No address spelling lets a display set a PLC physical input: the controller rebuilds the input image from its terminals every scan, so an external write survives less than one scan cycle. Retag the toggle to a C command bit in a display-owned 16-bit group, or to a V register for values, and let the rung combine it with the real field input.
You cannot tag a panel toggle to input X1, and no other spelling of that address fixes it. Every scan, the controller copies the state of its input terminals into an input image table, solves the program against that table, then writes the outputs. A write from the display into that image is replaced by the terminal state at the next input read. The toggle on the screen is doing what it was told; the PLC erases the result a fraction of a second later. Point the toggle at memory the hardware does not refresh: a C bit for a discrete command, a V register for a value, and put that command in the rung next to the real input.
Why the write cannot stick
The scan cycle repeats for as long as the controller is in RUN: read every input into the image table, execute the program from top to bottom, write the outputs. Scan references put a typical scan between 1 and 20 ms, with large programs approaching 100 ms. That number sets the lifetime of a foreign write in the input image: less than one scan, because the input read that starts the next scan overwrites it with the terminal state. No tag address, data type or driver setting changes this. The input image belongs to the terminals, and only the terminals.
Where the operator command goes
| The operator's job | Where the write goes |
|---|---|
| A discrete request: start, jog, mode select | One C bit in a display-write-only group |
| A value: setpoint, recipe number, timer preset | A V register the rung reads |
| Behave like a field device | Nowhere. Fix the field device or its wiring |
The command is a request. The rung keeps its permissives: physical inputs, timer done bits and interlocks stay between the request and the output. The same pattern carries changing a timer preset from the panel: the panel writes the preset register, the PLC decides when the timer runs. You can also gate whether a control is available at all, as in gating button visibility on a timer done bit.
Never let the operator bit stand in for a physical interlock or stop device. If the machine's design depends on that device, repair the circuit or the input channel, and take any change that touches a safety function through the machine builder's process.
Check the tag's address syntax and data type against the communication driver's C and V memory format before you download. An X-style label tells you nothing about those formats, and a mismatch surfaces as the kind of invalid address error you fix once.
One writer per 16-bit group
Allocate C bits in complete groups of 16 on a 16-bit boundary: the first group is C0-C17, the next is C20-C37. The boundaries look odd because these discrete addresses count in octal. C0-C7 is eight bits, C10-C17 is eight more, and no address contains an 8 or a 9. Eight plus eight is sixteen, one word.
Label each group with one owner: PLC-write-only for status the panel reads, display-write-only for commands the panel writes. Two writers in one group produce bits that revert or flicker, because a word update from one side carries the other side's bits back to old values, and the symptom shows only when both sides are active. When the program must cancel outstanding commands, clear a display-write-only group on first scan, and leave the PLC-owned groups out of that clear.
Example: moving a jog request from X1 to C0
The inputs:
- One jog rung on a conveyor, driven today by X1, a hardwired pushbutton on an input terminal. The button stays.
- The panel adds five operator commands, jog among them.
- No scan time has been measured. Scan references put typical programs between 1 and 20 ms; the example uses 10 ms from that range.
The rule in words: each operator command takes one C bit, all panel commands go in one display-write-only group of 16, and the first free boundary is C0. The rung becomes X1 OR the command bit, feeding the jog output through its existing permissives. The program clears the display group on first scan.
The arithmetic:
- Group capacity is 16 bits. Commands to place: 5. Sixteen minus 5 leaves 11 spare bits, so eleven more commands fit before the next boundary, C20-C37, is needed.
- Boundary check: C0-C7 is 8 bits, C10-C17 is 8 bits, and 8 plus 8 is 16. The group is word aligned.
- Input image, for contrast: at 10 ms per scan, 1,000 ms divided by 10 ms is 100 input reads per second. The same write aimed at X1 is overwritten within one scan, at most 10 ms after it lands. Aimed at C0, it holds until the panel or the program changes it.
The decision: retag the jog toggle to C0, mark group C0-C17 display-write-only, and put X1 OR C0 into the rung with the permissives that already gate the jog. Clear C0-C17 on first scan so a power restoration cannot restart the conveyor on a stale toggle. The limits to respect: one writer per group, and a group is full at 16 commands.
Three checks before you call it working
- The write lands. Open the C address in the controller's data view and flip the toggle. The bit follows the screen.
- The logic answers. With the bit set and the permissives true, the rung output turns on in the data view.
- The machine moves. The output channel and the device respond.
Watch X1 at the same time. It must track the pushbutton and stay untouched by the toggle; that is the confirmation that the input image still belongs to its terminal. If the command address never changes, the fault is in the tag: address syntax, data type, then the communication driver. If the command changes and the rung stays off, walk the permissives. If the rung output turns on and the machine is idle, follow the output channel and the device.
Decide the restart behavior on purpose
A maintained toggle can power up still set. The first-scan clear of the display group handles that: the operator raises the request again once the machine is ready. A momentary object writes only while it is touched, so the rung must latch the request when the action must persist, and how many scans a momentary button bit stays on sets the window the logic has to catch it. Write both choices into the project notes: the owner of each group, and what clears it.
When the job is display, not command
Status for the whole floor is a different job from the machine's operator panel. An offline shop-floor display plays notices and line status from its own card with no subscription and no PLC changes, which suits boards the whole shift reads from a distance. The panel stays the place an operator commands the machine.
Recommendation: Retag the toggle to one C bit in a display-write-only group of 16, C0-C17 first and C20-C37 next, keep X1 in the rung as the physical permissive, and clear the display-owned group on first scan. Point value commands at a V register instead of a bit. When the input itself is the suspected fault, troubleshoot the field device and its terminal, and leave the panel out of the repair.
Questions
Can any panel configuration write to a PLC physical input like X1?
No. The input image is rebuilt from the input terminals at the start of every scan, so an external write survives less than one scan cycle, too short to serve as a held command. Give the panel a C bit or a V register to write, and let the program drive the output from that command.
Why does C0-C17 hold 16 bits when the label runs to 17?
Those discrete addresses count in octal, so the digits run 0 through 7 and then 10 through 17, with no address containing an 8 or a 9. C0-C7 is eight bits and C10-C17 is eight more, sixteen in total, one word. Starting a group off a boundary, at C3 for instance, makes one word's update span two owners.
Should the operator command be a momentary or a maintained toggle?
A momentary object writes only while it is touched, so the rung must latch the request when the action must persist. A maintained toggle can still be set after a restart, so clear the display-owned group on first scan and make the operator raise the request again. State the choice in the project notes.
What does it mean when the command bit changes but the machine does not respond?
The write path works; the logic or the output does not. First walk the permissives in series with the command: physical inputs, timer and counter done bits, interlocks. If the rung output turns on and the machine stays idle, the fault is in the output channel or the device. Force nothing while the machine can move; read the data view instead.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.