Modbus RTU in the Control Cabinet: Correctly Setting Termination, Biasing and Addresses

→ Product category: Modbus I/O

 

Several Modbus I/O modules are mounted on the DIN rail, the supply voltage is present and each device works individually. However, as soon as all participants are connected to the RS485 line together, communication errors suddenly occur:

Timeouts, CRC errors or individual participants do not respond

The cause is often not the Modbus protocol itself, but the structure of the serial RS485 connection.

With Modbus RTU, several aspects must be correct at the same time:

• bus wiring and topology,
• termination,
• biasing or polarization,
• participant addresses,
• baud rate, parity and stop bits,
• EMC-compliant cable routing.

A Modbus participant can be configured completely correctly and still fail to communicate reliably if, for example, a termination resistor is activated on every device, two participants have the same address or a bus line has effectively been turned into a star topology.

A clean setup can be demonstrated particularly well with the DMB 96000 Modbus RTU Connection Module. The module connects the 24 V supply and Modbus lines A/B to the In-Rail Bus and features switchable termination and polarization resistors. Further suitable I/O modules can be found under Modbus I/O at ICS Schneider and under Panel and Control Cabinet Measuring Instruments.

What is Modbus RTU over RS485?

Modbus RTU is a serial communication protocol that is very commonly transmitted via an RS485 interface in industrial installations.

A segment typically consists of:

1 master + several slaves

or, in more modern terminology:

client/master + several server/slave devices

The master initiates communication.

A slave only responds to a suitable request

In normal Modbus RTU operation, it does not initiate communication on its own and does not communicate directly with another slave.

Modbus RTU is therefore suitable, for example, for:

• analog I/O modules,
• digital I/O modules,
• energy meters,
• transmitters,
• temperature modules,
• pressure sensors,
• panel meters.

Do not confuse RS485 with Modbus

Two terms are often treated as if they were identical:

RS485

and:

Modbus RTU

However, they describe different levels.

RS485 describes the electrical interface

This includes, among other things:

• differential signal transmission,
• multi-drop operation,
• bus line,
• termination,
• polarization or biasing.

Modbus RTU, on the other hand, describes the communication

including, for example:

• participant address,
• function code,
• data field,
• CRC checksum,
• telegram structure.

An error in the RS485 wiring can therefore prevent a correctly structured Modbus telegram from ever reaching the receiver reliably.

Connecting A/B lines correctly

A two-wire RS485 connection uses a differential pair.

On many devices, the terminals are designated:

A / B

Other documentation may use designations such as:

D0 / D1

or:

A− / B+

In practice, these designations are not always consistent across manufacturers

Therefore, wiring should never be carried out solely according to assumptions such as:

A = positive

or:

B = positive

.

Instead, the terminal designations and assignments in the operating manuals of the devices being used must be compared.

Typical error pattern with swapped lines

The participant:

• is supplied with power,
• shows no device fault,
• but does not respond to any Modbus request.

For troubleshooting, checking the A/B assignment should therefore be one of the first steps.

The correct bus topology

RS485 Modbus should generally be set up as a line or bus structure.

In simplified form:

Master ─ Participant 1 ─ Participant 2 ─ Participant 3 ─ Participant 4

The main line is also referred to as the:

trunk

.

A star structure, on the other hand, is unfavorable

for example:

            ┌ Participant 1
Master ─────┼ Participant 2
            ├ Participant 3
            └ Participant 4

The individual branches create additional line ends.

Signal reflections can occur there.

The problem becomes more severe with

• high baud rates,
• long cables,
• long branches,
• unsuitable cable,
• many participants.

Within a control cabinet, the bus should therefore be wired consistently from one participant to the next wherever possible.

Keep stub lines as short as possible

Not every participant can always be connected directly into the main line.

Short branches are therefore possible in principle.

For a stable bus, however

stub lines should be kept as short as practically possible.

