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Siemens S7 PLC Integration

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Overview

The Dilution Control System supports native communication with Siemens S7 PLCs using the S7-TCP (ISO-on-TCP) protocol. This provides high-performance direct access to PLC data blocks without additional gateways.

SNAP7 Library

Communication uses the SNAP7 open-source library, which provides S7 protocol implementation compatible with all S7 PLC families.

Supported Models

PLC Family Models Notes
S7-1500 All models Recommended, best performance
S7-1200 All models (V4.0+) Ensure PUT/GET access enabled
S7-300 CPU 31x, 31xC With Ethernet CP or integrated
S7-400 CPU 41x, 41xH With Ethernet CP

Configuration

PLC Prerequisites

Before connecting, configure the PLC in TIA Portal or STEP 7:

  1. Enable PUT/GET Access (S7-1200/1500)

    In TIA Portal: PLC Properties > Protection & Security > Connection mechanisms

    • Check "Permit access with PUT/GET communication"

  2. Configure Data Block Access

    For each DB used:

    • Disable "Optimized block access" (use standard access)
    • Set DB to "Non-retained" if appropriate

  3. Note the Rack/Slot Numbers

    • S7-1200: Rack 0, Slot 1
    • S7-1500: Rack 0, Slot 1
    • S7-300/400: Check hardware configuration

Dilution Control Configuration

Connection Settings

Protocol Siemens S7-TCP
IP Address 192.168.1.30 PLC IP address
Port 102 ISO-on-TCP port
Rack 0 Rack number
Slot 1 CPU slot

Data Blocks

Configure the data block addresses for setpoints and feedbacks:

Recommended DB Structure

DB100 - Setpoints (from Dilution Control)

DB100.DBD0 Zone 1 Setpoint REAL (0-100%)
DB100.DBD4 Zone 2 Setpoint REAL (0-100%)
... ... ...
DB100.DBD188 Zone 48 Setpoint REAL (0-100%)
DB100.DBW200 Global Mode INT
DB100.DBW202 Heartbeat INT

DB101 - Feedbacks (to Dilution Control)

DB101.DBD0 Zone 1 Feedback REAL (0-100%)
DB101.DBD4 Zone 2 Feedback REAL (0-100%)
... ... ...
DB101.DBD188 Zone 48 Feedback REAL (0-100%)
DB101.DBW200 System Status INT

Address Configuration

Data Block Settings

Setpoint DB Number 100
Setpoint Start Offset 0
Feedback DB Number 101
Feedback Start Offset 0
Number of Zones 48
Data Type REAL (4 bytes)

PLC Program Requirements

The PLC program should handle:

Setpoint Processing

  • Read setpoints from DB100
  • Apply limits and ramp rates
  • Output to analog modules controlling valves
  • Handle mode switching (Auto/Manual)

Feedback Processing

  • Read analog inputs from valve position sensors
  • Scale to 0-100% range
  • Write to DB101 for reading by Dilution Control

Watchdog Handling

// Check heartbeat in PLC program
IF DB100.Heartbeat = OldHeartbeat THEN
    HeartbeatTimeout := HeartbeatTimeout + CycleTime;
    IF HeartbeatTimeout > T#5s THEN
        // Communication lost - hold last positions
        CommLost := TRUE;
    END_IF;
ELSE
    OldHeartbeat := DB100.Heartbeat;
    HeartbeatTimeout := T#0s;
    CommLost := FALSE;
END_IF;

Testing

  1. Test Connection

    Click Test Connection. Verify "Connected to S7-xxx" message.

  2. Monitor Feedbacks

    Verify feedback values appear in the zone panel (0-100% range).

  3. Test Manual Control

    Set a zone to Manual, change setpoint, verify valve moves.

  4. Verify Heartbeat

    Monitor DB100.DBW202 in PLC - should increment every 500ms.

Troubleshooting

Problem Cause Solution
Connection refused PUT/GET not enabled Enable PUT/GET in TIA Portal
Connection timeout Wrong IP or port blocked Verify IP, check firewall for port 102
Wrong rack/slot Configuration mismatch S7-1200/1500: Rack 0, Slot 1
Read error on DB DB doesn't exist Create DB in PLC program
Values all zero Optimized block access Disable optimized access for DB