Physical Layer

This document specifies the physical layer options for Fusain protocol communication. For network-based transports, see TCP Transport and WebSocket Transport.

Overview

Fusain uses UART-based packet framing and supports multiple physical layer transports. The protocol remains unchanged across all physical layers - transceiver ICs handle the adaptation between Fusain packets and the physical bus. For routing between physical layers, see Packet Routing.

Physical Layer	Baud Rate	Status	Use Case
Plain UART	230400	Development only	Lab bench testing, prototyping
RS-485	115200	Production	Multi-appliance networks, long distances
LIN	19200	Production (default)	Single-wire retrofits

Transceiver Architecture

The MCU firmware operates identically regardless of physical layer. Transceiver ICs handle all physical layer details:

MCU (Fusain firmware)
    |
    | UART (Fusain packets)
    |
[Transceiver IC]
    |
    | LIN / RS-485 / None (plain UART)
    |
Physical Bus

The transceiver IC handles:

• Physical layer signaling (LIN break/sync, RS-485 differential, etc.)

• Frame fragmentation/reassembly (for LIN’s 8-byte frame limit)

• Fusain packet buffering and delivery

• Direction control (RS-485 DE/RE pins)

UART Configuration

All physical layers use the same UART parameters at the MCU level:

Parameter	Value
Data bits	8
Parity	None
Stop bits	1
Flow control	None
Baud rate	Physical layer dependent (see below)

Baud Rates by Physical Layer

Physical Layer	Baud Rate	Notes
Plain UART	230400	Direct MCU-to-MCU connection
RS-485	115200	Via RS-485 transceiver
LIN	19200	Via UART-to-LIN transceiver

Plain UART

Status: Development and prototyping only

Plain TTL UART provides direct connection between devices for development and testing. Not recommended for production due to limited distance and noise immunity.

Specifications

Parameter	Value
Maximum distance	~15m (depends on cable quality and environment)
Multi-drop support	No (point-to-point only)
Noise immunity	Low
Wiring	TX, RX, GND (3 wires minimum)

Use Cases

• Initial firmware development

• Protocol validation and debugging

• Bench testing with short cables

RS-485

Status: Production (multi-appliance networks)

RS-485 provides robust, differential signaling suitable for long distances and multi-drop networks. Use RS-485 when:

• Multiple appliances share one bus

• Cable runs exceed 40m

• Environment has high electrical noise

Specifications

Parameter	Value
Maximum distance	1200m (ideal conditions)
Typical distance	100m (guaranteed with CAT5)
Maximum nodes	32-256 (depends on transceiver unit load)
Noise immunity	High (differential signaling)
Wiring	A, B, GND (twisted pair)

Transmit Enable Control

RS-485 transceivers require driver enable (DE) and receiver enable (RE) control.

• Assert DE before transmission begins

• Wait for UART TX shift register to empty before de-asserting DE

• RE is typically active-low and tied inverse to DE

Timing Requirements

Parameter	Value
Inter-packet gap	500μs minimum (prevents collisions)
Turn-around time	1-10μs (transceiver dependent)

Multi-Drop Topology

RS-485 supports multiple devices on a single bus.

Daisy-Chain (Recommended)

[Controller]---[Appliance 1]---[Appliance 2]---[Appliance 3]
    120Ω                                            120Ω

• Devices connected in series along the main bus

• Short stubs (<30cm) from bus to each device

• Termination resistors (120Ω) at both physical ends only

• Best signal integrity and maximum distance

Star Topology (Not Recommended)

Star topology creates reflections at branch points and is not recommended for RS-485. If unavoidable, keep branch stubs under 30cm.

Termination

Requirements

• 120Ω resistors at both physical ends of the bus

• 1/4W or greater power rating

• Must match cable characteristic impedance

Correct Termination

[Controller]---[Appliance 1]---[Appliance 2]---[Appliance 3]
    120Ω                                            120Ω

Incorrect Termination

[Controller]---[Appliance 1]---[Appliance 2]---[Appliance 3]
    120Ω            120Ω            120Ω            120Ω
(Problem: Over-termination causes signal attenuation)

[Controller]---[Appliance 1]---[Appliance 2]---[Appliance 3]
    120Ω
(Problem: Reflections from unterminated far end)

Cable Specifications

Parameter	Specification
Cable type	Twisted pair (required)
Characteristic impedance	120Ω
Recommended cables	Industrial RS-485 cable, CAT5/CAT6 (one pair)
Avoid	Parallel untwisted wires

Shielding

• Shielded (STP): Recommended for industrial/noisy environments

• Unshielded (UTP): Acceptable for typical installations

Grounding and Biasing

Common-Mode Ground

RS-485 is differential, but common-mode voltage must stay within ±7V of device ground. Connect all device grounds together, or use isolated transceivers for ground isolation up to 2.5kV.

Fail-Safe Biasing

When no drivers are active, the bus is in an undefined state. Add bias resistors at the controller:

• A to +5V: 560Ω (pulls A high)

• B to GND: 560Ω (pulls B low)

This ensures the bus idles in a defined state.

Collision Avoidance

Fusain requires software collision avoidance on RS-485. See Communication Patterns for polling mode usage.

1. Discovery Phase

   DISCOVERY_REQUEST causes all appliances to respond. Each appliance MUST wait a random delay (0-50ms) before sending DEVICE_ANNOUNCE.

