HMI Alarm Configuration in TIA Portal: From NC Field Wiring to Discrete Alarms

• Wire alarm feedbacks as NC (closed-when-healthy) contacts so a broken wire raises an alarm instead of hiding one.
• Use %I addresses only in the input mapping FC; alarm logic and HMI tags read DB members, never %I or %M.
• Put the reset contact in the latch branch so an alarm can only be cleared after its cause has gone.
• Group fault bits into words in the PLC so one HMI trigger tag can carry sixteen discrete alarms.
• Alarm text should name the device, the condition and the first thing to check, not repeat the tag name.
• Acknowledgement is a design decision: not every message needs it, and everything needing it must be visible until it is done.

How HMI Alarms Are Triggered

WinCC on a panel does not scan the machine. It watches tags. Everything therefore depends on how the PLC presents fault information.

The usual approach is a fault word: a WORD or DWORD in a data block where each bit represents one condition. In the worked example below, bits 0 to 4 carry human-safety feedbacks, bits 5 to 9 electrical-safety feedbacks and bits 10 to 14 system-safety feedbacks. The panel monitors the word, and each bit becomes a discrete alarm with its own message text.

This is far more efficient than one HMI tag per fault. One word covers sixteen alarms with a single connection, communication load stays low, and the numbering in the PLC matches the numbering in the alarm list, which makes fault finding much faster.

Build the fault word carefully. Latch conditions that must not disappear before an operator sees them, and keep a separate first-out bit that records which fault stopped the machine. The first-out register, and how the collective fault then blocks a restart, are programmed step by step in first-out fault capture in PLC interlocks and permissives.

Field Wiring for Discrete Alarms: NC (Fail-Safe) Feedback Contacts

Every discrete alarm on the panel starts as a contact somewhere on the machine. In this lesson fifteen feedback signals are grouped into three categories: HS (Human Safety), ES (Electrical Safety) and SS (System Safety). All fifteen are wired closed-circuit: the contact is closed while the device is healthy, so the PLC input reads 1 in normal running and drops to 0 on a fault.

Why NC contact wiring for alarm feedback

• Broken wire is a fault, not silence. A cut core, a loose terminal or a pulled connector opens the circuit and raises the alarm. With a normally open contact the same broken wire would hide the next real fault.
• Loss of field supply is visible. If the 24 VDC feeding the field contacts fails, the inputs drop and the machine stops in a known state instead of running blind.
• One rule for every device. "Input 1 = healthy" for all fifteen signals keeps tag names, ladder logic and troubleshooting consistent. Name the tags _OK so the meaning is obvious in the watch table.

I/O allocation and contact selection

Choose the contact on each device that is closed when healthy. For breakers this is usually the auxiliary contact that closes when the breaker is ON; where the device also has a separate trip-indication contact, wire both in series so "switched OFF" and "tripped" both raise the alarm.

Good practice on the panel side: feed the field contacts from a dedicated, fused 24 VDC circuit, ferrule and number every core to its X1 terminal, and route field cables away from motor power cables. Because a single loss of the field supply will drop all fifteen inputs at once, many projects add one extra "field 24 V OK" input so the operator sees the root cause first rather than a flood of alarms. For hardware setup and tag creation, see Siemens PLC device configuration and tag creation.

PLC Program Structure: IO Mapping, Input Mapping FC and Alarm Mapping FC

The PLC side is built in three layers so that physical addresses live in exactly one place. The PLC tag table (the IO mapping page) names the inputs. FC_InputMapping copies each input tag into a global data block DB_IO. FC_AlarmMapping then reads only DB_IO members and writes the latched faults into "DB_Alarm".Alarm. From the input mapping onward nothing uses %I or %M: every operand is a symbolic DB member, the reset push button included.

Why an IO mapping layer between PLC tags and alarm mapping

• One place to change hardware. If a signal moves to another terminal or a DI card is replaced, you edit one network in FC_InputMapping; the alarm logic and the HMI do not change.
• One place to watch. During commissioning a single watch table on DB_IO shows every field signal by name, and the alarm FC can be tested from it.
• No hidden addresses. M memory can collide between programmers and, for a Word, hides a byte swap (%X0 of MW20 is physically M21.0). Optimized DB members have no absolute address, so slice access .%X0 is the only way to reach a bit and PLC and HMI numbering always agree.

