Wiring of the Modbus Network

Overview

This chapter describes how to connect an LTMR controller to an RS 485 Modbus network with an RJ45 or an open-style connector.

It presents three possible network topologies.

WARNING
LOSS OF CONTROL The designer of any control scheme must consider the potential failure modes of control paths and, for critical functions, provide a means to achieve an acceptable state during and after a path interruption. Examples of critical control functions are emergency stop and overtravel stop. Separate or redundant control paths must be provided for critical control functions. System control paths may include communication links. Consideration must be given to the implications of anticipated transmission delays or interruptions of the link.⁽¹⁾ Each implementation of an LTMR controller must be individually and thoroughly tested for proper operation before being placed into service. Failure to follow these instructions can result in death, serious injury, or equipment damage.

WARNING

LOSS OF CONTROL

The designer of any control scheme must consider the potential failure modes of control paths and, for critical functions, provide a means to achieve an acceptable state during and after a path interruption. Examples of critical control functions are emergency stop and overtravel stop.
Separate or redundant control paths must be provided for critical control functions.
System control paths may include communication links. Consideration must be given to the implications of anticipated transmission delays or interruptions of the link.⁽¹⁾
Each implementation of an LTMR controller must be individually and thoroughly tested for proper operation before being placed into service.

Failure to follow these instructions can result in death, serious injury, or equipment damage.

(1) For additional information, refer to NEMA ICS 1.1 (latest edition), Safety Guidelines for the Application, Installation, and Maintenance of Solid State Control.

Modbus Network Characteristics

Overview

The Modbus over Serial Line Specification and Implementation Guide, published on www.modbus.org, defines the characteristics of the Modbus protocol over serial line. The LTMR Modbus controller complies with this specification.

Modbus Network Standard Diagram

The standard diagram corresponds to the Modbus specification on the www.modbus.org site and in particular to the two-wire multidrop serial bus diagram.

The simplified diagram is as follows:

Characteristics for Connection to the RS 485 Bus

The RS 485 standard allows variants of some characteristics:

Polarization
Line terminator
Number of secondary devices
Bus length

Characteristics	Value
Maximum number of stations (without repeater)	32 stations (31 secondary devices)
Type of trunk cable	Single, shielded, twisted pair cable, with 120 Ω characteristic impedance, and at least a third conductor
Maximum bus length	1,000 m (3,300 ft) at 19,200 Baud
Maximum length of tap-offs	20 m (66 ft) for one tap-off 40 m (131 ft) divided by the number of tap-offs on the multiple junction box
Bus polarization	A 450 to 650 Ω pull-up resistor at the 5 V A 450 to 650 Ω pull-down resistor at the Common This polarization is recommended for the primary. There is no polarization at the RS 485 terminal on the LTMR controller.
Line terminator	A 120 Ω resistor +/- 5% at both ends of the bus
Common polarity	The common polarity is connected to the protective ground in at least one point on the bus.

Modbus Communication Port Wiring Terminal Characteristics

General

The main physical characteristics of a Modbus port are:

Physical interface	Multipoint 2-wire RS 485 - electrical networking
Connector	Terminal block and RJ45
Polarization	At primary level

Physical Interface and Connectors

The LTMR controller is equipped with two connector types, on the front face:

A female shielded RJ45 connector,
An open-style, pull-apart, terminal block.

The figure shows the LTMR front face with the Modbus connectors:

Both connectors are electrically identical. They follow the Modbus interoperability standards.

NOTE: The product must be connected through only one port. The use of the RJ45 connector is recommended.

RJ45 Connector Pinout

The LTMR controller is connected to the Modbus network with a shielded RJ45 connector in compliance with the following wiring:

Front view

The RJ45 wiring layout is:

Pin No.	Signal	Description
1	–	Not connected
2	–	Not connected
3	–	Not connected
4	D1 or D(B)	Transceiver terminal 1
5	D0 or D(A)	Transceiver terminal 0
6	–	Not connected
7	–	Not connected
8	0VL	Signal and power supply common

Open-Style Terminal Block

The LTMR controller has the following Modbus network plug-in terminals and pin assignments.

Pin	Signal	Description
1	D1 or D(B)	Transceiver terminal 1
2	D0 or D(A)	Transceiver terminal 0
3	S	Modbus shield pin
4	V-	Signal and power supply common
5	NC	Modbus VP pin (not connected)

Open-Style Terminal Block Characteristics

Connector	Five pins
Pitch	5.08 mm (0.2 in.)
Tightening torque	0.5 to 0.6 N•m (5 lb-in)
Flat screwdriver	3 mm (0.10 in.)

Wiring of the Modbus Network

Overview

The recommended way to connect an LTMR controller to a Modbus network on the RS 485 bus is the connection via the female shielded RJ45 connector.

This section describes three typical cases of connection of LTMR controllers to the bus via its RJ45 connector:

Connection of LTMR controllers installed in an enclosure via T-junction boxes.
Connection of LTMR controllers installed in withdrawable drawers via T-junction boxes.
Connection of LTMR controllers installed in withdrawable drawers via hardwired cables.