The official Modbus Serial Line specification allows certain branch lengths depending on the topology, but in a compact control cabinet there is normally no technical reason to create unnecessarily long stub lines.

An unfavorable example would be

a main line routed through the cable duct, from which each I/O module is connected via:

1 to 2 m of separate RS485 cable

.

A consistent daisy-chain connection or a dedicated bus system such as the In-Rail Bus of the DMB series is preferable.

What does termination do?

Electrical signals travel along a longer cable as electromagnetic waves.

If such a wave reaches the end of a line with a strongly mismatched impedance, part of the signal can be reflected.

These reflections can alter the signal waveform

and can cause, for example:

• false edges,
• telegram errors,
• CRC errors,
• sporadic communication interruptions.

A termination resistor at the end of the line reduces these reflections.

The termination is connected between the two data conductors

that is, directly across the differential pair.

Why only the two bus ends are terminated

An RS485 bus line has two physical ends.

That is exactly where the termination resistors are required.

In simplified form:

Termination — Participant — Participant — Participant — Termination

A common installation error is:

Every Modbus participant has a termination switch → switch all of them on

This is incorrect.

If, for example, five termination resistors are connected in parallel across the bus, the resulting load on the RS485 drivers increases significantly.

Possible consequences include

• insufficient differential voltage,
• unstable communication,
• communication works only with individual devices,
• sporadic failures.

Termination belongs at the two physical ends of the bus line – not automatically at the first and last participant according to device number or control cabinet position.

Which termination resistor is correct?

In industrial RS485 applications, a termination resistor matching the characteristic impedance of the bus cable is commonly used.

A widely used value is:

120 Ω

The Modbus Serial Line specification also mentions, for example:

150 Ω / 0.5 W

as one possible pure line termination.

For systems with polarization, it also describes a combination of:

120 Ω + 1 nF in series

.

For practical installations

the termination should not be designed using a generic resistor value if the device already has a defined internal termination.

The manufacturer specification for the:

• master,
• gateway,
• I/O system

is decisive.

On the DMB 96000, the termination resistors provided for the RS485 interface can be activated directly via DIP switches.

What does biasing or polarization mean?

When no RS485 participant is transmitting, the bus lines can temporarily be in a state where they are not actively driven.

Depending on the RS485 transceivers used, the receiver may then become more sensitive to interference.

Biasing creates a defined idle state

For this purpose, the line pair is biased via:

• a pull-up resistor,
• a pull-down resistor

to establish a defined differential level.

The Modbus specification also uses the term:

Line Polarization

for this.

Biasing and termination have different functions

Function	Purpose
Termination	Reduces signal reflections at the ends of the line
Biasing / polarization	Creates a defined bus level when no transmitter is active

The two terms must therefore not be confused.

Why biasing is normally activated only once

Another common mistake is:

Every device has bias resistors → switch all of them on

This changes the resistance conditions of the entire bus.

If polarization is required, the Modbus specification generally provides one resistor pair for the entire bus

This is typically located:

• at the master,
• at the gateway,
• at a central bus connection.

The Modbus specification specifies a resistance range of:

450 … 650 Ω

for the polarization concept described there.

Modern RS485 drivers may already include internal fail-safe functions

Biasing should therefore not simply be added without checking.

The documentation of the devices being used is decisive.

On the DMB 96000, both termination and polarization resistors can be activated via DIP switches. This allows the required function to be set at a defined point in the segment.

Assigning participant addresses correctly

Each Modbus slave requires a unique address within a serial bus.

The addresses available for individually addressed participants are generally:

1 … 247

.

Address 0 has a special function

It is reserved for:

broadcast

.

The addresses:

248 … 255

are reserved.

Two devices with the same address cause bus collisions

If the master sends a request to:

address 12

and two devices have this address, both attempt to respond to the same request.

The telegrams overlap.

