Modbus TCP is a widely used communication protocol for industrial and IoT devices. It enables communication between a master (usually a computer or controller) and one or more slave devices (such as sensors or controllers) over a TCP/IP network.

What is Modbus TCP?

Modbus TCP is an Ethernet-based version of the Modbus protocol.
It uses a client-server architecture.
Communication occurs over port 502.
It supports reading and writing to data areas called:

Holding Registers (HR) – for configuration and text fields
Input Registers (IR) – for sensor readings
Discrete Inputs (DI) – for digital status flags

Decoding Text in Holding Registers

Holding Registers store ASCII strings as 16-bit values. Each 16-bit register (2 bytes) stores two ASCII characters:

High byte → First character
Low byte → Second character

When decoding:

Convert each 16-bit register into two 8-bit bytes.
Strip padding bytes like 0x00 (null characters).

Example:
Uint16Array.from([0x4865, 0x6C6C, 0x6F00])

Index Hex High Byte Low Byte Characters
0 0x4865 0x48 = 'H' 0x65 = 'e' He
1 0x6C6C 0x6C = 'l' 0x6C = 'l' ll
2 0x6F00 0x6F = 'o' 0x00 = \0 o

Holding Registers

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Input Registers

Property	Start Address	Size	Description	Friendly Notation	Legacy Address
Temp1 (×10)	0	1	Temperature Probe 1	IR[0]	30001
Temp2 (×10)	1	1	Temperature Probe 2	IR[1]	30002

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Discrete Inputs

Property	Start Address	Size	Description	Friendly Notation	Legacy Address
Door1Open	0	1	Door 1 status	DI[0]	10001
Reserved	1–7	7	Unused	DI[1–7]	10002–10008
LightRelay	8	1	Light Relay	DI[8]	10009
Reserved	9	1	Unused	DI[9]	10010
HeaterDoor1	10	1	Heater Door 1	DI[10]	10011
Reserved	11–13	3	Unused	DI[11–13]	10012–10014
ExternalAlarm	14	1	External Alarm	DI[14]	10015
Reserved	15–16	2	Unused	DI[15–16]	10016–10017
PowerFail	17	1	Power Fail	DI[17]	10018
Panic	18	1	Panic	DI[18]	10019
Door1Opened	19	1	Door 1 Opened Alarm	DI[19]	10020
Reserved	20	1	Unused	DI[20]	10021
Room1Installed	21	1	Room 1 Installed	DI[21]	10022
Room2Installed	22	1	Room 2 Installed	DI[22]	10023
Reserved	23	1	Unused	DI[23]	10024
TempProbe1NotGood	24	1	Temp Probe 1 Not Good	DI[24]	10025
HighTempProbe1	25	1	High Temp Probe 1 Alarm	DI[25]	10026
LowTempProbe1	26	1	Low Temp Probe 1 Alarm	DI[26]	10027
TempProbe2NotGood	27	1	Temp Probe 2 Not Good	DI[27]	10028
HighTempProbe2	28	1	High Temp Probe 2 Alarm	DI[28]	10029
LowTempProbe2	29	1	Low Temp Probe 2 Alarm	DI[29]	10030
Reserved	30–45	16	Unused	DI[30–45]	10031–10046
Wifi	46	1	WiFi Status	DI[46]	10047
Reserved	47	1	Unused	DI[47]	10048
Server	48	1	Server Connection	DI[48]	10049
Bluetooth	49	1	Bluetooth Status	DI[49]	10050
BatteryNotGood	50	1	Battery Not Good	DI[50]	10051
Reserved	51–55	5	Unused	DI[51–55]	10052–10056

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Access ModBus – Cloud Configuration

1. Log in to your account

2. Click on device you want to configure

3. Click on “Modbus” button near the device card, under “Program” button

4. A configuration window will appear, providing detailed information and settings for the Modbus client.

Recommended Software

1. Windows – Mod 32/64 Based on your operating System https://www.modbustools.com/download.html
2. MAC – QModMaster/ModPoll – https://www.modbusdriver.com/modpoll.html
3. Web – Node.js + jsmodbus or modbus-serial