Danfoss RS 485 RTU Design guide
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 congure the
communication between EKD/EIM and a system
controller, using the Modbus RTU.
Features • Simplicity
• Standard Ethernet
• Open protocol
• Availability on many devices
© Danfoss | DCS (sw) | 2015.10
DKRCC.PS.R00.A1.02 | 520H10468 | 1
Design guide | Data Communication Modbus RS485 RTU
Part 1 - Hardware network specication
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Denitions and abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
RS-485 specic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Repeater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Cable connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Conductors in a daisy chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Ground connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Part 2 - Software specication and conguration
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
© Danfoss | DCS (sw) | 2015.10
DKRCC.PS.R00.A1.02 | 520H10468 | 2
Design guide | Data Communication Modbus RS485 RTU
Introduction
References
Denitions 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 specications, Modbus.org
EIA-485 RS-485 eldbus specication
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.
Typ e 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 oset. 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.”
© Danfoss | DCS (sw) | 2015.10
DKRCC.PS.R00.A1.02 | 520H10468 | 3
84N404.10
Design guide | Data Communication Modbus RS485 RTU
Wiring
Wiring characteristics
Wiring types
Recommendations
(Source Modbus.org)
The wires should have the following characteristics:
• Characteristic impedance: 120 Ohm +/- 10%;
• Specic 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 suciently wide to permit the maximum length
(1000 m ). AWG 22 is always sucient 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 dierential signals and use another conductor
(for example a second twisted pair) for connecting the ground.
Note!
Modbus
D+ (A+)
D– (B–)
Modbus connections
EKE EKD EIM
D+ A+ TxD+
D- B- TxD-
Danfoss
Fig. 1 Connection example
GND
Gnd Gnd
1
) Gnd has been omitted from EIM.
Connection of a twisted cable in a 2-wire Modbus system may cause damages.
1
)
© Danfoss | DCS (sw) | 2015.10
DKRCC.PS.R00.A1.02 | 520H10468 | 4
Danfoss
84B3210.10
Repeater
Design guide | Data Communication Modbus RS485 RTU
RS-485 specic
RS-485: Controllers
Repeater
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 dierential 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
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.
A repeater has no address.
A repeater from the company "Phoenix" can be used:
Danfoss code no. = 084B2240 (type AKA 222).
Master
Modbus
Master
Modbus
Slaves
max. 1000 m
Slaves Slaves
Danfoss
84B3210.10
© Danfoss | DCS (sw) | 2015.10
DKRCC.PS.R00.A1.02 | 520H10468 | 5
Min 10-15 cm
Danfoss
84B3206.10
Field BUSField BUS
Danfoss
84B3205.10
Field BUS
Stub
Danfoss
84B3203.10
Design guide | Data Communication Modbus RS485 RTU
Topology
Standard topology
Recommendations
The controller should be connected according to the bus topology. That means that the communication cable is wired from one controller to the next without branches.
Three conductors
T
001
002
T
n
Three conductors + shiels
T
001
Legend
Shield
Communication cable, 120 Ω impedance
T
120 Ω termination
002
T
n
Danfoss
84B3202.10
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).
Fig Fieldbus with ‘’STUB” 01
Fig Star topology
Do not use a ‘STAR topology’
- 0.0.0. - 0.0.0.
Fig Fieldbus with ‘’STUB” 02
Fig Fieldbus wire
Do not route the eldbus wires close to power
lines or wires leading to heavy loads
Danfoss
84B3204.10
© Danfoss | DCS (sw) | 2015.10
DKRCC.PS.R00.A1.02 | 520H10468 | 6
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