Step 1 – IO mapping page: PLC tags and their DB_IO members

Declare only the physical inputs in the PLC tag table, each with a symbolic name and comment. The same table is your input mapping list: every PLC tag has exactly one DB_IO member written by one network of FC_InputMapping.

Step 2 – DB_Alarm: the alarm word and reset bits

Create both data blocks as global, optimized DBs and keep Accessible from HMI/OPC UA ticked on the members the panel needs (Alarm, HMI_Reset, HMI_AckAll, PLC_Ack, Reset_Cmd and Any_Fault). For data block basics see Siemens block types: OB, FB, FC and DB.

Step 3 – FC_InputMapping: PLC tag to DB_IO

Keep this FC deliberately simple: one normally open contact and one coil per network, no logic. The input is copied as it is, so DB_IO keeps the "1 = healthy" meaning of the NC wiring.

Step 4 – FC_AlarmMapping: fault latch with reset push button

• Fault branch: a normally closed contact instruction -|/|- on the mapped DB_IO bit. The bit is 1 when healthy, so this contact passes power only when the field contact has opened.
• Seal-in (latch) branch: "DB_Alarm".Alarm.%X0 in series with a normally closed "DB_Alarm".Reset_Cmd contact, connected in parallel with the fault branch. Once set, the bit holds itself until Reset is pressed.
• Why reset goes in the latch branch: while the E-stop is still pressed the fault branch keeps the coil on, so pressing Reset cannot clear an alarm whose cause is still present. Use a momentary push button, never a maintained switch.
• Comments: title every network "HS-01 E-stop main panel → Alarm.%X0" so the wiring drawing, both FCs and the HMI alarm list share the same ID.

The same latch as one line of SCL, useful if you later move alarm handling into a function (see the SCL vs ladder comparison):

// HS-01 E-stop main panel → DB_Alarm.Alarm bit 0
"DB_Alarm".Alarm.%X0 := NOT "DB_IO".HS_EStop_Panel_OK
                        OR ("DB_Alarm".Alarm.%X0 AND NOT "DB_Alarm".Reset_Cmd);

Step 5 – Call order in OB1

OB1  Network 1   CALL "FC_InputMapping"    // %I tags → DB_IO
OB1  Network 2   CALL "FC_AlarmMapping"    // DB_IO → DB_Alarm.Alarm

Calling the input mapping first means the alarm logic always works on inputs from the current scan. "DB_Alarm".Any_Fault then feeds the red tower lamp and blocks the start command, as built in the start/stop seal-in with the Any_Fault interlock, which continues this project with a conveyor motor, a contactor feedback trip on %X15 and first-out capture.

Configuring Discrete Alarms in the HMI (WinCC)

Open HMI alarms → Discrete alarms in the project tree and add one row per fault bit.

Writing alarm text that helps

Compare two messages for the same bit. "M1_FLT true" tells the operator nothing. "Conveyor 1 motor overload tripped, check thermal relay in panel MCC-2" tells them the device, the condition and where to go. Text is where alarm systems succeed or fail.

You can embed live values in a message using output fields inside the alarm text, which is useful for showing the measured value that caused a trip.

HMI tags for the alarm word and reset

Drag the members from DB_Alarm into the HMI tag table rather than typing addresses. TIA Portal creates DB_Alarm_Alarm (Word) connected to "DB_Alarm".Alarm and DB_Alarm_HMI_Reset (Bool) connected to "DB_Alarm".HMI_Reset, both with symbolic access. Set the acquisition mode of DB_Alarm_Alarm to Cyclic continuous so alarms are detected even when no screen is displaying the tag. As in the PLC, no HMI tag points at %I or %M.

Alarm classes for HS, ES and SS

Create three user-defined alarm classes under HMI alarms → Alarm classes so the operator can tell the category at a glance and filter the alarm view by it.

The fifteen discrete alarms

Add one row per bit in HMI alarms → Discrete alarms. Leave the trigger mode on the rising edge, because the PLC bit goes to 1 when the fault latches. Prefixing each text with its HS/ES/SS ID ties the panel message back to the wiring drawing and the ladder network comment.

