A LogixPro vending-machine routine can clear the accumulated value with a CLR instruction, but instruction placement controls whether the final change value reaches the simulated outputs before the register returns to zero. The reset must execute only after the vend and output-update sequence has completed.
Clear the Integer Behind the BCD Display
Apply CLR to each 16-bit integer word that stores a value requiring reset. For the cited accumulator, use CLR N7:0. Clearing only the output bits does not reset the underlying integer; the next conversion can restore the previous display value.
BCD conversion: source N7:0 -> output image
Vend/change output processing
CLR N7:0LogixPro does not implement every RSLogix instruction. The available alternatives described in the evidence are RST for setting an Integer or Binary word address to zero and an unlatch instruction for resetting an individual bit. Use CLR when the target is a complete numeric word.
Place the Reset After the Final Value Is Used
- Complete the price and change calculation.
- Convert the result to BCD and write it to the output image.
- Perform the vend operation.
- Enable
CLR N7:0with a one-cycle completion condition so it does not clear the accumulator continuously. - Clear any other transaction words or latched state individually if they must start the next sale at zero.
If the clear executes before the BCD conversion, the display receives zero instead of the calculated change. If its rung remains continuously true, the register cannot accumulate the next transaction.
Remove Competing Writes in the Scan
The supplied ladder reportedly contains identical rungs 3 and 13. The corrected review identifies rung 13 as the last rung writing the outputs, so rung 3 is redundant and should be removed. When multiple rungs write the same destination during one scan, the last executed write determines the final output image.
| Symptom | Check | Action |
|---|---|---|
| Change appears but never resets | Accumulator remains nonzero after vending | Execute CLR N7:0 after output processing |
| Display clears before showing change | Clear rung precedes BCD conversion | Move or sequence the clear after the display write |
| Output does not match an earlier rung | Later rung writes the same destination | Retain one authoritative write path |
Separate Transaction Reset from Change Inventory
Resetting the customer's transaction value is not the same as tracking the machine's coin inventory. A no-change indicator requires persistent counts for the available denominations; those inventory values must survive each transaction and change only when coins enter or leave the machine. The evidence does not define storage addresses, starting quantities, or a change-selection algorithm, so those details must be assigned before implementing the indicator.
Verify the reset by inserting a value above 65, confirming that the calculated change reaches the indicators, completing the vend, and then confirming that N7:0 is zero. Start another transaction and verify that its accumulation begins from zero while the separate coin-inventory values remain intact.
FAQ
How do I reset N7:0 to zero in LogixPro?
Execute CLR N7:0 after the BCD value has been written to the output image and the vend sequence has used the result.
Why does my BCD display show zero instead of the calculated change?
The clear instruction is probably executing before the BCD conversion or output write. Sequence the display update first and clear the integer afterward.
Can CLR also reset the vending machine's no-change status?
Use CLR for complete numeric words and an unlatch instruction for individual bits, but do not clear persistent coin-inventory counts with each sale. Derive the no-change status from those separate inventory values.
