Dräger Polytron 8000 Series User manual

Technical Manual
Digital Communication -
Modbus RTU
Dräger Polytron® 8000 Series

2 Technical Manual |
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Technical Manual | Digital Communication - Modbus RTU 3
Contents
Digital Comm unication - Mo dbus RTUDräger Polytron®8000 Series
Contents
1Introduction .............................................................................................. 4
1.1 Target group ................................................................................... 4
1.2 General safety statements.............................................................. 4
1.3 Meaning of the warning notes......................................................... 4
1.4 Trademark ...................................................................................... 4
2Basic principles of the Modbus technology .......................................... 5
2.1 Definition of the Modbus protocol ................................................... 5
2.2 System overview............................................................................. 5
2.3 Data transmission ........................................................................... 8
3Installation ................................................................................................ 15
3.1 Technical data ................................................................................ 15
3.2 Cables............................................................................................. 15
3.3 Pin assignment ............................................................................... 16
3.4 Termination..................................................................................... 18
3.5 Opening the gas detector ............................................................... 19
3.6 Connecting the gas detector........................................................... 20
3.7 Closing the gas detector ................................................................. 20
4Commissioning ........................................................................................ 21
4.1 Checking installation and operation................................................ 21
4.2 Configuring the RS-485 interface ................................................... 21
4.3 Configuring the gas detector........................................................... 22
5Integration into process control systems.............................................. 27
5.1 General information ........................................................................ 27
5.2 Reading the gas concentration ....................................................... 28
5.3 Reading diagnostic and status information..................................... 29
5.4 Reading the date and time.............................................................. 32
6Troubleshooting ....................................................................................... 33
6.1 Error analysis.................................................................................. 33
6.2 Errors .............................................................................................. 33
7Annex ........................................................................................................ 39
7.1 Glossary.......................................................................................... 39
7.2 Annex 1 Polytron®8000 parameter list .......................................... 39

4 Technical Manual | Digital Communication - Modbus RTU
Introduction
1 Introduction
This document is a supplement to the instructions for use for the following gas
detectors:
– Dräger Polytron®8100 EC
– Dräger Polytron®8200 CAT
– Dräger Polytron®8310 IR
– Dräger Polytron®8700 IR
– Dräger Polytron®8720 IR
This document contains additional information on the Modbus RTU interface.
1.1 Target group
This document is intended for experts who are specialized in PLC programming,
certified electricians, or persons instructed by certified electricians. In addition, the
individuals have to be familiar with the applicable standards.
1.2 General safety statements
Carefully read the instructions for use before using the corresponding product. This
document does not replace the instructions for use.
1.3 Meaning of the warning notes
The following warning notes are used in this document to notify users of possible
dangers. The meanings of the warning notes are defined as follows:
1.4 Trademark
The following web page provides a list of the countries in which the trademarks are
registered: www.draeger.com/trademarks.
Alert icon Signal word Consequences in case of nonob-
servance
WARNING Indicates a potentially hazardous situation.
If not avoided, it could result in death or
serious injury.
CAUTION Indicates a potentially hazardous situation.
If not avoided, it could result in physical
injury. It may also be used to alert against
unsafe practices.
NOTICE Indicates a potentially hazardous situation.
If not avoided, it could result in damage to
the product or environment.
Trademark Owner
Polytron®Dräger

Technical Manual | Digital Communication - Modbus RTU 5
Basic principles of the Modbus technology
2 Basic principles of the Modbus technology
2.1 Definition of the Modbus protocol
The Modbus protocol was developed by Modicon in 1979. It is an open,
standardized communications protocol which is used in production, process, or
building automation.
Modbus is a protocol for transmitting messages between electronic devices. The
device requesting the information is the Modbus master. The devices sending the
information are the so-called slaves. There are one master and up to 247 slaves in
a standard Modbus network. Each slave is defined by its own address from 1 to
247.
Modbus RTU (Remote Terminal Unit) is a binary transmission protocol for serial
interfaces (RS-232 or RS-485). The original system was developed with a RS-232
interface. However, it was adapted for RS-485 interfaces for faster transmission
rates. In addition, the RS-485 interface is bus-compatible: multiple devices can be
connected to the data cables.
For this product, only the RS-485 interface is described.
For further information on the Modbus technology, also see www.modbus.org.
For further information on RS-485, also see ANSI/TIA/EIA-485-A.
2.2 System overview
Modbus RTU is based on a Master/Slave architecture. In a Master/Slave system,
the Master communicates with one or more slaves. It is a mono-Master system:
only one Master can be connected at any one time.
The slaves in a Modbus network cannot start communication. They can only
respond.

