A programmed constant cannot be written from the panel. Change the timer's preset operand to a writable V-memory address, drive it with a 32-bit BCD numeric entry that spans two words, and raise the entry's 255 range check to the process limit.
The panel cannot change the constant, and the timer can still be operator-adjustable. A programmed K constant is compiled into the timer instruction when the program downloads; a numeric entry writes memory, never code. The fix is to change the preset operand from the constant to a writable V-memory address, and let the panel's numeric entry write that address. A one-word preset carries up to 9999 counts; a two-word preset carries up to 99999999 counts. A 255-count ceiling on the entry is its rangeCheck setting, set to 255, and it lifts once you set it to the process limit. This page gives the address rule, the data type, the two reservation traps, and the checks that prove the write.
Why the panel cannot write the constant
A K constant is code. It is compiled into the timer instruction and changes only through an edit in the PLC programming software followed by a download. No numeric entry, address, or communications setting reaches it, because panel writes land in memory and the constant lives in the program. That settles the first decision for you: an operator-adjustable preset must live in a writable V-memory location, and the timer instruction must read its preset from there.
What the preset becomes instead
Replace the constant operand with a V-memory starting address. The numeric entry writes that address; the timer uses whatever the address holds as its preset. The widths must match on both sides:
| Preset width | Entry data type | Largest value | Words consumed |
|---|---|---|---|
| One word | 16-bit BCD | 9999 counts | 1 |
| Two words | BCD_INT_32 | 99999999 counts | 2 |
The capacities are binary-coded decimal arithmetic: a 16-bit word holds four BCD digits, so it tops out at 9999 counts, and two words hold eight digits. When the panel prompts for the numeric entry's address string, pick BCD_INT_32 as the data type for a two-word preset, and give the entry the same starting address the timer operand uses. Two words carrying one number is the layout a PLC float stored in two word registers uses, so the same spanning rules apply.
Where the 255 ceiling comes from
The memory was never the limit. A one-word entry accepts up to 9999 counts, so a value that stops at 255 counts stopped because the entry's rangeCheck is turned on and set to 255. A configured range check rejects or truncates the input at its limit, at the keypad, before any write reaches the PLC. Open the entry's configuration and set the permitted range to the process limits.
Set the high limit to the longest value the process actually runs, not to the data type limit: an entry allowed the full double-word range lets an operator dial a dwell the machine was never built to run. When the limit must hold even if the panel is reconfigured, clamp the numeric entry in ladder logic as well.
Reserve the second word and the paired timer element
A two-word preset consumes the word after its starting address. A preset at V2000 owns V2001, so nothing else may use that word: no second setpoint, no counter, no communications scratch value. Writes to either use corrupt both values. Timer elements pair the same way: a two-word timer on T0 takes T1 with it, and T1 stops being available as an independent timer.
Address suitability also depends on the CPU. One documented note marks V1400 as a usable preset address except on the 230 and 240 CPUs, so read the installed CPU's memory map before committing to any address.
What you need
- The PLC program, its programming software, and download access.
- The panel project and the numeric entry you will reconfigure.
- The memory map of the installed CPU, to find an unused word pair.
- The process limit for the preset, in counts of the timer's time base. On a 0.1 s time base, each count is 0.1 s, so a 9999-count one-word preset spans 999.9 s, about 16.7 minutes, and a 99999999-count two-word preset spans 9999999.9 s, about 115.7 days.
Example: entering a 12000-count dwell at V2000
Inputs:
- Preset the process runs: 12000 counts, from the machine's dwell specification, in counts of the timer's 0.1 s time base. That is a dwell of 12000 x 0.1 s = 1200 s, or 20 minutes.
- One-word capacity: 9999 counts, four BCD digits per 16-bit word, which on the 0.1 s time base is 999.9 s.
- Two-word capacity: 99999999 counts, eight BCD digits across a word pair.
- Entry ceiling today: 255 counts, the rangeCheck setting on the numeric entry, which on the 0.1 s time base is 25.5 s.
- Starting address: V2000, an unused word confirmed against the installed CPU's memory map.
Rule in words: the operator can enter a preset only when it fits the entry's data type and sits inside the rangeCheck limits. Both conditions must hold.
The arithmetic:
- 12000 > 9999 counts, so the one-word entry cannot carry it.
- 12000 > 255 counts, so the entry as configured today rejects it.
- 12000 counts < 99999999 counts, so a 32-bit BCD entry carries it with room.
Decision: configure the entry as BCD_INT_32 at V2000, reserve V2001, reserve the paired timer element, and set rangeCheck to a low limit of 0 counts and a high limit of 12000 counts, the longest dwell the process runs, 20 minutes on this time base. Two thresholds carry forward: any preset above 9999 counts forces the two-word entry, and any preset above 255 counts forces the rangeCheck change.
The changes in order
- Pick an unused V-memory word pair from the installed CPU's memory map and confirm nothing in the current program touches it.
- In the timer instruction, change the preset operand from the K constant to the starting address, V2000 in this example.
- Reserve the following word, V2001, and the paired timer element, T1 when you use T0.
- Download the PLC program; the operand change is a program change.
- In the panel project, point the numeric entry at the same starting address and choose BCD_INT_32 as its data type.
- Open the entry's rangeCheck and set the range to the process limits, 0 to 12000 counts in this example.
- Load the panel project to the panel.
- Test with the machine in a safe state and a known low-risk preset, and give the check at least 5 minutes so a long dwell finishes a full cycle.
How to check the write works
Enter a value above 255 counts and inside the new range. In the programming software, watch the preset words: both must change together as one value, and memory used elsewhere must stay untouched. Then confirm the timer runs the new duration and resets. Put the count on the panel too by showing the timer counting from the PLC's current value, so the operator sees the new preset take effect.
If the panel displays the number but memory does not change, the write is failing: check the entry's address, its data type, and the panel-to-PLC communications. If memory changes but the timer misbehaves, the operand or a reservation is at fault: confirm the operand names the starting address, V2001 is reserved, and the timer pairing is intact. A preset that falls back to zero after a restart is a separate fault; see a timer preset that resets to zero in the PLC.
Recommendation: for every timer an operator adjusts, move the preset to V-memory, drive it with a BCD_INT_32 entry, set the rangeCheck high limit to the process limit, and reserve the adjacent word and the paired timer element in the same edit. Keep K constants only on timers nobody but the programmer ever changes.
Questions
Why does the numeric entry stop at 255 when the timer accepts more?
The ceiling is the rangeCheck setting on that entry, turned on and set to 255. The panel enforces it at the keypad and rejects or truncates anything above the limit before a write reaches the PLC. The memory is not the constraint: one V-memory word holds up to 9999 counts, and a two-word preset holds up to 99999999 counts. Raise the limit to the process maximum.
What changes when I pick BCD_INT_32 for the entry?
The entry spans two V-memory words instead of one. You give one starting address in the address string, and the 32-bit data type makes the entry read and write the pair as a single number. That also means the word after your starting address belongs to the preset, so reserve it before anything else claims it.
Can the panel write the K constant if I just widen the entry?
No. A programmed constant is part of the downloaded PLC program, and panel writes only reach memory. Moving the preset is a one-time operand change followed by a program download. After that, operators adjust the value from the screen with no program change.
Why did another timer stop working after this change?
Check the pairings. A two-word preset at V2000 owns V2001, and a two-word timer on T0 takes T1 with it. An overlap on either side shows up as corrupted setpoints or a timer that miscounts while the panel write itself keeps working.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.