PID Loop Operation
Maintenance
and Troubleshooting
8–6
PID Loop Operation
Loop Setup Parameters
The DL05 PLC gets its PID loop processing instructions only from tables in
V-memory. A “PID instruction” type in RLL does not exist for the DirectLogic PLCs.
Instead, the CPU reads setup parameters from reserved V-memory locations.
Shown in the table below, you must program a value in V7640 to point to the main
loop table. Then you will need to program V7641 with the number of loops you want
the CPU to calculate. V7642 contains error flags which will be set if V7640 or V7641
are programmed improperly.
Address Setup Parameter Data type Ranges Read/Write
V7640 Loop Parameter
Table Pointer
Octal V1200 – V7377 write
V7641 Number of Loops BCD 0 – 4 write
V7642 Loop Error Flags Binary 0 or 1 read
If the number of loops is “0”, the loop controller task is turned off during the ladder
program scan. The loop controller will allow use of loops in ascending order,
beginning with 1. For example, you cannot use loop 1 and 4 while skipping 2 and 3.
The loop controller attempts to control the full number of loops specified in V7641.
The CPU reports any programming errors
of the setup parameters in V7640 and
V7641. It does this by setting the
appropriate bits in V7642 on
program-to-run mode transitions.
PID Error Flags, V7642
013456789101112131415 2Bit
If you use the DirectSOFT32 loop setup dialog box, its automatic range checking
prohibits possible setup errors. However, the setup parameters may be written using
other methods such as RLL, so the error flag register may be helpful in those cases.
The following table lists the errors reported in V7642.
Bit Error Description (0 = no error, 1 = error)
0 The starting address (in V7640) is out of the lower V-memory range.
1 The starting address (in V7640) is out of the upper V-memory range.
2 The number of loops selected (in V7641) is greater than 4.
3 The loop table extends past (straddles) the boundary at V7377. Use an
address closer to V1200.
As a quick check, if the CPU is in Run mode and V7642=0000, there are no
programming errors.
Loop Table and
Number of Loops
PID Error Flags
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