Industrial Fieldbus Protocols

This page outlines the practical aspects of supporting Modbus, PROFIBUS, PROFINET, and Interbus on EsoCore hardware and firmware. It focuses on physical interfaces, topology, addressing, timing, EMC, and certification considerations so designs are robust in industrial environments.

Dual Ethernet Port Architecture

EsoCore uses two physically separated Ethernet ports for true network isolation between operational technology (OT) and IT infrastructure:

Port A (Machine Network): PROFINET RT, Modbus TCP, on-device web UI (HTTPS), PLC communications
- Static IP (e.g., 192.168.10.x), connected to machine VLAN switch
- No internet access; isolated from corporate network
Port B (IT Network): HTTPS cloud sync, REST API, NTP time sync, OTA firmware updates
- DHCP (e.g., 10.x.x.x), connected to corporate network with firewall to internet

Each port has an independent PHY (KSZ8081RNACA), independent magnetics (Wurth 7499111121), and a separate MAC address. This provides:

True isolation: No VLAN hopping attacks; physical separation between OT and IT
Independent failure domains: Cloud outage does not affect machine communication; machine network issues do not disrupt cloud sync
Clear security boundary: Airgap between OT and IT; compliance with industries requiring physical network separation
Easier troubleshooting: Separate LEDs per port; Wireshark on one port does not show other port's traffic

See Dual Ethernet Ports for full architecture details, PHY selection rationale, and PCB layout guidelines.

How It Works

Port A carries all real-time and machine-facing traffic. PROFINET RT frames use 802.1Q priority tagging (PCP=6) for deterministic performance. Modbus TCP (port 502) and the on-device web UI (HTTPS, port 443) coexist on Port A with QoS ensuring RT priority.

Port B handles all IT-facing traffic. Cloud sync, OTA updates, NTP, and REST API access run on Port B, completely isolated from machine communications. This means cloud-side issues (DNS failures, certificate problems, high latency) never impact real-time machine control.

Network Load: Follow PI recommendations to keep Port A link utilization below 50%. EsoCore's web UI and diagnostics on Port A are designed with rate limiting to respect real-time cycle requirements.

Modbus RTU (RS‑485)

Physical Layer

RS‑485 half‑duplex, differential pair A/B plus reference GND
Isolated transceiver recommended for ESD/EMC and ground potential differences
3‑pin terminal block (A/B/G) with shield termination near chassis
Switchable 120 Ω termination + optional biasing (pull‑up/pull‑down)

Topology & Cabling

Linear bus (daisy chain), avoid stubs
Cable: twisted pair, 120 Ω characteristic impedance
Max nodes: 32 per segment without repeaters (more with modern transceivers)

Addressing & Framing

Node addressing: 1–247 (DIP switch or software configurable)
8‑E‑1 or 8‑N‑2 common, CRC‑16 integrity

Timing & Throughput

Typical 9600–115200 bps (up to 1 Mbps in some systems)
Silent interval between frames (3.5 character times) must be enforced

Firmware Notes

UART with DMA + precise inter‑frame timing
CRC‑16 table or hardware accelerator

BOM & Connectors

Isolated RS‑485 transceiver
3‑pin 5.08 mm terminal block
DIP switch for address and termination enable

Modbus TCP (Ethernet)

Physical Layer

Standard 10/100 Ethernet on Port A (Machine Network)
Shares Port A with web UI and PROFINET; isolated from Port B (IT Network)

Topology & Addressing

IP addressing via static IP on Port A (same IP as web UI and PROFINET on machine network)
TCP port 502

Firmware Notes

Lightweight TCP stack with Modbus application server
Same function codes as RTU with PDU carried over TCP
Coexists with PROFINET RT on Port A via QoS/priority handling
Multiple simultaneous client connections supported

PROFIBUS DP

Physical Layer

RS‑485 with PROFIBUS electrical levels
DB‑9 male with shield and integrated chassis connection
Termination selectable via switch at both ends of the segment

Topology & Performance

Linear bus with repeaters for large networks
9.6 kbps to 12 Mbps, timing tighter than generic RS‑485

