The operator needs three timed temperature runs—500 °C for 2 hours, 350 °C for 2 hours, and 180 °C for 1 hour—then the heater must stop and a completion light must turn on. Keep the temperature loop in the SL4848 and use the DL06 or CLICK PLC as the sequence controller. Send setpoints over Modbus RTU, time each selected run in the PLC, and make heater shutdown independent of the communicated setpoint.
Reject the quick fixes first
Do not build three separate PID loops in the PLC merely to change three setpoints. The SL4848 already performs temperature control; duplicating that loop creates competing outputs, extra tuning work, and an unclear owner for the heater.
| Quick fix | Why it fails | Use instead |
|---|---|---|
| Put all timing in the C-more screen | An HMI can disconnect, reboot, or change screens. The process sequence then depends on the operator interface. | Run the state and timers in the PLC. Use C-more for selection, commands, status, and alarm display. |
| Write the setpoint every PLC scan | Unnecessary serial traffic can increase retries and hide failed transactions. | Write once when a run or step begins, then confirm communication health. |
| Set the target to zero when time expires | A new setpoint is not a positive heater-isolation command. Communication loss can also prevent the write. | Issue the controller's documented stop or output-disable action and remove the heater run permissive through the machine's designed control path. |
| Start the two-hour timer with the Start button | Warm-up time consumes the required dwell unless that is the intended process definition. | Choose explicitly between elapsed-cycle timing and at-temperature dwell timing. |
Stop here if the heater has no independent overtemperature protection, contactor control, or approved shutdown path. PLC logic, C-more controls, and serial communication are not substitutes for the protective devices required by the machine design.
Check which device owns temperature control
Read the present SL4848 setpoint and process value while the machine is idle. If the controller already regulates its heater output from its local sensor, leave PID execution there. The PLC should select a recipe, transmit its setpoint, supervise progress, run the dwell timer, and command completion.
If the plan is to run PID in the CLICK, first identify the temperature input, input scaling, PLC scan behavior, output type, output update rate, loss-of-signal action, and required independent protection. None of those installation details are defined here, so moving the loop into the PLC is not the production-restoration path. Keep the established SL4848 loop until those engineering decisions and a new tuning exercise are complete.
The three listed conditions may mean either three operator-selectable recipes or three consecutive stages. The requested durations indicate three separate programs, so the basic branch is one selected run per Start command. If the process must execute all three consecutively, use the same state-machine pattern with three setpoint-write and dwell states; define the required order before commissioning.
Prove Modbus RTU communication before adding timers
Connect the PLC serial port to the SL4848 using Modbus RTU. Read the controller manual and PLC communication setup for the actual physical interface, station address, baud rate, parity, stop bits, setpoint register, process-value register, data type, register numbering convention, and scaling. Do not guess any of these fields.
- With the heater disabled, read a harmless controller value. Compare the returned value with the SL4848 display.
- If the read fails, inspect wiring polarity, common/reference requirements, serial format, slave address, timeout indications, and PLC communication status. Do not continue to setpoint writes.
- If the read succeeds but the number is wrong, resolve word size, signed representation, byte or word order, decimal scaling, and the difference between protocol offsets and displayed register numbers.
- Write a permitted test setpoint with the heater still disabled. Confirm that the controller display changes to the intended engineering value.
- Power-cycle only if the operating procedure permits it, then determine whether the controller retains the setpoint. The PLC should still write the selected recipe when a new run begins.
A successful PLC instruction is not enough by itself. Use the communication completion and error indications supplied by the selected DL06 or CLICK instruction. Latch a communication fault after the configured retry policy fails, block cycle timing, remove the heat permissive, and require an operator reset after communications recover.