2. Normal Operation

   Use addressing to send commands to specific appliances. Each appliance only responds when addressed.

3. Polling Mode

   For multi-appliance networks, use polling mode (TELEMETRY_CONFIG with interval_ms=0) to prevent broadcast collisions.

Node Capacity

RS-485 node limits depend on transceiver unit load (UL):

Transceiver Type	Unit Load	Max Nodes	Example ICs
Standard	1 UL	32	MAX485, SN75176
Low-power	1/2 UL	64	MAX3485
High-density	1/4 UL	128	ADM2682E
Ultra-high-density	1/8 UL	256	MAX22501E

Recommended Hardware

Transceivers

• MAX485: Basic, low cost

• MAX3485: Low power, 1/2 unit load

• ADM2682E: Isolated, 1/4 unit load

Cables

• Belden 3105A: Industrial RS-485

• CAT5e: Budget option (use one twisted pair)

Connectors

• RJ45: Ethernet-style

• Phoenix Contact: Industrial terminal blocks

LIN

Status: Production (default for single-appliance networks)

LIN (Local Interconnect Network) is the default physical layer for Fusain in production environments. Most heaters being retrofitted only have one wire available for appliance-to-controller communication.

Warning

LIN MUST NOT be used for multi-appliance networks. LIN’s single-master architecture and limited collision handling make it unsuitable for multi-drop configurations. Use RS-485 for multi-appliance networks.

Specifications

Parameter	Value
Maximum distance	~40m
Maximum nodes	2 (Fusain restriction: 1 controller + 1 appliance)
Noise immunity	Medium-high
Wiring	LIN (single wire) + GND
Termination	Built into transceiver ICs

Transceiver Architecture

The Fusain protocol runs unchanged at the MCU level. A UART-to-LIN transceiver IC handles all LIN physical layer requirements:

Controller MCU                          Appliance MCU
    |                                        |
    | UART (19.2 kbaud)                      | UART (19.2 kbaud)
    |                                        |
[UART-to-LIN IC]                        [UART-to-LIN IC]
    |                                        |
    +--------------- LIN Bus ----------------+
           (single wire + ground)

Transceiver Responsibilities

• Voltage Translation: Converts UART logic levels to LIN bus levels

• Slew Rate Control: Limits signal transitions for EMC compliance

• Wake/Sleep: Handles LIN bus wake-up and sleep modes

Recommended Transceiver ICs

IC	Interface	Features	Use Case
TJA1027	UART	Transparent UART-to-LIN, low standby current, automotive-grade	Recommended
TLIN1029	UART	Diagnostic features, integrated regulator	Industrial
MCP2003B	UART	Low-cost, basic functionality	Budget builds
NCV7321	SPI	SPI-controlled, flexible	Complex systems

Performance Characteristics

Metric	Value	Notes
Baud rate	19.2 kbaud	LIN standard
Telemetry interval	500ms minimum	See Link Bandwidth Requirements

Configuration

1. Baud Rate: 19.2 kbaud at MCU UART

2. Telemetry Intervals: 500ms minimum. See Link Bandwidth Requirements.

3. Discovery: Random delay (0-50ms) MUST be applied before responding to DISCOVERY_REQUEST for protocol consistency.

Wiring

• LIN Bus: Single wire (12V nominal, automotive-grade wire)

• Ground: Separate ground return

• Termination: Built into most transceiver ICs (no external resistors)

• Connectors: Automotive-grade (Deutsch, AMP, or equivalent)

Benefits

• Single-wire topology: Ideal for retrofits with limited wiring

• Automotive-grade reliability: Proven in harsh environments

• Built-in collision avoidance: LIN centralized scheduling

• Lower cost: Simpler than RS-485

• Standard compliance: LIN 2.0+ specification

Comparison Summary

Feature	Plain UART	RS-485	LIN	Notes
Distance	~15m	~1200m	~40m
Baud rate	230400	115200	19200
Multi-drop	No	Yes (32-256)	No	Fusain restriction
Wiring	3-wire	2-wire differential	1-wire + GND
Noise immunity	Low	High	Medium-high
Recommended use	Development	Production	Production
Typical use case	Lab testing	Multi-appliance	Single-wire retrofit

Recommendations

Production Deployments

1. Single-wire retrofits (most common): LIN

   • Use TJA1027 or equivalent transceiver

   • Configure 19.2 kbaud at MCU UART

   • Set telemetry interval to 500ms minimum

   • Ideal for heater retrofits with existing single-wire infrastructure

2. Multi-appliance networks or long distances (>40m): RS-485

   • Implement DE/RE control in firmware

   • Use daisy-chain topology with 120Ω termination at both ends

   • Add random delays (0-50ms) to DISCOVERY_REQUEST responses

   • Use polling mode for multi-appliance networks

Development and Prototyping

• Use plain UART for initial development

• Direct UART connection between devices

• Suitable for bench testing and firmware development

Protocol Configuration Summary

Environment	Physical Layer	Baud Rate	Telemetry Interval
Production (single-appliance)	LIN	19200	500ms
Production (multi-appliance)	RS-485	115200	100-500ms
Development	Plain UART	230400	100ms

All configurations use identical packet format. See Packet Format for framing, CRC, and addressing. See Implementation Guide for buffer sizing and timeouts.