Siemens PLC Wiring Guide for Roll Forming Machines (S7-1200 / S7-1500)

Siemens PLC Wiring Guide

S7-1200 & S7-1500 Wiring Architecture for Roll Forming Machines

Siemens PLC systems — particularly the S7-1200 and S7-1500 platforms — are widely used in:

• High-speed roofing roll forming lines

• Structural purlin systems

• Multi-profile production lines

• Automated coil processing equipment

Siemens systems are robust — but only when wired correctly.

Most instability issues are not programming problems.

They are:

• Incorrect I/O referencing

• Mixed commons

• Poor shield termination

• Improper analog grounding

• Encoder wiring errors

• Incorrect 24V distribution

This guide explains how to wire Siemens PLCs properly in roll forming environments.

1) Typical Siemens PLC Architecture in Roll Forming

A standard roll forming configuration may include:

• CPU (S7-1200 or S7-1500)

• Digital Input modules

• Digital Output modules

• Analog Input module

• High-Speed Counter (HSC)

• Communication module (Profinet)

• Safety relay or F-CPU (if required)

Word-Based System Flow:

Field Devices → I/O Modules → CPU → Outputs → Drives & Solenoids

2) Power Supply Wiring (24VDC Standard)

Siemens PLC systems operate on 24VDC.

Word-Based Power Flow:

• AC Supply → SMPS (24VDC) →
• • PLC CPU
• • I/O Modules
• • Sensors
• • Safety Relay

Critical Rule:

24V must be stable and properly distributed.

Avoid:

• Long shared common runs

• Undersized PSU

• Mixed control voltages

3) Digital Input Wiring (Siemens)

Most Siemens input modules are 24VDC PNP type.

Word-Based PNP Input Wiring:

+24V → Sensor → PLC Input Channel
M (0V) → PLC Common

Sensor supplies +24V to input when active.

Siemens inputs detect positive voltage.

Incorrect NPN sensor will not function properly.

4) Input Common Referencing

Siemens modules often group inputs by common terminals.

Engineering consideration:

All sensors sharing a module must share common 0V reference.

Do not mix external 24V supplies without shared reference.

Floating commons cause unstable input behavior.

5) Digital Output Wiring (Relay vs Transistor)

Siemens output modules may be:

• Relay output type

• Transistor output type

Relay Output Example

PLC Relay Contact → Contactor Coil → 0V

Can switch AC or DC loads.

Transistor Output Example (PNP)

PLC Output → Solenoid → 0V

Switches +24VDC.

Transistor outputs are preferred for high-speed control.

6) Solenoid & Inductive Load Protection

Hydraulic solenoids must include:

• Flyback diode (for DC coils)

Word-Based:

PLC Output → Fuse → Solenoid → 0V
Flyback Diode across solenoid terminals.

Without suppression, output module can fail.

7) Analog Input Wiring (4–20mA Preferred)

Siemens analog modules support:

• 4–20mA

• 0–10V

Preferred for roll forming:

4–20mA (better noise immunity)

Word-Based:

Transmitter + → AI+
Transmitter – → AI–

Shield grounded at cabinet end only.

Separate analog from motor wiring.

8) Encoder & High-Speed Counter Wiring

High-speed roofing lines require accurate length control.

Word-Based:

• Encoder A → HSC Input A
• Encoder B → HSC Input B
• 0V → Encoder 0V
• Shield → Earth bar

Use:

• Twisted pair shielded cable

• Separate trunking

• Proper shield clamp

Never run encoder cable parallel to VFD motor cable.

9) Profinet & Communication Wiring

Siemens systems typically use Profinet.

Requirements:

• Industrial Ethernet cable

• Shielded connectors

• Proper routing away from power cables

• Avoid sharp bends

Communication instability often due to poor cable routing.

10) Safety Integration with Siemens

Two common approaches:

1. Standard CPU + External Safety Relay

2. F-CPU (Failsafe PLC)

Standard configuration:

Safety Relay → Contactor Coil
Safety Relay Auxiliary → PLC Input

PLC monitors safety but does not control emergency stop directly.

11) Word-Based Safety Flow with Siemens

E-STOP Dual Channel → Safety Relay → Main Contactor

Safety Relay Auxiliary → Siemens DI

PLC Logic:

IF Safety_OK = TRUE
THEN Allow Start

Never wire E-stop directly into standard Siemens input as primary stop mechanism.

12) EMC & Shielding Best Practices

Inside cabinet:

Left trunking → Power
Right trunking → Signal

Rules:

• Encoder cables shielded

• Analog cables shielded

• Motor cables routed directly to exit

• Shield grounded at single defined point

Siemens modules are robust, but noise-sensitive in poor layouts.

13) Common Siemens Wiring Mistakes in Roll Forming

1. Mixing PNP/NPN sensors

2. No flyback diode on solenoids

3. Shared commons between multiple PSUs

4. Encoder cable routed with motor cables

5. No shield termination

6. Incorrect analog scaling configuration

7. Overloaded 24V PSU

8. Improper Profinet routing

Most “PLC faults” are wiring faults.

14) Commissioning Checklist (Siemens)

Before powering drives:

• Verify 24V at CPU

• Check I/O LED indicators

• Confirm input status in TIA Portal

• Test output manually (with caution)

• Confirm analog values stable

• Verify encoder count direction

Incorrect encoder polarity causes negative count.

15) High-Speed Roofing Considerations

At 50–60 m/min:

• Scan time optimization critical

• HSC must be configured properly

• Encoder cable integrity critical

• Output response time matters

Transistor outputs preferred for fast shear trigger.

16) Documentation Requirements

Supplier should provide:

• Full wiring schematic

• Terminal layout

• I/O list

• Network topology

• PLC program backup

No Siemens system should ship without documented I/O mapping.

17) Export Considerations

Ensure:

• 24V PSU rated for local supply

• Transformer taps set correctly

• 50Hz/60Hz compatibility

• Spare modules available locally

Siemens has strong global support — advantage for international buyers.

18) Buyer Strategy (30%)

Before purchasing a Siemens PLC roll forming machine, ask:

1. Is S7-1200 or S7-1500 used?

2. Are outputs relay or transistor type?

3. Is encoder connected via HSC module?

4. Is 4–20mA used for analog signals?

5. Is safety handled by relay or F-CPU?

6. Are Profinet cables shielded properly?

7. Is full TIA Portal backup provided?

8. Is I/O list documented clearly?

Red flag:

“Wiring diagram will be provided later.”

Professional suppliers deliver full documentation at shipment.

6 Frequently Asked Questions

1) Is S7-1200 suitable for roll forming?

Yes, for small to medium lines.

2) When should S7-1500 be used?

For larger, multi-drive, high-speed systems.

3) Are Siemens inputs PNP?

Most are PNP 24VDC — verify module specification.

4) Can encoder connect directly to standard input?

No. Use high-speed counter input.

5) Is safety built into standard Siemens CPU?

Not unless using F-series safety PLC.

6) What is biggest Siemens wiring mistake?

Poor EMC separation and incorrect sensor type selection.

Final Engineering Summary

Correct Siemens PLC wiring in roll forming machines requires:

• Stable 24VDC architecture

• Correct PNP sensor wiring

• Proper output protection

• Shielded encoder integration

• Clear signal segregation

• Proper safety relay integration

• Documented I/O mapping

Siemens PLCs provide industrial reliability — but only when wiring architecture supports:

• EMC stability

• High-speed synchronization

• Safe operation

• Global serviceability

Most system instability blamed on “software” originates in wiring discipline.