6 Technical Manual | Digital Communication - Modbus RTU
Basic principles of the Modbus technology
2.2.1 Topology
Minimum configuration
A Modbus system consists of at least the following components:
– A Modbus Master to control the data traffic.
– One or more slaves which are available upon request of the Master.
– The transmission medium, consisting of the bus cable and bus connector to
connect the individual participants.
Maximum configuration
A bus segment consists of a maximum of 32 field devices (active and passive). The
highest possible number of slaves that can be operated with one Modbus Master
across several segments is determined by the internal memory structure of the
master.
The bus cable can be separated anywhere and a new participant added with a bus
connector. At the end of a segment, the bus cable can be extended until the
predetermined segment length is reached. The length of a bus segment depends
on the set transmission speed. The transmission speed is primarily determined by
the system constellation (length of a segment, distributed inputs/outputs) and the
required scanning cycles of individual participants. The same transmission speed
has to be selected for all participants connected to the bus.
Connect Modbus devices in a line structure.
If there are more than 32 participants, the Modbus system can be extended by
connecting repeaters. This enables bridging longer distances than defined for the
transmission speed.
31044
1 Request of the Master
2 Response from a Slave
3Master
4Slave
5Slave
6Slave
12 3
456

Technical Manual | Digital Communication - Modbus RTU 7
Basic principles of the Modbus technology
2.2.2 Transmission technology
RS-485
RS-485 is the most common transmission technology. It uses a shielded, twisted 2-
wire line. Transmission rates between 9600 bit/s and 115200 bit/s can be selected.
It is defined when commissioning the system and is the same for all devices
connected to the bus. Up to 32 bus participants can be connected per segment.
The maximum permissible line length depends on the transmission rate.
Several different network topologies are possible:
– Backbone
–Star
–Ring
– Daisy chain
According to experience, a RS-485 network works best in a daisy chain topology. In
this topology, reflections on the network line are absorbed most effectively.
Characteristics of the bus connection
The most important characteristics of the bus connection with their physical and
electrical properties are:
31082
Serial RS-485 connection - not connected correctly
31072
Serial RS-485 connection - connected correctly
Network topology Linear bus; if required, with bus termination at
both ends
Transmission medium Shielded, twisted 2-wire copper cable
Line lengths (without repeaters) Maximum line length: 1000 m
Physical interface RS-485 with bus connector in the top hat rail,
can be installed on site
Addressing 1 to 247
(32 devices in one segment. Can be extended
to 247 with repeater)
Device 1
COM
+
-
+ -
+ -
COM COM
Device 2 Device 3
Device 1
COM
+
-
+ -
+ -
COM COM
Device 2 Device 3

8 Technical Manual | Digital Communication - Modbus RTU
Basic principles of the Modbus technology
2.3 Data transmission
There are 3 different data transmission modes in a Modbus system:
– Modbus ASCII
– Modbus RTU
– Modbus TCP
In Modbus ASCII, data is not transmitted as a binary sequence, but as ASCII code.
The data is directly human-readable, but the data throughput is lower than in
Modbus RTU.
Data transmission in Modbus RTU is in binary format. The binary format enables
faster data transmission. However, the data needs to be converted to a readable
format as it is not human-readable.
Modbus TCP is very similar to Modbus RTU. However, the data is transmitted in
TCP/IP packets.
2.3.1 Modbus protocol structure
Communication in a Modbus network is structured in several Layers according to
the ISO/OSI model. To enable communication across a variety of technical
systems, the individual layers have different tasks.
The Modbus protocol corresponds to layer 7 in the ISO/OSI model – the so-called
application layer. It enables client/server communication between different devices
that are connected via different buses or networks. The protocol defines a
procedure for the exchange of information. It is therefore independent of the lower
physical layers. For this reason, different implementations exist on different
communication media:
– Ethernet
– Radio links
– Fiber optic cable
– Mobile radio
ISO/OSI layer Protocol Unit
7 Application layer Application-ori-
ented
Modbus Data
6 Presentation layer
5 Session layer
4 Transport layer Transport-ori-
ented
TCP
(Transmission Con-
trol Protocol)
Segments
3 Network layer IP
(Internet Protocol)
Packets
2 Data link layer Ethernet (802.3),
WLAN (802.11)
Frames
1 Physical layer Bits

