Modbus

Modbus is simple, ubiquitous and completely untyped: the device gives you 16-bit registers and it is your job to know what they mean. QUBIQ's Modbus support is about making that mapping explicit and repeatable.

Configuration

Field	Description
Name	Connection identifier.
Host	Device IP or hostname.
Port	502 for Modbus TCP; the gateway's port for RTU-over-TCP.
Framing mode	tcp (Modbus TCP), rtu (RTU framing over a serial gateway), ascii.
Unit ID	The slave/unit address. 1 unless the device or gateway says otherwise.
Byte order	How multi-register values are assembled — see below.
Connect timeout (ms)	How long to wait for the socket.
Request timeout (ms)	How long to wait for a response before failing the read.
Probe address / quantity	The register the connection test reads to prove the link.

Byte and word order

The single most common source of wrong Modbus values. A 32-bit float spans two registers, and vendors disagree on the order of both the words and the bytes within them.

Order	Also called	Registers for 1.0f (0x3F800000)
Big-endian	ABCD	0x3F80, 0x0000
Little-endian	DCBA	0x0000, 0x803F
Big-endian byte swap	BADC	0x803F, 0x0000
Little-endian byte swap	CDAB	0x0000, 0x3F80

If a value reads as an absurd number (4.6e-41, 1.7e38), the order is wrong — not the address. Read a register whose value you know, and try orders until it matches.

Register addressing

Modbus has four address spaces. Which one you are reading is part of the address.

Space	Function code	Access	Typical notation
Coils	1 / 5 / 15	read/write bit	0xxxx
Discrete inputs	2	read bit	1xxxx
Input registers	4	read 16-bit	3xxxx
Holding registers	3 / 6 / 16	read/write 16-bit	4xxxx

Off-by-one

Vendor documentation is inconsistent about whether addresses are 0-based (protocol) or 1-based (documentation). Holding register "40001" in a manual is protocol address 0. If everything is shifted by exactly one, this is why.

Binding tags

Field	Notes
Register	The register space (HR, IR, Coil, DI), the number, and 1-based addressing (datasheet) if you are typing the number from a manual. They compose the address, e.g. HR:100.
Byte / Word Order	ABCD (default), CDAB, BADC, DCBA — see byte and word order. The width comes from the tag's data type.
Poll Rate (ms)	Poll period, 1000 by default.

Modbus is strictly request/response: every Modbus tag is polled. The deadband is the tag's own, on its History tab.

Poll rate and register grouping

QUBIQ groups contiguous registers into single requests. Two consequences:

• Contiguous addresses are much cheaper than scattered ones. If you can influence the device's register map, block related values together.
• Do not poll faster than the device can answer. A small PLC serving a serial gateway may manage a few requests per second. Polling every 100 ms produces timeouts, not fresher data.

Start at 1000 ms and reduce only where a faster value demonstrably matters.

Verifying a register

Use Read registers in the connection's editor (needs View connections). It performs a one-shot read of a register range, so you can confirm an address and byte order before building tags on it.

Scaling

Devices frequently transmit scaled integers: temperature as tenths of a degree, pressure as hundredths of a bar. Do not scale in every widget — configure it once on the tag:

Raw low   0        Eng low   0.0
Raw high  10000    Eng high  100.0

Scaling is applied after the read and before history and alarms, so everything downstream sees engineering units. → Tag properties

Writing

Writable tags issue function code 5/6/15/16 depending on the target space and width. A write is acknowledged by the device; a failure surfaces as a rejected write, not a silent no-op.

Gate device writes with a write-security level on the tag so only sufficiently privileged runtime operators can drive equipment. → LiveView authentication

Troubleshooting

Symptom	Check
Connection test times out	Wrong port; some devices use RTU-over-TCP on a non-502 port. Check the unit ID too.
Illegal data address	The register does not exist, or the off-by-one above, or the wrong address space.
Illegal function	The device does not support that function code — often read-only registers being written.
Values are nonsense floats	Byte/word order.
Values are exactly 10× or 100× off	Missing scaling.
Intermittent timeouts	Polling faster than the device or serial gateway can serve. Slow down.
One tag Bad, the rest fine	That register is outside a valid block; the grouped read fails for it only.

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