Possible symptoms include

• CRC errors,
• no valid response,
• sporadic communication,
• other participants also appear to be temporarily affected.

Assigning and documenting all participant addresses uniquely is therefore one of the most important commissioning steps.

Why 247 addresses do not automatically mean 247 RS485 devices

A common misconception is:

Modbus allows addresses 1 … 247 → therefore 247 devices can always be operated on one RS485 segment.

This is not generally correct.

The Modbus address is a protocol characteristic

The permissible number of electrical participants also depends on the RS485 interface.

The Modbus Serial Line specification generally states:

32 devices without a repeater

as a safely supported basis for a conventional RS485 multipoint bus.

Modern transceivers, however, load the bus significantly less

They are available, for example, as:

1/2 Unit Load

1/4 Unit Load

or:

1/8 Unit Load

.

This can significantly increase the number of participants.

Example: DMB series

ICS specifies a:

1/8-load RS485 interface

for various DRAGO DMB modules.

According to the product specification, this allows up to:

247 DRAGO Modbus devices

in one segment without an additional repeater.

This specification applies to the correspondingly designed system and must not be transferred generally to arbitrary RS485 devices from different manufacturers.

Baud rate, parity and stop bits

Even a completely correctly wired RS485 bus will not work if the serial communication parameters do not match.

All participants in a Modbus RTU segment must be configured consistently

This includes, in particular:

• baud rate,
• data bits,
• parity,
• stop bits.

Example

Master:

19200 baud / 8 data bits / even / 1 stop bit

Slave:

9600 baud / 8 data bits / even / 1 stop bit

Result:

no communication

An incorrect parity setting alone is also sufficient

For example:

Master = Even

Slave = None

The telegrams are then not interpreted correctly.

Depending on the version, the DMB I/O modules support baud rates of:

300 … 115200 baud

as well as different parity configurations.

Signal reference and common conductor

RS485 operates differentially.

However, this does not mean that potential differences between different devices can be unlimited.

In larger installations, a common signal reference may be required

The Modbus specification refers to this as:

Common

or signal common.

It must not be confused with the cable shield.

For the actual wiring

the manufacturer’s specifications for the RS485 interfaces being used must be observed.

Potential management is particularly important with:

• galvanically isolated interfaces,
• different power supply systems,
• multiple control cabinets.

Shielding and EMC

A Modbus bus can work perfectly on a test bench and suddenly show sporadic errors after installation in the machine.

A common cause is electromagnetic interference.

A suitable twisted-pair cable should be used for RS485

The two data conductors form a pair.

This means that coupled interference acts as similarly as possible on both conductors and is largely rejected by the differential receiver.

Additional shielding may also be required

The specific shield connection depends on:

• machine design,
• equipotential bonding,
• device manufacturer,
• EMC environment.

The shield should not be connected arbitrarily to signal ground or the data conductors.

Distance from frequency converters and motor cables

Particularly problematic sources of interference for communication cables include:

• frequency converters,
• motor cables carrying PWM voltages,
• contactors,
• braking resistors,
• power cables with high rates of current change.

RS485 cables should therefore not be routed in parallel with interference-generating power cables over long distances

If a crossing is necessary, routing the cables approximately at right angles is generally preferable to a long parallel run.

In the control cabinet, this means

where possible:

• route communication cables separately from motor cables,
• maintain sufficient distance,
• use separate cable ducts for signal and power cables,
• implement the shielding concept consistently.

Communication works, but the measured value is still wrong

If a Modbus participant responds, the physical communication is already largely functional.

Nevertheless, the value being read can still be incorrect.

The cause is then often at protocol or data level

For example:

• incorrect register address,
• incorrect function code,
• 32-bit value with incorrect byte order,
• floating-point value interpreted as an integer,
• scaling factor not taken into account,
• offset between 0-based and 1-based register representation.

Example

The data sheet specifies:

Holding Register 40001

However, the PLC software being used expects a start address of:

0

or:

1

depending on its addressing logic.