Technical Manual | Digital Communication - Modbus RTU 9
Basic principles of the Modbus technology
Application layer – layer 7
The application layer provides functions for the application. It forms the connection
to the lower layers. Software for communication within the scope of the specific
application (measuring system, etc.) is provided by the manufacturers. In addition,
there are a number of terminal programs to enable communication between the
participants in a Modbus network. The terminal programs (e.g., Modbus Poll)
perform all functions of layers 5 to 7 for the user.
Physical layer – layer 1
The physical layer is the lowest layer and provides mechanical, electrical, and other
functional tools for establishing connections. These connections are activated,
deactivated, and used to transmit bits. These procedures serve to transmit, for
example, electrical signals, optical signals, electromagnetic waves, or sound.
Among others, the following devices and network components are assigned to the
physical layer:
– Antenna
– Repeaters
– Connector
– Socket for the network cable
–Hub
– Transceiver
–T-piece
– Terminator
Data transmission via Modbus RTU is digital, purely electrical, based on voltage
levels. Normally, 9-pin connectors and sockets are used. The pin assignment is not
standardized. Therefore always observe the manufacturer's documentation when
using different devices.
Transmission rates
The transmission rate defines the speed at which data is transmitted. The
transmission parameters have to be the same for all devices connected to a bus.
Parameter RS-485/EIA485
Number of receivers 32
Maximum line length 1200 m
Common-mode input voltage -7 to +12 V
Input resistance of the receiver 12 kOhm
Input sensitivity of the receiver +/- 200 mV
Differential data transmission Yes

10 Technical Manual | Digital Communication - Modbus RTU
Basic principles of the Modbus technology
The following transmission rates are available for RS-485 applications:
Operating principle
Communication in a Modbus network follows a simple request/response
mechanism: the Master (1) sends a request (Telegram) (2) to the Slave (4). The
function code contained in the telegram tells the Slave which function to execute.
The Modbus application protocol defines the format in which the request initiated by
the Master is sent.
There are 3 different communication cases:
Communication
If a cable pair is used, transmission takes place in half-duplex mode: data can flow
alternately in both directions, but not simultaneously.
If two cable pairs are used, full-duplex mode is also possible: data is transmitted in
both directions at the same time.
1200 bit/s
2400 bit/s
4800 bit/s
9600 bit/s
19200 bit/s
38400 bit/s
56000 bit/s
115200 bit/s
31033
1Master
2 Request from Master to Slave
3 Slave response
4Slave
The device (Slave) receives a
valid telegram with a valid
request.
If it can answer this, it sends a response telegram
with the function code of this request.
The device (Slave) receives an
invalid telegram (e.g., CRC
error).
It does not answer. The application (Master) has
to correct the error within a defined time (timeout).
The device (Slave) receives a
valid telegram with an invalid
request.
It answers with a Modbus exception code (Excep-
tion). The application (Master) has to correct the
error.
1 2 3 4

