If you are supplying, buying, or commissioning an industrial automation system, the system architecture drawing is one of the most important documents you need to produce and understand.
It shows every major device, how they connect, and which communication protocol links them. Without it, you cannot troubleshoot a network fault or verify that the system was built correctly.
System architecture in industrial automation is not optional. You need it before you write a single line of PLC code, before you order a network switch, and before commissioning.
We will cover what a system architecture drawing is, what devices you must include, how to design the drawing step by step, how to choose communication protocols and network topology, and how to document spare ports and redundancy correctly.
What Is System Architecture in Industrial Automation?
System architecture is a design drawing that shows which devices are used in your automation system and how they are interconnected.
You must understand what system architecture is not. It does not show electrical panel wiring or PLC ladder logic. It shows only the major communication devices and the data paths between them.
You must show: PLC (CPU and IO modules), SCADA servers, HMI panels, IO modules, network switches, VFDs, energy meters, and any device that has a communication port.
What you do not include are things like contactor coils, fuse terminals, cable trays, or power supplies. Those belong in the electrical panel drawing, not the system architecture.
System Architecture in Industrial Automation: The Layer Model
Before you start drawing, understand the layer model. Every device belongs to one of six layers. This tells you which devices communicate with each other and in which direction data flows.
| Layer | Devices at This Layer | Typical Protocol |
|---|---|---|
| Layer 5: Enterprise | ERP systems, MES, cloud platforms | OPC UA, REST API |
| Layer 4: Supervision | SCADA server, historian, engineering workstation | Ethernet, OPC UA |
| Layer 3: Control | PLC, DCS, PAC | Ethernet/IP, PROFINET, Modbus TCP |
| Layer 2: HMI | Touchscreen panels, local operator stations | Ethernet, RS-485 |
| Layer 1: IO and Smart Devices | Remote IO modules, VFDs, energy meters, positioners | PROFIBUS, Modbus RTU, CANopen |
| Layer 0: Field | Sensors, transmitters, actuators, final elements | 4-20 mA, HART, Digital IO |
Your system architecture drawing must show devices from Layer 0 to Layer 4 at minimum. Include Layer 5 only if your system integrates with a business software system like SAP.
Step 1: List All Your Devices Before You Draw Anything
The first step in building a system architecture in industrial automation is your device list. Do not open a drawing tool until you have a complete list.
Go through your automation requirements document and list every device that has a communication port. For each device, capture four things: the device type, model number, quantity, and the communication ports available.
Step 2: Know the Communication Protocols Before You Draw Lines
You must know the communication protocol for every connection before you draw a single line. A line without a protocol label is not a complete drawing.
Do not assume that every device connects by Ethernet. Many automation devices use serial protocols. Get this right before you start drawing.
Step 3: Draw the Architecture Starting from the PLC
Your system architecture in industrial automation drawing starts from the PLC. Place it at the centre, because more devices hang off it than any other single component.
Draw the PLC CPU block first. Then draw each IO module: local IO on the rack, and remote IO on the fieldbus. Label every module with its model number and IO type.
In a small system with one PLC and one HMI, draw a direct Ethernet line between them with no switch needed.
In a system with multiple devices on the same Ethernet segment, all those devices connect to the switch, not directly to each other.
Once the PLC and switch are placed, draw the SCADA server and connect it to the same switch. Label the connection protocol: S7 (Siemens), EtherNet/IP (Allen-Bradley), or Modbus TCP.
Then draw the HMI panels and connect them the same way.
After the supervision layer, draw the serial bus devices below the PLC. Draw the RS-485 bus line for each Modbus network and label each device with its model number and slave address.
Step 4: Choose the Right Network Topology
Define the network topology for your system architecture in industrial automation once you know your devices and protocols. The topology is the physical arrangement of the network.
Step 5: Document Spare Ports and Future Expansion
This step is one that many engineers skip, and they regret it later. Every communication port that is not currently used must be labelled as spare in your system architecture drawing.
When a maintenance engineer needs to add a VFD later, the drawing must show which ports are spare. Without this, they may attempt to use a port already allocated to something else.
For the PLC, document DI, DO, AI, and AO channels separately: installed, used, and spare. See the IO modules in industrial automation guide for correct IO sizing.
Comparing System Architecture for Small, Medium, and Large Systems
| Aspect | Small System (1 PLC, 1 HMI) | Medium System (1-3 PLCs, SCADA) | Large System (Multiple PLCs, DCS) |
|---|---|---|---|
| Typical topology | Direct Ethernet between PLC and HMI. No switch needed in many cases. | Star Ethernet with managed switch. SCADA server on same network as PLC. | Ring Ethernet with multiple switches. Separate control and supervision networks with a firewall. |
| PLC to SCADA protocol | Proprietary driver (S7, EtherNet/IP) over Ethernet. | OPC UA preferred for vendor-neutral data exchange. Redundant SCADA servers. | OPC UA to MES and ERP. Separate OT and IT networks. IEC 62443 security zones. |
| Remote IO | Local IO only, plugged directly into PLC rack. | Mix of local and remote IO on PROFIBUS or PROFINET drops. | Fully distributed IO across large plant areas. IO in Ex-rated enclosures for hazardous areas. |
| Redundancy | Not typically required. Single CPU, single power supply. | Redundant power supplies. Possibly redundant PLC CPU for critical processes. | Fully redundant PLCs (hot standby), redundant network switches, redundant SCADA servers. |
| Serial bus devices | 1 to 2 Modbus devices on RS-485 (energy meter, VFD). | Multiple Modbus RTU networks and possibly PROFIBUS DP for remote IO. | Legacy PROFIBUS retained for installed base. New devices on PROFINET. Serial bridges where needed. |
Watch: Complete Guide to the Automation Architecture Stack
System Architecture in Industrial Automation Questions
Related Articles on This Site
- What Is a PLC and How Does It Work?
- IO Modules in Industrial Automation Explained
- HART Protocol: How It Works in Industrial Automation
- PLC Redundancy and Hot Standby Configuration
- Why 4-20 mA Is the Best Signal for Industrial Automation
External References
- How to Design a System Architecture in Industrial Automation | Inst Tools
- Industrial Automation Architecture Layers | Medium
What We Learn Today
- System architecture in industrial automation shows every communication device and the protocols connecting them. It must be produced before any other design document. It includes PLC, IO modules, SCADA, HMI, network switches, and smart field devices. Electrical panel components are not included.
- Start by listing all devices with model numbers and communication ports. Then identify the protocol for each connection: Ethernet for PLC to SCADA, PROFINET or PROFIBUS for remote IO, Modbus RTU for serial devices, OPC UA for enterprise integration.
- Label every connection with its protocol name and port. Mark every unused port as spare. Document DI, DO, AI, and AO counts separately for future expansion. A drawing without complete labelling fails its purpose as a commissioning and troubleshooting reference.