Siemens Implementation

Modbus RTU for Siemens

Serial communication protocol using binary encoding over RS-232/RS-485. Most widely used industrial protocol worldwide.

Type: Serial CommunicationSpeed: Up to 115.2 kbpsReleased: 1979Organization: Modbus Organization

115.2 kbps max

Bandwidth

247

Max Nodes

1200m (RS-485)

Max Distance

Yes

Real-Time

Protocol Overview

Siemens S7-1200/1500 support Modbus RTU via CM 1241 communication modules. TIA Portal V13+ provides MB_CLIENT (master) and MB_SERVER (slave) function blocks with comprehensive diagnostics.

When to Use Modbus RTU

Choose Modbus RTU for simple applications, legacy integrations, cost-sensitive projects, or when connecting basic I/O devices over short to medium distances.

Siemens Hardware Requirements

Compatible Communication Modules

CM 1241 RS232 (6ES7241-1CH30-1XB0)

CM 1241 RS422/485 (6ES7241-1CH32-1XB0)

CB 1241 RS485 (6ES7241-1AH32-0XB0)

CP 341 (6ES7341-1AH02-0AE0 - S7-300)

CP 441-2 (6ES7441-2AA05-0AE0 - S7-400)

Wiring Details

RS-485: Terminal A (Data+), Terminal B (Data-), twisted pair 120Ω characteristic impedance. Ground shield at one point only.

Termination

120Ω resistors at both physical bus ends. CM 1241 has switchable termination via DIP switch.

Siemens Software & Programming

Function Blocks & Instructions

MB_MASTER (legacy)

MB_SLAVE (legacy)

MB_CLIENT (TIA V13+, recommended)

MB_SERVER (TIA V13+, recommended)

PORT_CFG

Siemens Code Example

MB_CLIENT(
  REQ := TriggerRead,
  MB_MODE := 0,  // 0=Read, 1=Write
  MB_DATA_ADDR := 40001,
  MB_DATA_LEN := 10,
  MB_DATA_PTR := DataBuffer,
  DONE => ReadDone,
  ERROR => ReadError,
  STATUS => StatusWord
);

Siemens-Specific Implementation Notes

TIA Portal integrates Modbus configuration directly in hardware configuration. Supports both polling and event-driven modes. STATUS word provides detailed error diagnostics (80C8H=timeout, 80A1H=parameter error).

Technical Specifications

Physical Layer

Physical Layer: RS-232, RS-422, RS-485
Cable Type: RS-485 twisted pair, RS-232
Topology: Bus, Star, Tree
Power Over Cable: No

Data Layer

Data Link: Master/Slave
Encoding: Binary (RTU mode)
Error Detection: CRC-16
Frame Size: 256 bytes max

Performance

Scan Time: 10-100ms typical
Deterministic: Yes
Real-Time: Yes

Certification

Certification Required: No
License Cost: Free (open standard)
Version: Current

Advantages & Disadvantages

Advantages

✓Simple and easy to implement
✓Very low cost
✓Excellent industry support
✓Reliable in harsh environments
✓Minimal hardware requirements
✓Open standard
✓Wide device compatibility

Disadvantages

✗Limited speed (115.2 kbps max)
✗No built-in security
✗Master/slave only (no peer-to-peer)
✗Limited diagnostic capabilities
✗No standardized device profiles
✗Requires proper termination

Common Use Cases & Applications

Simple sensor and actuator networks

Building automation

Energy management systems

Water/wastewater treatment

Legacy equipment integration

Small to medium manufacturing lines

Adoption & Ecosystem Context

Adoption LevelVery High

Primary-source market share30-40% of industrial protocols

Geographic PresenceWorldwide

Primary Industries

ManufacturingBuilding AutomationEnergyWater Treatment

Supported Message Types

Read Coils

Read Discrete Inputs

Read Holding Registers

Write Single Coil

Write Single Register

Write Multiple Registers

Supported PLC Brands

Siemens Allen-Bradley Mitsubishi Schneider Electric Beckhoff ABB Rockwell

Phoenix Contact

Wago

Protocol Comparisons

Modbus RTU vs PROFINET

Modbus RTU is simpler and cheaper; PROFINET is faster and more capable

View Detailed Comparison →

Quick Links

Alternative Protocols

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