Addressing & Configuration

Node address 0–126, usually via rotary/DIP switch or software tool
GSD file describes device to the PLC/engineering tool

Firmware Notes

Real‑time deterministic stack required for high baud rates
State machine: parameterization → configuration → data exchange

BOM & Connectors

PROFIBUS‑compliant RS‑485 transceiver
DB‑9 male with shield
DIP switch for address/termination

PROFINET (Device)

Physical Layer & Topology

Standard Ethernet (RJ45 with magnetics) on Port A (Machine Network)
Dual-port design: Port A carries PROFINET RT alongside web UI and Modbus TCP; Port B is isolated for IT traffic
Star or line topology via switches; dual-port design supports future MRP (Media Redundancy Protocol) capability

Real‑Time Classes

RT (Real Time): cyclic I/O over VLAN‑tagged frames (EtherType 0x8892, PCP=6)
IRT (Isochronous Real Time): tighter jitter with special switch silicon (future option)

Addressing & Discovery

IP via DCP set name/IP, discovery with LLDP
GSDML file describes device to engineering tools
Port A IP address: Web UI, Modbus TCP, and PROFINET all accessible via Port A's IP on the machine network

Firmware Notes

Prioritized traffic, bounded cycle times (2–4 ms typical for RT)
Alarms, diagnostics, records; SNMP/LLDP for topology
QoS handling ensures RT frames take priority over best-effort IP traffic on Port A
Rate-limited web UI and diagnostics to maintain <50% link utilization on Port A

BOM & Connectors

Port A Ethernet PHY (KSZ8081RNACA) and MagJack (Wurth 7499111121) for all machine-facing protocols
Port B Ethernet PHY and MagJack are independent and dedicated to IT traffic
Dual-port design supports future MRP (Media Redundancy Protocol) for line topology redundancy

Interbus

Background

Interbus is an obsolete fieldbus protocol still found on some older Bosch Rexroth controller machines. While Interbus modules are increasingly hard to source, customer demand for legacy equipment support requires EsoCore to provide connectivity.

Physical Layer

RS-485 differential signaling at the physical layer
Dedicated isolated RS-485 transceiver (ADM2582EBRWZ) separate from Modbus RTU and PROFIBUS interfaces
4-pin terminal block connector (A/B/GND/Shield) with switchable 120 ohm termination
Cable: shielded twisted pair, 120 ohm characteristic impedance

Protocol Characteristics

Master/slave architecture with summation-frame approach
Deterministic cyclic behavior: strict cycle timing with defined scan rates
Frame construction: summation frame collects data from all slaves in a single bus cycle
Not a generic RS-485 protocol: despite using RS-485 at the physical layer, Interbus has its own protocol timing, frame structure, and diagnostics that differ fundamentally from Modbus RTU

Firmware Considerations

Adding native Interbus support is feasible in principle but non-trivial due to:

Strict cycle timing: master scheduling must meet deterministic deadlines
Frame construction/parsing: summation-frame protocol requires precise assembly and disassembly of multi-slave data
Diagnostics: Interbus has its own diagnostic channel and fault reporting mechanisms
Scarcity of tooling: modern, maintained embedded stacks and test equipment for Interbus are rare
Validation: limited availability of Interbus slave devices for testing and certification

BOM & Connectors

Isolated RS-485 transceiver (ADM2582EBRWZ) dedicated to Interbus
4-pin pluggable terminal block (Degson 2EDGR-5.0-04P / 2EDGK-5.0-04P)
120 ohm termination resistor (switchable)

EMC, Safety and Certification

Cable shield bonding at the connector to chassis ground
ESD arrays on data lines; common‑mode chokes for USB/Ethernet where needed
Creepage/clearance and isolation for RS‑485 and power domains
Follow conformance test plans: Modbus interoperability tests, PROFIBUS (PIT), PROFINET (PNO) certification

References

See the Hardware Requirements for mechanical and connector details
See the BOM for specific transceivers, connectors, and switches