Industrial Communication

A machine is rarely one box. A PLC talks to drives, remote I/O, meters, HMIs and other controllers, and increasingly to databases and the cloud. Each link uses a protocol (the agreed message format and rules) over a physical layer (the cable and electrical signals). This guide covers both, from the basics of how a serial character is built to industrial Ethernet, OPC UA and MQTT, with the hands-on part that most references leave out: connection recipes for the common PLCs, cabling and grounding, network security, and how to troubleshoot a link with a meter, a USB adapter and a sniffer. Every number (a frame size, a cycle time, a voltage, a cable length) is worked out so you can reuse the method.

Start here: what do you need?

I need to…	Go to
understand layers, topologies, latency and how protocols fit together	Networking Fundamentals
wire RS-232 or RS-485, set baud rate and parity, terminate and bias a bus	Serial Communication: RS-232, RS-422 and RS-485
set up Modbus RTU or TCP, read a register map, decode a frame, fix a word-order problem	Modbus RTU, ASCII and TCP
understand or maintain PROFIBUS, CANopen, DeviceNet, AS-Interface, IO-Link or HART	Fieldbuses and Device Networks
design or fix an EtherNet/IP, PROFINET or EtherCAT network, switches, multicast and rings	Industrial Ethernet: EtherNet/IP, PROFINET and EtherCAT
choose IP addresses and masks, subnets, gateways, ports and VLANs	IP Addressing, Subnets and VLANs
send data to SCADA, a historian or the cloud with OPC UA or MQTT	OPC UA, MQTT and Industrial IoT
connect a particular PLC, drive or HMI: Allen-Bradley, Siemens, Mitsubishi, Omron and others	Connecting PLCs, Drives and HMIs
choose cable and connectors, route and shield them, or fit fiber	Cabling, Grounding and Noise
protect a control network: zones, firewalls, remote access, hardening	Industrial Network Security
find out why a link does not work, with a method and symptom tables	Troubleshooting Industrial Networks
use USB adapters, loopback plugs, serial sniffers, Wireshark and a scope	Sniffers, Analyzers and Serial Tools

A suggested order

If you are new to industrial networks, read the pages in this order. Each builds on the one before, and the previous and next links at the top and bottom of every page follow it. If you already work with networks, jump to the page you need.

1. Networking Fundamentals: Layers and protocol stacks, network levels, topologies, messaging models, speed, latency and determinism.
2. Serial Communication: RS-232, RS-422 and RS-485: How a serial character is built, the three electrical standards, DB9 pinouts and null modems, and a correct RS-485 bus.
3. Modbus RTU, ASCII and TCP: Frames, function codes, addressing, data formats, exceptions, timing and gateways, with computed examples.
4. Fieldbuses and Device Networks: PROFIBUS, CAN, CANopen, DeviceNet, AS-Interface, IO-Link, HART and CC-Link: how each works and where it fits.
5. Industrial Ethernet: EtherNet/IP, PROFINET and EtherCAT: The Ethernet physical layer, the real-time protocols, switches, VLANs, multicast and ring redundancy.
6. IP Addressing, Subnets and VLANs: Addresses and masks, subnet math, gateways, DHCP and BOOTP, ports, VLANs and a machine-network design example.
7. OPC UA, MQTT and Industrial IoT: The information model, client/server and publish/subscribe, Sparkplug, gateways and edge architectures.
8. Connecting PLCs, Drives and HMIs: Step-by-step connection recipes for Allen-Bradley, Siemens, Mitsubishi, Omron, IDEC, Unitronics, AutomationDirect and CODESYS.
9. Cabling, Grounding and Noise: Cable types and connectors, shielding and grounding, routing, fiber, and an installation checklist.
10. Industrial Network Security: Zones and conduits, segmentation, firewalls, remote access, hardening and the IEC 62443 approach.
11. Troubleshooting Industrial Networks: A layered method, symptom tables for serial, Modbus and Ethernet faults, diagnostics and a commissioning checklist.
12. Sniffers, Analyzers and Serial Tools: USB serial adapters, loopback plugs, terminal and Modbus tools, serial and Ethernet sniffing, Wireshark and scope checks.

