STI interrupt subroutine. The STI
routine should have a time interval equal to the setting of the PID "loop
update" parameter (PD10:0.LUT). Set the STI period in word STI:0.SPM.
For example, if the loop update time contains the value 10 (for 100 ms),
then the STI time interval must also equal 100 (for 100 ms).
When using timed mode, your processor scan time
NOTE
should be at least ten times faster than the loop
update time to prevent timing inaccuracies or
disturbances.
Loop Update Time (LUT)
Tuning Parameter Address
Descriptions
LUT - Loop Update Time PD10:0.LUT word (INT)
The loop update time (word 13) is the time interval between PID
calculations. The entry is in 0.01 second intervals. Enter a loop update
time five to ten times faster than the natural period of the load. The
natural period of the load is determined by setting the reset and rate
parameters to zero and then increasing the gain until the output begins to
oscillate. When in STI mode, this value must equal the STI time interval
value loaded in STI:0.SPM. The valid range is 0.01 to 10.24 seconds.
Data Range Type
Format
binary 0 or 1 control
Data Format Range
1 to 1024
User Program
Access
read/write
Type User Program
Access
control read/write