A device that is communicating correctly can therefore appear to deliver incorrect data.

Termination and biasing do not solve register problems – just as changing the register address will not solve a physically unstable RS485 bus.

Typical error patterns

Observation	Possible cause	Recommended check
No participant responds	A/B swapped, incorrect baud rate or master incorrectly configured	check bus parameters and data lines
Only one participant responds	other devices have incorrect addresses or configuration	test devices individually
Two devices work individually, but not together	same slave address	check addresses
Communication works only with a short cable	termination or cable unsuitable	check bus ends and cable type
Many CRC errors	interference, reflections or collisions	check termination, topology, EMC and duplicate addresses
Errors occur only while the motor is running	EMC coupling from frequency converter or motor cable	check cable routing and shielding
Bus becomes worse when termination is activated	too many termination resistors active	terminate only the two bus ends
Communication is sporadically unstable while idle	no defined idle level	check manufacturer requirements for biasing
Bus stops working after an expansion	star topology or excessively long stub line introduced	check topology
One participant responds only occasionally	marginal signal level, EMC or contact problem	check terminals, supply and RS485 signal
Response is received, but value is obviously wrong	incorrect register, data type or byte order	check Modbus register description
Replacement device does not respond	factory setting for address or baud rate differs	recheck communication parameters

Systematically commissioning a Modbus bus

1. Define the bus structure: Plan the main line as a line or daisy chain.
2. Determine device positions: Clearly define the physical bus ends.
3. Use suitable RS485 cable: Consider a twisted data pair and the required shielding concept.
4. Check A/B assignment: Compare manufacturer documentation and do not wire solely according to assumed plus/minus designations.
5. Check the common concept: Implement the signal reference according to the devices being used.
6. Activate termination: Only at the two physical bus ends.
7. Check biasing: Activate it only if required by the system and do not activate it multiple times in the segment.
8. Assign participant addresses: Use each address from 1 to 247 only once.
9. Document the address: Enter the participant number directly in the circuit diagram or device list.
10. Set baud rate: Use the same transmission rate for all participants.
11. Set parity and stop bits: Standardize the parameters throughout the segment.
12. Check the power supply: Verify the voltage at every I/O module.
13. Connect only a few participants initially: Test the master with one slave.
14. Expand the segment step by step: Add further participants one at a time.
15. Observe communication errors: Do not ignore timeouts or CRC errors.
16. Check stub lines: Avoid unnecessarily long branches.
17. Check the EMC situation: Pay particular attention to cable routing near converters and motor cables.
18. Check registers: Only evaluate data types and scaling once communication is stable.
19. Document the final state: Record address, baud rate, parity, termination and biasing.

Practical example with several I/O modules

Several sensor signals are to be transferred to a PLC via Modbus RTU in a control cabinet.

The following are used, for example:

• one DMB 96000 connection module,
• two DMB 96700 DI/DO modules,
• two DMB 96400 analog I/O modules.

The supply voltage is

24 V DC

Modbus and the power supply are fed into the In-Rail Bus via the DMB 96000.

The participants are assigned, for example

DMB 96700 No. 1 → address 11

DMB 96700 No. 2 → address 12

DMB 96400 No. 1 → address 13

DMB 96400 No. 2 → address 14

All modules are configured, for example, to:

19200 baud

with identical parity and stop bit settings.

The physical bus line ends at the I/O system

In this case, the corresponding termination can be activated on the DMB 96000, provided that this module actually forms the end of the entire RS485 segment.

The second termination is located at the opposite physical end of the RS485 bus, for example at the master or gateway.

Biasing

is activated only at the designated point in the overall system.

The decisive factor is not how many I/O modules are mounted on the DIN rail, but how the complete RS485 bus is structured from the first physical line end to the last.

Suitable Modbus I/O products from ICS Schneider

DMB 96000 Modbus RTU Connection Module

The DMB 96000 is particularly suitable for the topic of this technical article.