The big picture

Networks are arranged in levels. Fast, deterministic links connect the controller to field devices; standard Ethernet joins controllers and supervisory systems; IT-style protocols carry data upwards. The boundaries are not strict: Ethernet-based protocols now reach down to the field level.

Protocols at a glance

Protocol	Medium	Speed	Model	Best for	Page
RS-232	DB9 cable, 15 m	to 115 kbit/s	Point to point	Programming ports, instruments	Serial
RS-485 / Modbus RTU	Twisted pair, 1 200 m	9.6 to 115 kbit/s	Master / slave	Meters, drives, sensors	Modbus
Modbus TCP	Ethernet	100 Mbit/s	Client / server	Simple device integration	Modbus
PROFIBUS DP	RS-485, violet cable	9.6 kbit/s to 12 Mbit/s	Master / slave	Existing Siemens and process plants	Fieldbuses
CANopen / DeviceNet	CAN bus	125 kbit/s to 1 Mbit/s	Multi-master, producer/consumer	Machines, drives, mobile equipment	Fieldbuses
AS-Interface, IO-Link	Flat cable; 3-wire M12	cycles of 0.4 to 10 ms	Master / slave	Binary and smart sensors	Fieldbuses
EtherNet/IP	Ethernet	100 Mbit/s or 1 Gbit/s	Producer / consumer	Rockwell and many drives	Industrial Ethernet
PROFINET RT / IRT	Ethernet	100 Mbit/s or 1 Gbit/s	Controller / devices	Siemens and many others; motion with IRT	Industrial Ethernet
EtherCAT	Ethernet	100 Mbit/s	Master / slaves, one frame	Fast motion and many I/O points	Industrial Ethernet
OPC UA	TCP, Ethernet	100 Mbit/s and up	Client / server, PubSub	SCADA, MES, vendor-neutral integration	OPC UA and MQTT
MQTT	TCP, Ethernet or cellular	any	Publish / subscribe	Gateways and the cloud	OPC UA and MQTT

Quick reference numbers

Topic	Number or rule	Page
Serial character time	bits per character ÷ baud: 10 bits at 9 600 baud = 1.04 ms (11 bits at 8E1 = 1.15 ms)	Serial
Modbus RTU frame gap	at least 3.5 character times (1.75 ms fixed above 19 200 baud)	Modbus
Modbus CRC-16	polynomial A001 (hex), start FFFF, sent low byte first	Modbus
Modbus address	register 40001 is wire address 0; function 03 reads, 06 and 16 write	Modbus
RS-485 limits	32 unit loads; 1 200 m at 9 600 baud; about 60 Ω across A and B with two 120 Ω terminators	Serial
Ethernet copper	100 m per channel; 100 Mbit/s uses pins 1, 2, 3, 6	Cabling
Usable addresses in a subnet	2(32 − prefix) − 2: /24 = 254, /26 = 62, /28 = 14	IP Addressing
Common ports	Modbus TCP 502; EtherNet/IP 44818 and 2222; S7 102; FINS 9600; OPC UA 4840; MQTT 1883 and 8883	IP Addressing
Frame time at 100 Mbit/s	84 bytes (minimum frame with preamble and gap) = 6.7 µs; 1 538 bytes = 123 µs	Fundamentals
Network load	packets per second × bytes per packet × 8; keep under 30 to 50 percent of a link	Industrial Ethernet
Fiber link budget	transmit power − sensitivity − losses (fiber, connectors, splices) = margin; keep at least 3 dB	Cabling
HART	1 200 / 2 200 Hz, ±0.5 mA on a 4–20 mA loop, at least 230 Ω	Fieldbuses

Tools to carry

Tool	What for	Page
Multimeter	Idle voltage, termination resistance, supply voltage, continuity	Sniffers and tools
USB-RS232 and USB-RS485 adapters (isolated)	A COM port on a laptop for terminals, Modbus tools and sniffing	Sniffers and tools
Loopback plug, breakout box, null modem adapter	Prove the PC side; swap pins without a soldering iron	Sniffers and tools
Terminal and Modbus software	Talk to a device without a PLC	Sniffers and tools
Cable tester	Wire map, length, opens, shorts	Cabling
Laptop with Wireshark	See Ethernet traffic: queries, replies, resets	Sniffers and tools
Oscilloscope	Signal quality on serial lines	Troubleshooting
Spare patch cords, terminators, connectors	Quick swaps	Cabling
The address list and register maps	Everything else depends on them	Troubleshooting

Calculators

• Modbus Frame and CRC Calculator: Build Modbus RTU requests, decode captured frames, check a CRC-16 and see the frame timing.
• IP and Subnet Calculator: Network and broadcast addresses, host range, same-subnet check and subnet splitting.
• 4–20 mA / 0–10 V Analog Scaling Tool: Convert a raw analog signal to engineering units and back.