Modbus Wiring Rules

The following wiring rules must be respected in order to reduce disturbance due to EMC on the behavior of the LTMR controller:

Keep a distance as large as possible between the communication cable and the power or control cables (recommended 30 cm or 11.8 in.).
Cross over the Modbus cables and the power cables at right angles, if necessary.
Install the communication cables as close as possible to the grounded plate.
Do not bend or damage the cables. The minimum bending radius is 10 times the cable diameter.
Avoid sharp angles of paths or passage of the cable.
Use the recommended cables only.
All RJ45 connectors must be metallic.
A Modbus cable must be shielded:
- The cable shield must be connected to a protective ground.
- The connection of the cable shield to the protective ground must be as short as possible.
- Connect together all the shields, if necessary.
- Perform the grounding of the shield with a collar.
When the LTMR controller is installed in a withdrawable drawer:
- Connect together all the shield contacts of the withdrawable drawer part of the auxiliary connector to the ground of the withdrawable drawer to create an electromagnetic barrier. Refer to the Okken Communications Cabling & Wiring Guide (available on request).
- Do not connect the cable shield at the fixed part of the auxiliary connector.
Place a line terminator at each end of the bus to avoid malfunctions on the communication bus. A line terminator is generally already integrated in the primary device.
Wire the bus between each connector directly, without intermediate terminal blocks.
The common polarity (0 V) must be connected directly to protective ground, preferably at one point only for the entire bus. In general, this point is chosen either on the primary device or on the polarization device.

For more information, refer to the Electrical Installation Guide (available in English only), chapter ElectroMagnetic Compatibility (EMC).

NOTICE
COMMUNICATION MALFUNCTION Respect all the wiring and grounding rules in order to avoid communication malfunctions due to EMC disturbance. Failure to follow these instructions can result in equipment damage.

LTMR Controllers Installed in an Enclosure

The wiring diagram for connection of LTMR controllers installed in an enclosure to the RS 485 bus via the RJ45 connector is as follows:

1 Primary (PLC, PC, or communication module) with line terminator

2 Modbus shielded cable with two RJ45 connectors VW3 A8 306 R••

3 Grounding of the Modbus cable shield

4 Modbus T-junction boxes VW3 A8 306 TF•• (with cable)

5 Grounding of the Modbus T-junction boxes

6 Line terminator for RJ45 plug VW3 A8 306 R (120 Ω)

LTMR Controllers Installed in a Blokset or Okken Motor Control Switchboard

The installation of LTMR controllers in withdrawable drawers of a switchboard presents constraints specific to the type of switchboard:

For installation of LTMR controllers in an Okken switchboard, refer to the Okken Communications Cabling & Wiring Guide (available on request).
For installation of LTMR controllers in a Blokset switchboard, refer to the Blokset Communications Cabling & Wiring Guide (available on request).
For installation of LTMR controllers in other types of switchboard, follow the specific EMC instructions described in this manual and refer to the relative instructions specific to your type of switchboard.

LTMR Controllers Installed in Withdrawable Drawers With Hardwired Cables

The wiring diagram for connection of LTMR controllers installed in withdrawable drawers to the RS 485 bus via the RJ45 connector and hardwired cables is as follows:

1 Primary (PLC, PC, or communication module) with line terminator

2 Modbus shielded cable TSX CSA •00

3 Modbus shielded cable with one RJ45 connector VW3 A8 306 D30

4 Grounding of the Modbus cable shield

5 Withdrawable drawer

6 Withdrawable drawer part of the auxiliary connector

7 Fixed part of the auxiliary connector

8 Line terminator VW3 A8 306 DR (120 Ω)

LTMR Controllers Installed in Withdrawable Drawers With T-Junction Boxes

The wiring diagram for connection of LTMR controllers installed in withdrawable drawers to the RS 485 bus via the RJ45 connector and T-junction boxes is as follows:

1 Primary (PLC, PC, or communication module) with line terminator

2 Modbus shielded cable with two RJ45 connectors VW3 A8 306 R

3 Modbus shielded cable with one RJ45 connector VW3 A8 306 D30

4 Grounding of the Modbus cable shield

5 Withdrawable drawer

6 Withdrawable drawer part of the auxiliary connector

7 Fixed part of the auxiliary connector

8 Modbus T-junction boxes VW3 A8 306 TF• (with cable)

9 Grounding of the Modbus T-junction boxes

10 Line terminator VW3 A8 306 R (120 Ω)

List of Modbus Accessories

Designation	Description	Reference Number
T-junction boxes	Box with two RJ45 female connector for trunk cable and an integrated 0.3 m (1 ft) cable with one RJ45 male connector for tap-off	VW3 A8 306 TF03
T-junction boxes	Box with two RJ45 female connector for trunk cable and an integrated 1 m (3.2 ft) cable with one RJ45 male connector for tap-off	VW3 A8 306 TF10
Line terminator for RJ45 connector	R = 120 Ω	VW3 A8 306 R
Line terminator for open-style connector	R = 120 Ω	VW3 A8 306 DR

List of Modbus Cables

Designation	Length	Reference Number
Shielded cable for Modbus bus, with two RJ45 connectors	0.3 m (1 ft)	VW3 A8 306 R03
	1 m (3.2 ft)	VW3 A8 306 R10
	3 m (9.8 ft)	VW3 A8 306 R30
Shielded cable for Modbus bus, with one RJ45 connectors and one stripped end	3 m (9.8 ft)	VW3 A8 306 D30
Shielded cable for Modbus bus, with two stripped ends	100 m (320 ft)	TSX CSA 100
	200 m (640 ft)	TSX CSA 200
	500 m (1600 ft)	TSX CSA 500
Belden cable	–	–