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Modbus RTU vs TCP — what's what

Modbus TCP is Modbus wrapped in Ethernet and TCP/IP. Modbus RTU is the original version — serial communication over RS-485 or RS-232, bytes transmitted in binary with no network header.

In industrial practice, RTU still dominates field devices — sensors, transducers, meters, I/O modules. TCP shows up mainly where Ethernet is already present: controllers, HMI panels, SCADA systems.

RTU frame structure

Unlike TCP, the RTU frame has no MBAP header. Frame start and end are identified by silence on the line (minimum 3.5 character times).

[Address: 1B] [Function Code: 1B] [Data: N bytes] [CRC: 2B]

Address — slave address (1–247). Address 0 is broadcast (no response).
CRC — CRC-16/IBM, calculated from Address through the last Data byte. Transmitted little-endian (LSB first).

Frame example — read 2 registers from address 0, slave 1

Request:

01 03 00 00 00 02 C4 0B
│  │  │     │     └─ CRC (little-endian: C4, 0B)
│  │  └─────┴── start address 0x0000, count 0x0002
│  └── FC 03 = Read Holding Registers
└── slave address = 1

Response (2 registers = 4 bytes of data):

01 03 04 01 F4 00 00 F3 B0
│  │  │  └──────────┘  └─ CRC
│  │  └── Byte Count = 4
│  └── FC 03
└── slave address = 1

RS-485 — physical layer basics

RS-485 is a differential standard: two lines, A and B. Logic state "1" when A > B by at least 200 mV, "0" when B > A.

Key characteristics:

Half-duplex — cannot transmit and receive at the same time
Up to 32 devices on one bus (more with repeaters)
Maximum 1200 m at 100 kbps (shorter at higher speeds)
Requires 120 Ω termination at both ends of the bus

First connection in Python

from pymodbus.client import ModbusSerialClient

client = ModbusSerialClient(
    port="COM8",          # or "/dev/ttyUSB0" on Linux
    baudrate=9600,
    bytesize=8,
    parity="N",           # None — most common
    stopbits=1,
    timeout=1
)

client.connect()

result = client.read_holding_registers(address=0, count=10, slave=1)

if not result.isError():
    print(result.registers)

client.close()

Parameters (baudrate, parity, stopbits) must be identical on both sides. Check the device documentation — some manufacturers use Even parity or 2 stop bits.

Pitfalls

Missing termination — the most common problem. Without 120 Ω at the bus ends, signal reflections appear and communication works unstably or only on short cables.
Transmit direction — USB-to-RS485 converters require switching direction (RTS). Cheap converters do this automatically with a delay — if a frame gets cut off, try rtscts=True or a different converter.
Address 0 — broadcast, the slave doesn't respond. If you get a timeout at address 0, check whether you're sending a broadcast instead of a read.
Inter-byte gap — RTU requires silence between bytes no longer than 1.5 character times. Slow PCs can struggle to maintain timing at high baud rates.