It performs, among other things:

• connection of the 24 V power supply,
• connection of Modbus lines A and B,
• connection to the In-Rail Bus,
• distribution of power supply and Modbus to further modules,
• switchable RS485 termination resistors,
• switchable polarization resistors.

This makes it possible to explain termination and biasing directly using a practical control cabinet setup.

DMB 96700 4-Channel DI/DO Module

The DMB 96700 has four independently configurable digital I/O channels.

ICS specifies, among other things:

• Modbus RTU via RS485,
• addresses 1 … 247,
• baud rates from 300 to 115200,
• different parity configurations,
• digital inputs and outputs,
• frequency, counter, pulse and PWM functions,
• In-Rail Bus for Modbus and power supply.

DMB 96400 Analog I/O Module

The DMB 96400 provides two analog inputs and two analog outputs.

The channels can be used for industrial current and voltage signals.

This module also communicates via:

Modbus RTU / RS485

and is therefore suitable for decentralized or compact control cabinet I/O systems.

DMB 96100 Standard Signal Input Module

For pure measurement tasks, the DMB 96100 is also available.

It processes, among other signals:

• 0 … 20 mA,
• 4 … 20 mA,
• 0 … 10 V,
• 0 … 5 V

and provides the measured result via Modbus RTU.

Further modules can be found under Modbus I/O at ICS Schneider.

Conclusion

A reliable Modbus RTU bus is not created solely by setting registers or participant addresses correctly.

RS485 requires a clean line structure

The bus should be designed as a trunk or daisy chain. Long star-shaped branches should be avoided.

Termination belongs only at the two bus ends

Too many termination resistors unnecessarily load the line and can impair communication just as much as completely missing termination.

Biasing is not the same as termination

If required, polarization creates a defined idle state for the line pair and is generally implemented only at one designated point in the segment.

Every slave address must be unique

Addresses from 1 to 247 are available for individually addressed Modbus participants. Address 0 is reserved for broadcast.

247 addresses do not automatically mean 247 electrical participants

The permissible number of participants depends on the loading of the RS485 transceivers. Systems with 1/8 Unit Load can support significantly more devices than conventional RS485 interfaces.

Baud rate and character format must match

A single mismatch in baud rate, parity or stop bits is enough to prevent a participant from communicating.

EMC is part of bus planning

Frequency converters, motor cables and power contactors can interfere with an electrically correct bus installation during operation.

For practical applications

Plan the bus as a line → check A/B assignment using the device documentation → keep stub lines short → terminate only the two physical bus ends → activate biasing only at the designated point → assign unique addresses → standardize baud rate, parity and stop bits → route the RS485 cable in an EMC-compliant manner → test one participant first → expand the bus step by step → evaluate CRC errors and timeouts → only once communication is stable, check registers, data types and scaling.

FAQ: Correctly Setting Up Modbus RTU in the Control Cabinet

What is Modbus RTU?

Modbus RTU is a serial industrial protocol that is commonly transmitted via an RS485 interface.

Is RS485 the same as Modbus?

No. RS485 describes the electrical transmission layer, while Modbus RTU describes the communication protocol used on top of it.

How is Modbus RTU typically wired?

With two-wire RS485, a linear bus structure or daisy chain is typically used.

Can Modbus RS485 be wired in a star topology?

A star structure is unfavorable for a conventional RS485 bus because it creates additional line ends and can cause reflections.

What does daisy chain mean?

Daisy chain means that the bus line is continued from one participant to the next instead of routing a separate long cable from the same point to each participant.

How long may stub lines be?

This depends on the baud rate, cable and overall topology. In a control cabinet, stub lines should generally be kept as short as practically possible.

What is RS485 termination?

Termination means terminating the bus line with a suitable impedance in order to reduce signal reflections at the ends of the line.

Where must the termination resistors be installed?

