For a P2K counter configured with a BCD current-value tag, use UPKB to unpack the value into the selected display output tags. Before changing logic, confirm what signal format the display accepts and how its eight inputs are assigned.
Inspect the counter value before changing logic
A quick fix is to wire the counter value straight to output bits or to turn on outputs by comparing the count against individual numbers. Those approaches can fail when the tag represents the count in a different format than the display expects. A binary value and a BCD value can represent the same count with different bit patterns.
Find the counter's current-value tag and inspect its configured data type in the programming software. The relevant question is whether that value is BCD, as opposed to another numeric representation. Do not infer the format from the value shown on a screen: a displayed decimal number does not tell you how the PLC stores its bits.
- Record the current-value tag and its data type.
- Watch the tag while the counter increments, and compare its displayed numeric value with the counter's actual count.
- Check whether the tag changes when the counter is reset or reaches its configured limit.
Proceed when the tag you intend to use is confirmed as the live counter value and its data type is known.
Confirm the display expects BCD input signals
Check the display's input specification and terminal assignment before mapping PLC outputs. A device described as a BCD display generally receives coded digit signals, but the term alone does not establish whether this display accepts packed BCD, separate digit groups, or a different interface. The display's manual and wiring diagram decide that.
| Check | What to establish | Why it matters |
|---|---|---|
| Input encoding | Whether the display expects BCD and how its inputs encode digits | A bit pattern meant for another encoding can show the wrong digit. |
| Input assignment | Which display input corresponds to each PLC signal | The bit order and digit grouping must match at both ends. |
| Electrical interface | Display input voltage/current requirements and PLC output type | A correct logic value cannot compensate for an incompatible electrical interface. |
| Number of digits | How the display uses its eight input signals | Eight signals may be assigned to multiple digit groups or other functions; do not assume their purpose. |
If the display expects binary, serial, or another format, stop and select a conversion method that matches that interface; UPKB only unpacks bits. Continue when the display input format and all eight signal assignments are documented.
Set the BCD value and map UPKB destinations
When the counter's current-value tag is configured as BCD, the suggested conversion path is to use UPKB (Unpack Bits) and assign the resulting bit states to the output tags selected for the display. The instruction exposes individual bits; it does not replace checking that the source value has the format the display needs.
- Back up the current project and identify the counter's current-value tag.
- Set or confirm that tag's data type as BCD if the display input specification calls for BCD.
- Insert
UPKBwith the counter value as its source and the intended output tags as destinations. - Use the programming software's instruction help or project documentation to confirm source-bit order, destination order, and the number of bits transferred.
- Map each destination to the display input identified in the wiring diagram; label the tags by signal or digit function.
Do not guess bit order from tag names or from a single count. Confirm the instruction's displayed mapping and verify it against the display terminal assignments. The mapping is ready when each destination tag has a documented display input and the source value is confirmed as BCD.
Connect output tags to compatible output points
Output tags in the program do not by themselves establish that the display is electrically connected. Confirm that each tag is assigned to the intended physical output point, and compare the PLC output module's output type and ratings with the display input requirements. Read these values from the module and display specifications; do not substitute a logic-level test for an electrical compatibility check.
Check common or return wiring, polarity, and any required interface circuitry against both wiring diagrams. Keep the display disconnected or use an approved test arrangement while checking logic if the actual output circuit could affect connected equipment. Avoid relying on forced outputs as the normal operating logic: forces can mask a bad tag mapping and may leave the display showing a value unrelated to the counter.
With the counter stopped or in a controlled test condition, monitor the destination tags and the corresponding output indicators. Confirm each tag operates the intended output point and that the electrical signal at the display input matches its specification before connecting the complete signal set.
Prove the count through digit changes and reset
A single correct display reading does not prove the whole mapping. Exercise the counter through values that change each digit group and through the reset behavior used by the application. Watch the counter value, unpacked tags, output indicators, and displayed value together so a formatting error can be separated from a wiring or assignment error.
- Test the zero or reset value and record the counter tag, destination tags, and displayed result.
- Increment through values that change each displayed digit; confirm the corresponding input group changes in the documented bit order.
- Test the counter's configured limit and reset transition, checking for stale output states or a display value that does not return as expected.
- Compare the displayed number with the PLC counter value at each test point, then remove temporary test conditions and confirm normal counting.
If the PLC tags are correct but the display is wrong, recheck input encoding, terminal order, polarity, and electrical compatibility. If the tag itself is wrong, diagnose the counter configuration or program before changing wiring. Release the display for operation only after the end-to-end reading follows the counter through increments and reset.
Keep a temporary restore separate from the permanent repair
If production needs a temporary display while the correct mapping is being established, mark the display as unavailable or use an approved alternate indication. Do not leave manually forced bits or hand-switched outputs as the production fix; they will not track later counts and can conceal a fault.
For the permanent repair, save the confirmed data type, UPKB source and destination mapping, physical output assignments, and display terminal mapping in the project records. Have another qualified person check the mapping against the display documentation before returning the installation to normal use. The repair is complete when the saved project reproduces the verified count-to-display behavior after a restart or download using the site's normal procedure.
Frequently asked questions
Can I connect the counter value directly to eight outputs?
Only if the counter's bit representation already matches the display's required inputs and the output assignments are correct. For a BCD current-value tag, use UPKB to unpack bits into the selected output tags, then verify the bit order.
Does UPKB convert a binary count into BCD?
UPKB unpacks bits from its source value. Configure or confirm the counter value's BCD data type when the display expects BCD; unpacking alone does not select the encoding.
Can eight output signals drive any BCD display?
No. Check the display's input encoding, digit assignment, and electrical requirements. Eight signals do not identify the display's digit grouping or prove compatibility with the PLC output module.
Does a correct PLC tag value prove the display wiring is right?
No. Compare each destination tag with its assigned physical output and display terminal, then verify the electrical signal and the displayed count through digit changes and reset.
When should I stop and contact official support?
Stop if the software's UPKB mapping or the display's signal requirements cannot be identified, or if the specified electrical interface is incompatible with the available outputs. Contact official support for the PLC software or display manufacturer with the project version, tag data type, instruction mapping, module details, and display wiring information.