Technical Manual | Digital Communication - Modbus RTU 11
Basic principles of the Modbus technology
Communication in the Modbus network occurs through so-called polling. The
Master sends a telegram (Request) to the Slave. It then receives the Response.
Data transmission in Modbus RTU is in binary format. A data telegram consists of
several blocks (Frames):
– One address block (1 byte = 1 character)
– One function block (1 byte = 1 character)
– One data block (variable)
– One checksum block (CRC = Cyclic Redundancy Check)
Between 2 telegrams there has to be a break with a length of at least
3.5 characters. The corresponding wait time depends on the transmission rate. If a
transmission break of more than 1.5 characters occurs during telegram
transmission, the transmission process is considered incomplete and should be
aborted.
General form of a Modbus telegram
Address
The address specifies which device is contacted (Master - Slave) or which device
responds. The address is a number between 1 and 126. The address 0 is used for
messages to all devices (Broadcast), provided that the selected function supports
this.
The configuration of the address depends on the respective device and the system
architecture.
Device address Function code Data CRC check
8bits 8bits n*8bits 16bits
31032
Field Length Description Master Slave
Address 1 byte Identifies the target
device
Set by the Mas-
ter
Copied from the
request into the
response
CRC16 2 byte Checksum (CRC16) Calculated by
the Master
Calculated by the
Slave
Address Function
code Data CRC
Modbus telegram
Frame 1 Frame 1 Frame 1
8 bits 8 bits N x 8 bits 16 bits≥ 3.5 char
Start
At least 3.5 char At least 3.5 char 3.5 char
4.5 char
≥3.5 char
End

12 Technical Manual | Digital Communication - Modbus RTU
Basic principles of the Modbus technology
Function code
The function code specifies the purpose of the data transmission. It defines the
command to be executed by the Slave.
Broadcast messages are only permitted with function codes 03 and 16. Function
code 08 is only supported for RS-485.
FC03 Read Holding Registers
Request
Response
Com-
mand
Function code Description
011)
1) This command is not supported by Polytron®8000.
Read Coils Read back several digital outputs
021) Read Discrete Inputs Read several digital inputs
03 Read Holding Registers Read several analog inputs (and out-
puts)
041) Read Input Registers Read several analog inputs (and out-
puts)
051) Write Single Coil Write a single digital output
061) Write Single Register Write a single analog output
071) Read Exception Status
081) Diagnostics Check communication between Master
and Slave
16 Write Multiple Registers Write a new value to several Slave reg-
isters
231) Read/Write Multiple Registers Simultaneously read and write 1 to max.
118 registers in one telegram
xx Up to 255 function codes,
depending on the device
Function code 1 byte 0x03
Starting address 2 byte 0x0000 to 0xFFFF
Number of registers 2 byte 1 to 125 (0x7D)
Function code 1 byte 0x03
Byte count 1 byte 2 * number of registers
Holding register value (2 * number of
registers) bytes
2 to 250

Technical Manual | Digital Communication - Modbus RTU 13
Basic principles of the Modbus technology
Error
FC16 Write Multiple Register
Request
Response
Error
Data
The Data field contains the information to be transmitted. It is subdivided into
registers, the number of registers to be transmitted, and, if necessary, information
that has been read or has to be saved. Data is normally transmitted as a multiple of
16-bit registers.
Cyclic Redundancy Check – CRC (checksum)
The CRC is a 16-digit checksum calculated based on all bytes of a telegram. To be
able to detect transmission errors, it is generated by the Master and also calculated
by the receiver. If the CRC value does not match the transmitted sum, the device
will request the telegram to be transmitted again.
If the field device executes the requested function, it will send a response telegram
to the Master. The telegram contains the following parameters:
– Address of the device that was contacted and is now replying
– Function code
– The requested data
– CRC value
Modbus Exception Codes
If the Slave detects an error in the request telegram from the Master, it returns an
error message to the Master. The error message consists of the following elements:
– Slave address
Error code 1 byte 0x83
Exception code 1 byte 01 or 02 or 03 or 04
Function code 1 byte 0x10
Starting address 2 byte 0x0000 to 0xFFFF
Number of registers 2 byte 0x0001 to 0x007B
Byte count 1 byte (2 * number of
registers) bytes
Register value (2 * number of
registers) bytes
Value
Function code 1 byte 0x10
Subfunction 2 byte 0x0000 to 0xFFFF
Number of registers 2 byte 1 to 123 (0x7B)
Error code 1 byte 0x90
Exception code 1 byte 01 or 02 or 03 or 04

