Modbus RTU vs Modbus TCP: what actually differs, and which to use

Same command, different wrapper

Modbus dates to 1979 and is deliberately plain: "read N registers starting at address A from device X" is most of it. RTU and TCP are two envelopes for that same letter.

Because of that, function codes are shared. When a manual says "read with 03, write with 16", it applies to both transports.

Wiring and topology

RTU usually runs over two RS-485 wires. Devices chain along one pair, which is cheap and reaches well past a kilometre at 9600 bps. The cost is that everyone shares the line, so the master can only talk to one device at a time.

• Fit **120 Ω terminators** at both ends of the line. Without them reflections cause intermittent errors — fine on a short bench run, broken once the cable gets long.
• Get A/B (D+/D-) polarity right. Vendors label these inconsistently, so swapping the pair is the fastest test.
• A large **ground potential difference** between devices makes the link flaky. Run a common signal ground with the pair.
• Slave addresses run 1–247. Two devices sharing an address will collide on every reply.

TCP gives each device an IP and a port on a switch. If the plant network already exists there is almost no new cabling, and several clients can talk to the same device at once. In exchange you get the 100 m per-segment limit and switches and IPs to manage.

How much faster is it, really?

What you feel is not raw bit rate but the time for one round trip. Work it out for RTU at 9600 bps: with 8N1 each byte carries 10 bits, so 960 bytes per second — roughly 1 ms per byte.

request 8 bytes + response 9 bytes = 17 bytes  ~ 18 ms
+ inter-frame silence (3.5 chars) x2          ~  7 ms
+ device processing                           ~  5-20 ms
──────────────────────────────────────────────────────
one round trip ~ 30-45 ms  ->  20-30 per second

With ten slaves that is two or three updates per device per second. Plenty for temperature or flow; not enough when you need tens of milliseconds. On Ethernet the same exchange is typically 1–5 ms and devices can be polled in parallel, so update periods drop to single digits.

The addressing trap

Most lost hours are not about the protocol but the address. When a manual says 40001, the number that goes on the wire is 0. The 40001–49999 notation is a legacy way of naming holding registers; frames carry a zero-based offset.

In Python, only two lines change

Since the command is the same, so is the code. With pymodbus only the client constructor differs.

# pip install pymodbus
from pymodbus.client import ModbusSerialClient, ModbusTcpClient

# RTU — serial port
client = ModbusSerialClient(port="COM3", baudrate=9600,
                            parity="N", stopbits=1, bytesize=8)

# TCP — IP address (this line instead of the one above)
# client = ModbusTcpClient("192.168.0.50", port=502)

client.connect()
rr = client.read_holding_registers(address=0, count=2, slave=1)
print(rr.registers)          # e.g. [258, 16000]
client.close()

To practise without hardware, run a Modbus server on your PC and point a client at it on the same machine. The robot communication track walks through exactly that setup.

Which one should you pick?

With a gateway, the Unit ID is the key: the value in the TCP request is passed through as the RS-485 slave address behind it, so it has to match the device address to get an answer.

When nothing answers

RTU

1. 1Match the serial parameters exactly — baud rate, parity, stop bits. 9600 8N1 against 9600 8E1 gives you nothing at all.
2. 2Swap A/B. Vendor labelling is inconsistent and this is a common cause.
3. 3Check for duplicate slave addresses, and whether the manual's addresses are 0- or 1-based.
4. 4Check the 120 Ω terminators at both ends and the common ground — the usual suspects behind intermittent errors.
5. 5Still nothing: connect a single slave directly. If one works and several fail, it is wiring or termination.

TCP

1. 1ping the device first, then check that port 502 is open — firewalls block it regularly.
2. 2Check the Unit ID. Some devices expect something other than 1, and behind a gateway it must match the serial address.
3. 3Check the connection limit. Cheap devices accept one or two sockets and refuse you if another program already holds them.
4. 4Capture with Wireshark: is the request going out unanswered, or coming back as an exception (function code + 0x80)? Exception 02 means a bad address, 03 a bad quantity.

FAQ