Modbus 6-Digit Addressing Scheme Guide¶

Quick Reference

6-Digit Address Format: [Type][5-Digit Register Number]

Examples: 400001, 300025, 000013, 165536

Overview¶

The 6-digit addressing scheme is an industry-standard format used by Modbus Monitor tools to combine both the register type and register address into a single, easy-to-use number. This eliminates the confusion between function codes and addresses that often troubles users new to Modbus.

Common Confusion Alert

The first digit is NOT the Modbus function code! It's a register type indicator. Many users confuse this prefix with function codes, leading to addressing errors.

Why Use 6-Digit Addressing?¶

Traditional Modbus Confusion:¶

User thinks: "I need Function Code 3, Address 123"
Reality: FC03 with address 123 could mean:
- Register 123 (0-based) = Physical register 124
- Register 123 (1-based) = Physical register 123
- Different devices may interpret this differently!

6-Digit Clarity:¶

User enters: "400123"
System knows: Holding register #123, automatically handles:
- Correct function code (FC03 for read, FC06/16 for write)
- Proper addressing mode (0-based vs 1-based)
- Register type validation

Format Structure¶

graph TD
    A["6-Digit Address<br/>400123"] --> B["Type Prefix<br/>4"]
    A --> C["Register Number<br/>00123"]

    B --> D["Holding Register<br/>Read/Write Access"]
    C --> E["Register #123<br/>(123rd register)"]

    D --> F["Function Codes:<br/>FC03 (Read)<br/>FC06/16 (Write)"]
    E --> G["Physical Location:<br/>0-based: 122<br/>1-based: 123"]

    style A fill:#e3f2fd,stroke:#1976d2,stroke-width:3px,font-size:16px
    style B fill:#fff3e0,stroke:#f57c00,stroke-width:2px
    style C fill:#f3e5f5,stroke:#7b1fa2,stroke-width:2px
    style D fill:#e8f5e9,stroke:#388e3c,stroke-width:2px
    style E fill:#ffebee,stroke:#c62828,stroke-width:2px

Register Type Prefixes¶

Prefix	Register Type	Access	Function Codes	Address Range	Physical Memory
0	Coils	Read/Write	FC01 (Read) FC05 (Write Single) FC15 (Write Multiple)	`000001` - `065536`	Digital outputs, Boolean values
1	Discrete Inputs	Read Only	FC02 (Read)	`100001` - `165536`	Digital inputs, Boolean status
3	Input Registers	Read Only	FC04 (Read)	`300001` - `365536`	Analog inputs, sensor data
4	Holding Registers	Read/Write	FC03 (Read) FC06 (Write Single) FC16 (Write Multiple)	`400001` - `465536`	Analog outputs, setpoints, configuration

Memory Layout Analogy

Think of Modbus memory like a filing cabinet:

Coils (0xxxxx): Light switches - ON/OFF states you can control
Discrete Inputs (1xxxxx): Sensors - ON/OFF states you can only read
Input Registers (3xxxxx): Gauges - Analog values you can only read
Holding Registers (4xxxxx): Control knobs - Analog values you can read and adjust

Complete Address Examples¶

Holding Registers (4xxxxx) - Most Common¶

6-Digit Address	Meaning	Function Code	Use Case
`400001`	1^st holding register	FC03 (Read) / FC06 (Write)	First setpoint or control value
`400025`	25^th holding register	FC03 (Read) / FC06 (Write)	Temperature setpoint
`400100`	100^th holding register	FC03 (Read) / FC06 (Write)	Configuration parameter
`465536`	Last possible holding register	FC03 (Read) / FC06 (Write)	Maximum address

Input Registers (3xxxxx) - Sensor Data¶

6-Digit Address	Meaning	Function Code	Use Case
`300001`	1^st input register	FC04 (Read)	Temperature sensor
`300010`	10^th input register	FC04 (Read)	Pressure reading
`300050`	50^th input register	FC04 (Read)	Flow rate measurement

Coils (0xxxxx) - Digital Outputs¶

6-Digit Address	Meaning	Function Code	Use Case
`000001`	1^st coil	FC01 (Read) / FC05 (Write)	Motor start/stop
`000008`	8^th coil	FC01 (Read) / FC05 (Write)	Alarm acknowledgment
`000016`	16^th coil	FC01 (Read) / FC05 (Write)	Valve control

Discrete Inputs (1xxxxx) - Digital Inputs¶

6-Digit Address	Meaning	Function Code	Use Case
`100001`	1^st discrete input	FC02 (Read)	Emergency stop button
`100005`	5^th discrete input	FC02 (Read)	Door open sensor
`100012`	12^th discrete input	FC02 (Read)	Motor running status

Converting from Other Tools¶

From Modbus Poll/Similar Tools:¶

Your Tool Setup	6-Digit Equivalent	Explanation
FC03, Address 1	`400001`	Holding register 1
FC03, Address 123	`400123`	Holding register 123
FC04, Address 5	`300005`	Input register 5
FC01, Address 13	`000013`	Coil 13
FC02, Address 8	`100008`	Discrete input 8

Quick Conversion Formula:¶

Function Code → Prefix Mapping:
- FC01/05/15 (Coils) → Add "0" prefix
- FC02 (Discrete Inputs) → Add "1" prefix  
- FC03/06/16 (Holding Registers) → Add "4" prefix
- FC04 (Input Registers) → Add "3" prefix

Example: FC03, Address 567 → "4" + "00567" = 400567

Zero-Based vs One-Based Addressing¶

Critical Concept for Proper Communication

The same 6-digit address may point to different physical registers depending on the device's addressing mode.