14 Technical Manual | Digital Communication - Modbus RTU
Basic principles of the Modbus technology
– Function code
– Error code (Exception Code)
– CRC (checksum)
To mark the error message, the leading bit of the returned function code is set. The
error code (Exception Code) is used to transmit the error cause to the Master.
Addressing of the registers
* The register address is transmitted as a 16-bit word. The first register is assigned
the address value 0.
Please note: another notation, where the register addresses are counted starting
with 1, is also common in Modbus applications. In that case, this address (register
address - 1) is transmitted in the Modbus telegram.
Excep-
tion code
Description
01 ILLEGAL_FUNCTION
The function code sent by the Master is not supported by the Slave.
02 ILLEGAL_DATA_ADDRESS
The register addressed by the Master is not assigned (does not exist)
or the length of the requested data is too long.
03 ILLEGAL_DATA_VALUE
The Master is attempting to write to a register that allows only read
access. The value appearing in the data field is not permissible: e.g.,
range limits exceeded or wrong data format.
04 SLAVE_DEVICE_FAILURE
The Slave did not respond to the request telegram from the Master or
an error occurred when processing the request telegram.
Register Data type Access Address
range*
Information
Discrete Input Bit Read only 1XXXX Not used by the device
Coils Bit Read/write 2XXXX Not used by the device
Input Register 16-bit word Read only 3XXXX This data can only be
read by the device
Holding Reg-
ister
16-bit word Read/write 4XXXX This data can be read
by the device and
changed by the appli-
cation

Technical Manual | Digital Communication - Modbus RTU 15
Installation
3 Installation
The following chapters describe the installation and configuration of the
Dräger Polytron®8000 gas detector with Modbus RTU interface (hereafter referred
to as "gas detector").
3.1 Technical data
The following values apply for the gas detector:
3.2 Cables
3.2.1 Cable type
Various cable types (type designations A to D) for different applications are
available on the market to interconnect the devices and connect them to network
elements (e.g., segment couplers, links, and repeaters). Use a type A cable for RS-
485 transmission.
The maximum length of the cables that can be used in a Modbus system is
determined by various factors. Different limitations apply for the cables and the
individual leads.
Use shielded, twisted 2-wire cables and observe the maximum cable length.
Parameter RS-485/EIA485
Number of receivers 32
Maximum line length 1000 m
Common-mode input voltage -20 to +25 V
Input resistance of the receiver 12 kOhm
Input sensitivity of the receiver +/- 200 mV
Differential data transmission Yes
Terminal Tightening
torque in Nm
Cable cross-sec-
tion in AWG
Cable cross-sec-
tion in mm²
Power supply
(2-pin)
0.5 to 0.8 24 - 12 0.2 to 2.5
Modbus RTU
(2-pin)
0.5 to 0.8 24 - 12 0.2 to 2.5
Parameter Condition Min.
value
Max.
value
Unit
Common-mode input voltage
range
-20 30 V
Differential output voltage
Data signal A-B or B-A
Load R=54 ohms 1.5 - V
Differential input voltage
Data signal A-B or B-A
Common-mode
voltage range
-20 V to +25 V
200 - mV

16 Technical Manual | Digital Communication - Modbus RTU
Installation
3.2.2 Cable lengths
The following factors influence the cable length
– Transmission rate
–Cabletype
– Number of directly connected devices (series connection)
– Network configuration (2-wire or 4-wire)
The maximum line length is 1000 m, at a transmission rate of 19200 bit/s. It
depends on the data transmission speed and the number and lengths of the
individual leads.
3.2.3 Increasing cable lengths with repeaters
Repeaters can be integrated in the system to extend the Modbus transmission
cable. No more than 3 repeaters per system are permitted. The cable length can be
extended by the factor 4, i.e., to max. 4000 m.
3.2.4 Grounding and shielding
Use a shielded twisted cable for connections via the RS-485 interface. To achieve a
sufficient shielding effect, connect a large area of the shield at both ends of the
cable to housing or cabinet parts.
Connect the shielding of the connector to protective ground in at least one point.
The legal EMC requirements are only met if the shielding is grounded at the
control unit at one end.
The shield is capacitively connected to the grounding of the housing via pin 4
(shield) of the 4-pin connector.
3.3 Pin assignment
For further information on the electrical installation, see the instructions for use.
3.3.1 Power supply
41083
Condition Min. value Max. value Unit
Power supply 10 30 V
Current consumption at 24 V - 350 mA
PWR+
PWR-
SCR
VDC
+
-