Acknowledge and reset on the HMI

Add a RESET button to the alarm screen and, on its Press event, use the system function SetBitWhileKeyPressed on DB_Alarm_HMI_Reset. Keep the two operator actions separate in training: acknowledge confirms on the panel that the operator has seen the message, while reset clears the latched bit in the PLC once the device is healthy. In the alarm view an unacknowledged active alarm shows I (incoming), an acknowledged active alarm IA, and an alarm that has gone but is still waiting for acknowledgement IO; once it is both gone and acknowledged it leaves the active list. For button events and status lamps see HMI buttons, lamps and auto/manual mode.

Interactive HMI Alarm Test: Simulate Wiring, Latch, Acknowledge and Reset

Use this simulator to rehearse the whole chain before touching real hardware. Click a field device to open its NC contact (fault or broken wire), watch the input drop in DB_IO, the latch set in "DB_Alarm".Alarm, and the message arrive on the HMI alarm view. Then close the contact, acknowledge on the panel and press RESET, either the field push button or the HMI button. The rung monitor shows power flow through the selected FC_AlarmMapping network in real time.

Build the Same HMI Screens in TIA Portal: Connect, Configure and Design Step by Step

Follow these steps in order and the panel will look and behave like the simulator above. The colours, grid and hierarchy behind these screens are explained in HMI screen design best practices. The design targets a 7-inch TP700 Comfort panel (800 × 480 pixels) with WinCC Comfort in TIA Portal V17 to V19; menu names can differ slightly between versions. Every object is listed with its name, position, size, animation and event, so a student can rebuild the screens exactly. The numbers ① – ⑰ in the layout figures match the object tables.

Part A – Add the HMI and connect it to the PLC

1. In the project tree choose Add new device → HMI → SIMATIC Comfort Panel → 7" Display → TP700 Comfort and pick the device version that matches your panel firmware.
2. In the HMI device wizard, under PLC connections, browse and select your CPU. Click Finish (the screens are built manually below, so the wizard's default screens are not needed).
3. Open Devices & networks → Network view. Check that the CPU and the panel sit on the same PROFINET subnet (green line, e.g. PN/IE_1) and that the Connections tab lists HMI_Connection_1. If not, select Connections → HMI connection and drag from the CPU interface to the panel interface.
4. Set IP addresses in the same subnet: CPU 192.168.0.1, panel 192.168.0.2, subnet mask 255.255.255.0. On the real panel, enter the same address under Control Panel → Network and Dial-Up Connections and enable PROFINET in Transfer settings.
5. In DB_IO and DB_Alarm, make sure Accessible from HMI/OPC UA is ticked for the members used below. With S7-1200/1500 inside one TIA project the integrated connection is symbolic, so PUT/GET access is not needed.
6. Compile the PLC (Software, rebuild all) so the HMI can see the DB members.

Part B – HMI tags

Open HMI tags → Show all tags, create a tag table Alarm_Tags, and drag DB_Alarm and DB_IO from Program blocks into it. TIA Portal names the tags DB_Alarm_Alarm, DB_IO_HS_EStop_Panel_OK and so on and connects them symbolically. Then set the acquisition modes and add the internal tag HMI_Filter.

Part C – Alarm classes, discrete alarms and runtime settings

1. HMI alarms → Alarm classes: add HS_Safety, ES_Electrical and SS_System, each with acknowledgement required. Set the incoming background colours to light red (#FEE2E2), light orange (#FFEDD5) and light blue (#E0F2FE), and keep the status texts I, O and A.
2. HMI alarms → Discrete alarms: create the fifteen alarms from the table above: trigger tag DB_Alarm_Alarm, trigger bit 0 – 14, matching class and text.
3. For each discrete alarm open Properties → Acknowledgement and set the PLC acknowledgement tag to DB_Alarm_PLC_Ack with the same bit number as the trigger bit. This is what makes ACK ALL work: while the button is held, FC_AlarmMapping copies the alarm word into PLC_Ack, and every active alarm is acknowledged from the PLC.
4. Runtime settings: set the start screen to Alarm_Screen and, under Alarms, enable the use of alarm class colours so the alarm view rows show the class colours.