At the two physical ends of the RS485 bus line.

Does every Modbus participant need termination?

No. That would be a typical wiring error. On a normal passive two-wire bus, only the two ends are terminated.

What happens if too many termination resistors are used?

The RS485 drivers are loaded more heavily and the differential voltage can become too low. This can result in unstable communication.

What happens without termination resistors?

With longer cables or higher data rates, reflections can distort the signal and cause telegram errors.

Are termination resistors always 120 Ω?

Not necessarily. The value must suit the bus system and cable being used. If a device has integrated termination, the manufacturer’s specification should be followed.

What does biasing mean in RS485?

Biasing or polarization holds the RS485 line pair at a defined idle level when no transmitter is active.

Is biasing the same as termination?

No. Termination reduces reflections, while biasing establishes a defined idle state.

Does every device need biasing?

No. If polarization is required, it is normally implemented only once for the complete segment.

Where should the biasing be located?

Typically at the master, gateway or a designated bus connection. The specific manufacturer documentation is decisive.

Which Modbus addresses are available?

Addresses 1 to 247 are available for individually addressed slave devices.

What does Modbus address 0 mean?

Address 0 is reserved for broadcast telegrams and is not a normal individual slave address.

Can two devices use the same Modbus address?

No. Each slave requires a unique address on a serial bus.

What happens if two devices have the same address?

Several devices may respond at the same time. The telegrams collide and the master does not receive a valid response.

Can I always connect 247 devices to an RS485 bus?

No. 247 is the number of available Modbus slave addresses. The electrically permissible number of participants depends on the RS485 transceivers being used.

How many devices does conventional RS485 Modbus allow?

The Modbus Serial Line specification states 32 devices without a repeater as a general basis. Modern transceivers with a lower Unit Load allow larger numbers of participants.

Why can DMB modules support up to 247 participants?

ICS specifies a 1/8-load RS485 interface for corresponding DRAGO modules. According to the product specification, this allows up to 247 such devices to be operated in one segment.

Do all devices have to use the same baud rate?

Yes. The master and all slaves in a segment must operate with matching communication parameters.

Does the parity also have to match?

Yes. Different parity settings prevent telegrams from being evaluated correctly.

What does 9600 8E1 mean?

It means 9600 baud, 8 data bits, even parity and 1 stop bit.

Why is my Modbus participant not responding?

Common causes include an incorrect address, baud rate, parity, swapped data lines, missing power supply or incorrect bus wiring.

Why does Modbus work only when the frequency converter is stopped?

This indicates an EMC problem. Cable routing, distance from motor cables, shielding and equipotential bonding should be checked.

Why am I getting many CRC errors?

Possible causes include signal reflections, EMC interference, bus collisions, poor contacts or an unsuitable topology.

What is a CRC error?

Modbus RTU uses a CRC checksum to detect corrupted telegrams. If the received checksum is incorrect, the telegram is discarded.

Can an incorrect register cause a timeout?

Depending on the device, an invalid request can trigger a Modbus exception. A complete timeout, however, more often indicates that no valid response was received at all.

Why is the Modbus value incorrect even though communication works?

In this case, the register address, data type, byte order, scaling factor and function code should be checked.

What is the advantage of the DMB 96000?

The DMB 96000 feeds the 24 V supply and Modbus RTU into the In-Rail Bus system and features switchable termination and polarization resistors.

Where can I find the DMB 96000 at ICS Schneider?

Further information is available under DMB 96000 Modbus RTU Connection Module at ICS Schneider.

Which digital Modbus I/O modules does ICS offer?

One example is the DMB 96700 4-Channel DI/DO Module.

Are analog Modbus I/O modules also available?

Yes. Examples include the DMB 96400 with analog inputs and outputs and the DMB 96100 for standard signals.

Where can I find further Modbus I/O products?

An overview is available under Modbus I/O at ICS Schneider.