Technical Manual | Digital Communication - Modbus RTU 17
Installation
3.3.2 Signal lines for explosion protection type Ex d
The terminal of the PCB unit within the Ex d housing is assigned as follows:
3.3.3 Signal lines for explosion protection type Ex e
The terminal of the PCB within the docking station for explosion protection type Ex
e is assigned as follows (Fig. E3).
41082
4-pin connector
Assignment Data-A Data-B GND Shield
Function Signal A Signal B Ground Cable shield
DATA A
DATA B
Ground
Shield
+ / Data A
- / Data B
Ground
36810
4-pin terminal X2
Assignment FB + FB - COM SCR
Function Signal A Signal B Ground Cable shield
PWR+
PWR-
SCR
Digital: Fieldbus FF / Profibus PA, Modbus RTU
Power
VDC
+
-
FIELDBUS (X2)
PWR+
PWR-
SCR
E1
FB+
FB-
COM
SCR
E2
E3
E2
Fieldbus FF / Profibus PA
E3
Modbus RTU
FB +
FB -
COM
SCR
FB +
FB -
COM
SCR
FB+ / Data A
FB- / Data B
+ / Data A
- / Data B
Ground
E
E1

18 Technical Manual | Digital Communication - Modbus RTU
Installation
3.3.3.1 Rewiring of the PCB unit in the Ex d housing
The following information is only required if the Ex d housing has to be rewired (e.g.
when replacing the PCB unit in the Ex d housing).
Only the cable cores with ferrules are required for the gas detectors with Modbus
RTU interface. The other cable cores are inoperative. To be able to distinguish
between the cable cores with the same color, they were marked with heat-shrink
tubes. The "Remote sensor" connector is not present in every version.
3.4 Termination
The termination serves to suppress signal reflections on the bus cable. The
termination is switched off ex-factory.
44115
4-pin connector of the PCB unit in the Ex d housing
Assignment Data-A Data-B GND Shield
Cables Blue with mark-
ing
Blue without
marking
Gray with-
out marking
Gray with mark-
ing
Data-A
Data-B
GND
Shield
PWR +
PWR -
Remote sensor

Technical Manual | Digital Communication - Modbus RTU 19
Installation
Turning the termination on and off
To turn the termination on, slide the sliding switch towards the center of the PCB. To
turn the termination off, slide the sliding switch towards the edge of the PCB.
3.5 Opening the gas detector
1. Loosen the set screw (6).
2. Unscrew the lid (1) and take it off the gas detector.
3. Turn the handle (2) upward and pull out the PCB unit (3) containing the main
electronics.
31428
1 Sliding switch for termination (indicated position: termination turned on)
Off On
1
32174
4
1
3
5
2
6

20 Technical Manual | Digital Communication - Modbus RTU
Installation
3.6 Connecting the gas detector
Preconditions:
– Cable bushings are installed at the gas detector as described in the respective
instructions for use.
1. Insert the fieldbus cable and the power supply cable into the cable bushing.
2. Strip the cores of the cables.
3. Twist the shielding of the fieldbus cable.
4. If required, add ferrules and crimp them.
3.7 Closing the gas detector
1. Make sure that the following connections are correctly established:
a. The wiring screws are tightened to the correct torque.
b. All cable connections are secured with screws.
c. The sensor's plug-in coupling is inserted.
d. The grounding cable, which leads out of the housing, is connected to the
PCB unit with the eye,
2. Insert the PCB unit back in the housing.
3. Screw the cover back on with the correct torque until it is correctly fitted and
tighten the set screw.
Fig. 1 Seal the Ex d housing with the correct torque
40515
≥ 44 LB IN
≥ 5 Nm
≥ 8 LB IN
≥ 0.7 Nm
E