Danfoss RS 485 RTU Design guide

0 (0)

Design guide

DATA COMMUNICATION MODBUS RS 485 RTU

For product type - EKD / EIM controllers

This User guide document provides general information on the setup of Modbus RS-485 networks and explains how to configure the communication between EKD/EIM and a system controller, using the Modbus RTU.

Features	•	Simplicity
	•	Standard Ethernet
	•	Open protocol

• Availability on many devices

DKRCC.PS.R00.A1.02 | 520H10468 | 1

Design guide | Data Communication Modbus RS485 RTU

Part 1 - Hardware network specification
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
Definitions and abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
RS-485 specific . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
Repeater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
Cable connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
Conductors in a daisy chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
Ground connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
Part 2 - Software specification and configuration
Introduction of RS-485 Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
Modbus RTU message. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
Modbus RTU message structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
How to access parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
RS485 bus function codes overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
EKD examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
EIM examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
Appendix 1 EIM Modbus conversion table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
Appendix 2 EKD Modbus table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
Appendix 3 Modbus exception code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
Appendix 4 Modbus tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18

DKRCC.PS.R00.A1.02 | 520H10468 | 2

Design guide | Data Communication Modbus RS485 RTU

Introduction

References

Definitions and abbreviations

Making a network means to connect devices together with a communication bus. But to make reliable and high performance networks strict rules must be followed.

Modbus specifications, Modbus.org

EIA-485 RS-485 fieldbus specification

RTU	Remote Terminal Unit
SW	Software
HW	Hardware
AWG	American Wire Gauge
EKE / EIM / EKD	Superheat Controller/driver
Parameter	The parameter number indicates the number of a given parameter.
Value	Indicates the factory settings.
Type	Group. Group (number) indicates the group the relevant parameter
	belongs to. The group number is only of importance for presentation
	in AKM PC software.
PNU	Short for Parameter NUmber. In Modbus terminology it corresponds to the
	register number which is also often referred to as the offset. The PNU numbers
	can have values in the range from 1 to 65535. The corresponding Modbus
	address is found by subtracting 1 from the PNU number. For instance PNU
	number 117 would correspond to Modbus address 116.”

DKRCC.PS.R00.A1.02 | 520H10468 | 3

Design guide | Data Communication Modbus RS485 RTU

Wiring

Wiring characteristics

Wiring types

Recommendations

(Source Modbus.org)

Note!

The wires should have the following characteristics:

•Characteristic impedance: 120 Ohm +/- 10%;

•Specific resistance depending on network length.

•Cable must be with Screen if the bus cable exceeds 3m.

•The cable is connected from controller to controller and no branches are allowed on the cable

•Each shield must be grounded at one side only.

Two types of wires can be used based on required ruggedness:

1.Twisted pair with ground: short leads, no power lines in proximity.

2.Twisted pair + ground and shield: long leads, disturbed environment.

When choosing a transmission line for RS-485, it is necessary to examine the required distance of the cable and the data rate of the system.

An RS485-Modbus must use a balanced pair (for D+-D-) and a third wire (for the Common/Gnd). For RS485-Modbus, Wire Gauge must be chosen sufficiently wide to permit the maximum length (1000 m ). AWG 22 is always sufficient for the Modbus Data.

Category 5 cables may operate for RS485-Modbus, to a maximum length of 600m.

For the balanced pairs used in an RS485-system, a Characteristic Impedance with a value higher than 100 Ohms may be preferred, especially for 19200 and higher baud rates.

Use one twisted pair of conductors for connecting the differential signals and use another conductor (for example a second twisted pair) for connecting the ground.

	Modbus	Modbus connections
	D+ (A+)	EKE	EKD	EIM
	D+ (A+)
	D– (B–)	D+	A+	TxD+
	GND	D-	B-	TxD-
84N404.10		Gnd	Gnd	1)
Danfoss
Fig. 1	Connection example	1) Gnd has been omitted from EIM.

Connection of a twisted cable in a 2-wire Modbus system may cause damages.

DKRCC.PS.R00.A1.02 | 520H10468 | 4

Design guide | Data Communication Modbus RS485 RTU

RS-485 specific	The RS-485 consists of three wires:
	•	RS-485-A (D+);
	•	RS-485-B (D-);
	•	GND.
	The two wires RS-485-A (D+) and RS-485-B (D-) propagate a differential communication signal.
	In addition there is the ground wire for the common mode voltage reference.
	The recommended maximum Modbus cable length between the EKE(/EKD/EIM) and the system
	controller should not exceed 1000 meters (3300 feet).
		Wire length

		Length (m)	Max. baudrate	Min. Wire size	Suggested types

		1000	125 K	AWG22	Belden 3106A / 3107A

RS-485: Controllers	The maximum number of controllers that can be connected to a Modbus line is 120.
	One repearter must be added for every 32 controllers. If the data communication cable runs through
	an electrically noisy environment which impairs the data signal, one or more repeaters must be
	added to stabilize the signal.
Repeater	A repeater has no address.
	A repeater from the company "Phoenix" can be used:
	Danfoss code no. = 084B2240 (type AKA 222).

Modbus

Master

Slaves

max. 1000 m

<![if ! IE]>

<![endif]>Danfoss 84B3210.10

Modbus	Repeater
Modbus
Slaves	Slaves

Master

<![if ! IE]>

<![endif]>Danfoss 84B3210.10

DKRCC.PS.R00.A1.02 | 520H10468 | 5

Design guide | Data Communication Modbus RS485 RTU

Topology

Standard topology

The controller should be connected according to the bus topology. That means that the communica-

tion cable is wired from one controller to the next without branches.

Three conductors

002

001

Three conductors + shiels

002

001

Legend

<![if ! IE]>

<![endif]>Danfoss 84B3202.10

Shield

Communication cable, 120 impedance

120 termination

Recommendations

Avoid making stubs on the line.

If stubs are present in the network they should be kept as short as possible

(<0.3 m at 1 Mbit; <3 m at 50 kbit).

	<![if ! IE]> <![endif]>Danfoss 84B3203.10	Field BUS
	<![if ! IE]> <![endif]>Danfoss 84B3203.10	Stub
		Stub

	<![if ! IE]> <![endif]>Danfoss 84B3204.10
- 0.0.0.	- 0.0.0.

Fig Fieldbus with ‘’STUB” 01

Fig Fieldbus with ‘’STUB” 02

Field BUS	Field BUS
	<![if ! IE]> <![endif]>Danfoss 84B3205.10

Fig Star topology

Do not use a ‘STAR topology’

Min 10-15 cm

<![if ! IE]>

<![endif]>Danfoss 84B3206.10

Fig Fieldbus wire

Do not route the fieldbus wires close to power lines or wires leading to heavy loads

DKRCC.PS.R00.A1.02 | 520H10468 | 6

+ 12 hidden pages