PID Loop Operation
Maintenance
and Troubleshooting
8–20
PID Loop Operation
The loop controller in the DL05 PLC has the ability to directly access analog input
and output values independent of the ladder logic scan. These values represent the
process variable (PV) and the control output. The Direct Access feature makes it
possible for the loop controller to perform closed-loop control while the CPU is in
Program Mode.
The loop controller can read the analog PV value in the selected data format directly
from the desired analog input module and write the control output value in the same or
a different data format to the desired analog output. The Direct Access feature, when
enabled, accesses the analog values only once per PID calculation for each
respective loop. The ladder logic, however, may simultaneously access the same
analog input data by the standard method (LD instruction) or by the pointer method
whenever the CPU is in Run Mode.
NOTE: If PID direct access is used, do not use the analog output pointer logic to send
data to the outputs.
You may optionally configure each loop to access its analog I/O (PV and control
output) by placing proper values in the associated loop table registers. The figure
below shows the loop table parameters at V+36 and V+37 and their role in direct
access to the analog values.
Process Variable V+03
Loop
Calculation
S
Error
+
-
Control Output V+05Setpoint V+02
Loop Table
V2036 Slot / channel number for PV0F XX
V2037 Slot / channel number for Output0F XX
0F XX
Channel number, 1 to 4
Stands for Option Module.Slot
You may program these loop table parameters directly, or use the appropriate
DirectSOFT32 dialog box for easy configuring. For example, a value of “0F02” in
register V2036 directs the loop controller to read the PV data from channel 1 of the
analog input module. A value of “0000” in either register tells the loop controller not to
access the corresponding analog value directly. In that case, ladder logic must
transfer the value between the loop table and the analog input module.
If the PV or control output values require some math manipulation by ladder logic,
then it will not be possible to use the direct access function of the loop controller. In
this case, ladder logic will have to perform the math and transfer the data from the
analog module as required.
NOTE: The loop controller restricts the transfer of analog data to or from the module
to one method. In other words, you must designate the analog module for direct
access or ladder logic access.
Direct Access
to Analog I/O
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