Understanding the Difference:¶

Address Mode	`400001` Points To	`400000` Points To
One-Based	1^st physical register	Invalid address
Zero-Based	2^nd physical register	1^st physical register

Device Examples:¶

graph TD
    A["Device Documentation Says"] --> B["First Holding Register is 'Register 0'<br/>(Zero-Based)"]
    A --> C["First Holding Register is 'Register 1'<br/>(One-Based)"]

    B --> D["Use: 400000<br/>for first register"]
    C --> E["Use: 400001<br/>for first register"]

    style A fill:#e3f2fd,stroke:#1976d2,stroke-width:2px
    style B fill:#ffebee,stroke:#c62828,stroke-width:2px
    style C fill:#e8f5e9,stroke:#388e3c,stroke-width:2px
    style D fill:#ffebee,stroke:#c62828,stroke-width:2px
    style E fill:#e8f5e9,stroke:#388e3c,stroke-width:2px

How to Determine Your Device's Mode:¶

Check device documentation - Look for "Register 0" (zero-based) or "Register 1" (one-based)
Test with known values - Read a register with a known value
Look for address examples - Manufacturer examples often show the convention
Contact manufacturer - When in doubt, ask technical support

Common Device Patterns:¶

Device Type	Typical Mode	Example
PLCs (Allen-Bradley, Siemens)	One-based	First register = 400001
Meters/Instruments	One-based	First register = 400001
Embedded devices	Zero-based	First register = 400000
Custom hardware	Varies	Check documentation

Common Mistakes and Solutions¶

❌ Mistake 1: Using Function Code as Prefix¶

Wrong: "I need FC03, so I'll use 300001"
Right: "I need holding registers, so I'll use 400001"

Solution: Remember prefix indicates register type, not function code.

❌ Mistake 2: Mixing Addressing Modes¶

Wrong: Device uses 0-based, but you enter 400001 for first register
Result: You're actually reading the second register!

Solution: Test with one register first, verify you get expected data.

❌ Mistake 3: Invalid Prefix Combinations¶

Wrong: Using 200001 (prefix "2" doesn't exist)
Wrong: Using 500001 (prefix "5" doesn't exist)

Solution: Only use prefixes 0, 1, 3, or 4.

❌ Mistake 4: Address Range Errors¶

Wrong: 400066000 (exceeds 5-digit limit)
Wrong: 4000000 (too many digits)

Solution: Maximum register number is 65536 (e.g., 465536).

Advanced Topics¶

Multiple Register Reads¶

When reading consecutive registers, you can often specify:

Start Address: 400001
Count: 10

Result: Reads registers 400001 through 400010

Bit-Level Access¶

Some tools allow bit access within registers:

Address: 400001.5
Meaning: Bit 5 of holding register 1

Block Optimization¶

For efficiency, group related addresses:

Good: 400001-400010 (consecutive block)
Inefficient: 400001, 400005, 400009 (scattered reads)

Troubleshooting Guide¶

No Response from Device¶

Verify addressing mode - Try both 400000 and 400001 for first register
Check register type - Ensure you're using correct prefix (0,1,3,4)
Confirm device support - Not all devices implement all register types
Test with minimal address - Start with 400001 or 400000

Wrong Data Returned¶

Address offset issue - Wrong addressing mode (0-based vs 1-based)
Register type mismatch - Using 3xxxxx when device expects 4xxxxx
Endian/byte order - Data format issue (separate from addressing)

Address Rejected by Device¶

Exceeds device range - Device may only support 1-1000, not full 65536
Invalid register type - Device doesn't implement that register type
Reserved addresses - Some addresses may be reserved/protected

Best Practices¶

✅ Do:¶

Start simple - Test with 400001 first
Document working addresses - Keep a list of validated addresses
Use consecutive blocks - More efficient than scattered reads
Verify addressing mode - Test first register to confirm 0-based vs 1-based
Check device limits - Not all devices support the full 65536 range

❌ Don't:¶

Assume function codes - Let the tool handle function code selection
Mix addressing modes - Stick to one mode throughout your project
Skip documentation - Always check device manuals for addressing conventions
Use invalid prefixes - Only 0, 1, 3, 4 are valid
Exceed register limits - Respect device-specific address ranges

Reference Links¶

Video Tutorial: Modbus Monitor YouTube Channel
Modbus Specification: Wikipedia Modbus Article
Interactive Tools: Use address conversion utilities in Modbus Monitor XPF
Additional Help: See product-specific guides for detailed examples

This guide applies to both Modbus Monitor XPF and Modbus Monitor Android applications. The 6-digit addressing scheme ensures consistent, clear communication with Modbus devices while eliminating common addressing confusion.