Decide when each timer starts
Take two readings: the transmitted setpoint and the measured process value. Then select the timing rule that matches the process requirement.
| Timer rule | Start condition | Effect |
|---|---|---|
| Elapsed-cycle time | The SL4848 accepts the selected setpoint and the run is enabled. | Warm-up is included in the 2-hour or 1-hour period. |
| At-temperature dwell | The process value enters a configured acceptance band around the target. | The full period is spent near the target; warm-up is additional. |
The required acceptance band and out-of-band behavior are process decisions. Obtain the band from the process specification. Decide whether leaving the band pauses the timer, resets it, or merely alarms; then implement that rule visibly in the PLC rather than burying it in the HMI.
For an at-temperature dwell, compare the process value with the selected setpoint plus and minus that specified band. Start or enable the timer only when the comparison is true. Display separate states such as heating and dwelling so the operator does not mistake warm-up for a stalled timer.
Sequence the three selectable recipes
| Selection | Setpoint | Duration |
|---|---|---|
| Program 1 | 500 °C | 2 hours |
| Program 2 | 350 °C | 2 hours |
| Program 3 | 180 °C | 1 hour |
Store the setpoint and duration as one recipe record selected from C-more. Validate the selection before Start, then copy it into active PLC values. This freezes the running recipe so an HMI edit cannot silently change a cycle already in progress.
- Idle: heater run permissive off, completion light off, and recipe selection enabled.
- Write setpoint: send the active temperature to the documented SL4848 setpoint register. Advance only after successful transaction completion.
- Heat: enable the designed heater run path. For elapsed-cycle timing, proceed directly to timing. For at-temperature timing, wait until the process value meets the specified acceptance rule.
- Time: accumulate the selected 2-hour or 1-hour duration. Keep Stop, overtemperature, sensor-fault, and communication-fault handling active.
- Complete: remove the heater run permissive, perform the documented controller stop or disable action where applicable, and turn on the completion light.
- Reset: require acknowledgement before returning to Idle. Do not let a maintained Start command launch another cycle automatically.
Define restart behavior after power loss before releasing the program. The conservative default is to return with heat disabled and require the operator to inspect the load and start a new cycle; resuming a partially completed thermal dwell requires documented process authorization and retained-state design.
Apply the fix and verify the resolving branch
- Disable heater power through the normal maintenance procedure and prove read/write communication.
- Enter the three recipe values exactly as listed. Lock recipe editing to the intended operator access level.
- Test each selection with shortened commissioning durations while the heater remains disabled. Confirm state transitions, timer basis, Stop behavior, reset behavior, and completion-light operation.
- Interrupt the serial link during setpoint write, heating, and timing. Each test must block or stop heating according to the approved fault response and must not indicate a completed batch.
- Restore the serial link and confirm that communication recovery alone cannot restart the heater.
- Enable the heater and run a supervised test. Compare the C-more display, SL4848 display, active recipe, process value, elapsed time, and physical heater command.
- At timer completion, verify heater shutdown at the actual output path and verify that the completion light remains on until acknowledgement.
Get production running with one verified recipe first, then validate the remaining selections under the same test sheet. Do not bypass a communication fault, temperature-sensor fault, overtemperature device, or heater contactor problem to finish software commissioning.
FAQ
Can I run all three temperature programs from one CLICK PLC?
Yes. Store 500 °C for 2 hours, 350 °C for 2 hours, and 180 °C for 1 hour as selectable recipe records, then use one common state sequence to execute the selected record.
Can I use the C-more timer instead of a PLC timer?
Use the PLC timer for the production sequence. Let C-more select the recipe and display state, process value, setpoint, remaining time, completion, and faults.
Does the two-hour timer start when I press Start?
Only for elapsed-cycle timing. For a true two-hour temperature dwell, start timing when the measured temperature enters the process-approved band around 500 °C or 350 °C, and define what happens if it leaves that band.
When should I stop troubleshooting and escalate?
Stop if the SL4848 register map, serial electrical interface, scaling, controller fault response, or heater shutdown path cannot be identified from the installed documentation. Keep the heater disabled and contact official AutomationDirect support with the exact PLC, C-more, and controller configurations plus the communication status and observed register values.