Manual mode logic first decodes the selector: Manual_Mode = manual contact AND NOT auto contact, with a selector fault if both or neither are on for 0.5 s. In manual, each motor is controlled individually with hold-to-run jog buttons: forward runs only while the button is held, reverse is blocked while forward is active, and all permissives (safety, MPCB, alarms) still apply.
- Decode the selector with both contacts and detect selector faults
- Jog = hold-to-run: no seal-in in manual
- Forward/reverse interlocked in software (and with contactor auxiliaries)
- Leaving manual stops every manual command at once
Manual mode is where people stand closest to the machine. That is why it uses hold-to-run, keeps every interlock active and never lets auto sequence bits drive outputs.
Step 1: Decode the Mode Selector
| DI_Auto_Sel | DI_Manual_Sel | Result | Reason |
|---|---|---|---|
| 1 | 0 | Sts_Auto_Mode | Normal auto |
| 0 | 1 | Sts_Manual_Mode | Normal manual |
| 0 | 0 | No mode, Alm_Selector after 0.5 s | Selector between positions or wire broken |
| 1 | 1 | No mode, Alm_Selector after 0.5 s | Short circuit or wrong selector type |
Sts_Auto_Mode := DI_Auto_Sel AND NOT DI_Manual_Sel AND NOT Alm_Selector;
Sts_Manual_Mode := DI_Manual_Sel AND NOT DI_Auto_Sel AND NOT Alm_Selector;
T_Sel(IN := DI_Auto_Sel = DI_Manual_Sel, PT := T#500ms);
IF T_Sel.Q THEN Alm_Selector := TRUE; END_IF;Step 2: Program Hold-to-Run Jog with Interlocks
Manual jog simulator
Select Manual, then hold Jog FWD or Jog REV. Try both together, switch to Auto while jogging, and press the E-stop.
Green contact = passing power. Orange moving wire = rung is true and the output is energised.
In the full program the jog rungs write Cmd_Jog_Fwd / Cmd_Jog_Rev, Motor_Logic combines them with auto commands and Output_Mapping applies MPCB and alarm interlocks (Labs 6 and 13). Also fit electrical interlocks with the contactor auxiliary NC contacts.
Step 3: Rules for Manual Mode
| Rule | Implementation |
|---|---|
| Hold-to-run only | No seal-in contacts on manual commands |
| Both buttons pressed = nothing | NC contact of the opposite button in each rung |
| Direction change needs a pause | Optional: TOF on the opposite contactor feedback (0.5 s) |
| Auto sequence frozen | Sequence steps held in manual; sequence reset when entering auto |
| All safety permissives stay active | Same Relay_OK / MPCB / alarm contacts as in auto |
| Visible mode | Amber flash on tower lamp + manual lamp (Lab 14) |
Step 4: Test Manual Mode
| ID | Check | How to check | Pass criterion | Result |
|---|---|---|---|---|
| Manual mode tests | ||||
| MM-01 | Selector | Turn selector both ways | Correct mode, manual lamp follows | |
| MM-02 | Selector fault | Hold between positions | Alm_Selector after 0.5 s | |
| MM-03 | Jog FWD | Hold, then release | Runs only while held | |
| MM-04 | Both buttons | Hold FWD and REV | Motor stays off | |
| MM-05 | Mode change | Switch to auto while jogging | Motor stops | |
Practice Exercise
Add a second manual function: jog at reduced speed using the ACS580 constant speed (Lab 20). Decide which speed reference manual mode should use and add it to the mode table.
Troubleshooting
| Symptom | Likely cause | Fix |
|---|---|---|
| Motor keeps running after release | Seal-in copied from start/stop rung | Remove seal-in in manual rungs |
| Both contactors pull in | Missing cross interlock | Add NC contact of the other direction (software + hardware) |
| Neither mode active | Selector wiring or DI_Manual_Sel mapping | Check inputs in I/O Mapping online |
| Auto restarts after manual | Sequence not reset on mode change | Reset step to 0 when manual selected |
Frequently asked questions
What is manual mode in PLC programming?
A mode where an operator or technician controls each actuator individually, normally with hold-to-run buttons, while safety and protection interlocks stay active.
How do I interlock forward and reverse in ladder?
Put an NC contact of the reverse output in the forward rung and vice versa, and add NC contacts of the opposite push button. Also interlock the contactors electrically.
Why use hold-to-run for jogging?
The motor stops as soon as the operator releases the button, which keeps the person in control while close to the machine.
How do I detect a faulty auto/manual selector?
Read both selector contacts. If both or neither are TRUE for more than a short time, raise a selector alarm and allow no mode.