Part D – Screen template and Alarm_Screen

1. Screen management → Templates: rename Template_1 to Tpl_Main and draw objects ①, ②, ③ and ⑪. Placing them in the template makes the header, tower lamp and navigation identical on every screen.
2. Screens → Add new screen Alarm_Screen, select template Tpl_Main, background #E9EEF3.
3. Draw objects ④ – ⑩ from the toolbox (Controls → Alarm view, Elements → Button, Basic objects → Text field) and enter the values from the table. Use Properties → Properties → Layout for exact X, Y, width and height.
4. Build the filter in this order: place AV_All and its Btn_Ack_All, configure both, then copy and paste three times, rename, change the enabled alarm class and the visibility range. Keep all four views and ACK buttons at exactly the same position.

Why four alarm views instead of one: a single WinCC Comfort alarm view cannot change its alarm-class filter at runtime, so each filter button simply makes a different preconfigured view visible. Each view has its own ACK button because AlarmViewAcknowledgeAlarm acknowledges the alarm selected in one specific view.

Part E – Field_Status screen

1. Add screen Field_Status with template Tpl_Main.
2. Build one signal row (white rectangle, lamp ⑫, two text fields ⑬), configure the lamp animation, then group, copy and paste it for all fifteen signals and change the tag and texts. Column headings use the HS / ES / SS colours.
3. Add the alarm-word block ⑭ and ⑮: configure Bit_X0 once, then copy it fifteen times and change only the bit number in the animation.
4. Add the status lamps ⑯ and the RESET button ⑰.

Part F – Compile, download and test

1. Select the HMI device and run Compile → Software (rebuild all); fix every error and check the warnings for unconnected tags.
2. Without hardware: start the CPU in S7-PLCSIM, then select the HMI and choose Online → Simulation → Start. Toggle the inputs %I0.0 – %I1.7 from a PLCSIM SIM table to play the role of the field contacts.
3. With hardware: download the PLC, then the HMI (Download to device, PG/PC interface on the PROFINET network card).
4. Run the same cases as the simulator: open a contact, see the lamp turn red and the alarm arrive, close it, ACK, RESET; try RESET with the fault active; test each filter button and ACK ALL. Then sign off the loop-test sheet in the lab.

Troubleshooting Discrete Alarms: Symptom, Cause and First Check

Most alarm problems found during commissioning fall into a handful of patterns. Work from the field towards the panel: DI card LED, then DB_IO, then "DB_Alarm".Alarm, then the HMI tag. The first layer where the value is wrong is where the fault is.

For deeper CPU-side diagnosis, including the diagnostic buffer and watch tables, see Siemens PLC troubleshooting.

Analog Alarms and Limit Monitoring

Analog alarms watch a numeric tag and trigger when it crosses a limit. Open HMI alarms → Analog alarms and configure the trigger tag, the limit value and the limit mode.

• Higher: triggers when the value rises above the limit, for high temperature or pressure.
• Lower: triggers when the value falls below the limit, for low level or low flow.
• Delay: the value must stay past the limit for this long before the message appears.
• Dead band: a percentage or absolute hysteresis so a value hovering on the limit does not flood the list.

Delay and dead band are the two settings that decide whether the alarm system is usable. Without them, a signal sitting exactly at its limit generates hundreds of come-and-go messages in a shift, and operators stop reading the list. The reasoning behind these choices is expanded in SCADA alarm management.

Set four limits on important measurements where it makes sense: low low, low, high and high high, each in a different alarm class, so an early warning looks different from a trip condition.

Alarm Classes, Acknowledgement and the Alarm View

An alarm class defines behaviour for a whole group of messages: display colours for came, went and acknowledged states, whether acknowledgement is required, and whether the message is archived.

Acknowledgement can be written back to the PLC using an acknowledgement tag, which lets the control program require an operator action before a fault is reset. That is a much better pattern than allowing an automatic reset that hides the fault before anyone reads it.

Placing the alarm view

Add a full alarm view on a dedicated alarm screen with columns for time, alarm class, text and status, and enable filtering so an operator can show only unacknowledged errors. Then add a single-line alarm view to the screen template so the highest priority active message is visible on every screen in the project.

Keep an alarm history available as well, so a supervisor can see what happened during the night shift rather than only what is active now.