Safety and security

• Electrical safety first. Panels contain mains and drive voltages. Isolate and verify before you touch anything, and follow your site’s lockout rules.
• Do not experiment on a running machine. Do not force, write or reset anything you do not understand, and never connect a sniffer that can transmit to a live control network.
• Communications are not safety devices. An emergency stop must act through hard-wired or certified safety hardware; a network can lose a message.
• Protect the network. Keep control networks off the internet, segment them, change default passwords and use a single controlled path for remote access: see Industrial Network Security.
• Get permission. Plugging a laptop into a production network needs approval on most sites.

FAQ

Where should I start if the link does not work?

Check the physical layer first (power, cable, link lights, polarity), then the settings (addresses, baud rate, names), then prove the device with a PC and a protocol tool, and only then look at the controller program. The page on Troubleshooting Industrial Networks gives a step-by-step method.

What is the difference between Modbus RTU and Modbus TCP?

They carry the same requests and data tables. RTU runs over a serial line (usually RS-485), uses a CRC and silent gaps to frame messages, and handles one transaction at a time. TCP runs over Ethernet, drops the CRC and slave address in favor of a header, and allows several clients. See Modbus RTU, ASCII and TCP.

Why are there 120 ohm resistors on an RS-485 bus?

The cable acts as a transmission line with an impedance of about 120 Ω. An unterminated end reflects the signal back and corrupts data at higher speeds or longer lengths. Resistors at the two ends match the cable and absorb the signal.

Is Ethernet fast enough for motion control?

Standard TCP/IP Ethernet is not deterministic enough. Real-time industrial Ethernet protocols (EtherCAT, PROFINET IRT, EtherNet/IP with CIP Motion) use special mechanisms to give predictable, sub-millisecond cycles on the same style of cable.

Which is better: a fieldbus or Ethernet?

For new designs Ethernet-based networks are the usual choice: they are faster, use standard cabling and tools, and connect easily to IT systems. Fieldbuses remain a sensible choice when extending an existing installation or where the devices you need support only them.

Do I need a managed switch?

For a small network with a few devices and no real-time requirement, an unmanaged industrial switch is often enough. For larger networks, EtherNet/IP I/O, ring redundancy, or when you need diagnostics and VLANs, use managed switches.

Should the PLC be the Modbus master?

Usually, yes: the PLC polls the drives, meters and sensors. An HMI or SCADA system then polls the PLC, which acts as a Modbus server. Only one master may be active on a serial bus.

References

• Modbus Organization, Modbus Application Protocol Specification V1.1b3, Modbus over Serial Line Specification and Implementation Guide V1.02, and Modbus Messaging on TCP/IP Implementation Guide V1.0b (modbus.org).
• TIA/EIA-485-A and TIA/EIA-232-F, electrical characteristics of serial interfaces; Texas Instruments application notes on RS-485 design.
• PROFIBUS & PROFINET International, PROFIBUS Installation Guideline and PROFINET design guidelines (profibus.com).
• ODVA, EtherNet/IP and DeviceNet specifications (odva.org).
• EtherCAT Technology Group (ethercat.org); CAN in Automation, CANopen application layer (CiA 301); IO-Link Community specification; FieldComm Group, HART.
• OPC Foundation, OPC UA specifications (IEC 62541); OASIS, MQTT Version 5.0; Eclipse Foundation, Sparkplug specification.
• IEC 62443 series, Industrial communication networks: network and system security; IEC 61918, Installation of communication networks in industrial premises.

Last reviewed: October 2026. Speeds, lengths and node counts are typical values: confirm against the specification and the manufacturer’s documentation for the devices you use.