SILAREXHigh Performance
Multi-Channel NDIR Gas Sensor
Description of Module andCommunication for Firmware
Version 2.12

Page 1

Contents
1

2

3

General ............................................................................................................................................ 4
1.1

For your safety......................................................................................................................... 4

1.2

Intended use ............................................................................................................................ 4

1.3

Loss of warranty / liability / disclaimer ................................................................................... 5

Measurement cell with hose connections ...................................................................................... 6
2.1

Hose connection / hose material ............................................................................................ 6

2.2

Gas flow ................................................................................................................................... 6

2.3

Mounting / installation site ..................................................................................................... 6

Electrical connection ....................................................................................................................... 7
3.1

Current consumption .............................................................................................................. 74

LED status display ............................................................................................................................ 8

5

Pressure compensation (internal cell pressure ) ............................................................................. 8

6

Data interfaces ................................................................................................................................ 97

6.1

RS 485 mode ............................................................................................................................ 9

6.2

Signal profiles .......................................................................................................................... 9

6.3

Data exchange between master and Silarex (slave)................................................................ 9

6.4

RS 485 termination (bus terminating resistor) ...................................................................... 10

6.5

RS 232 mode via RS 485 interface ......................................................................................... 11

Modbus communication via RS 485 interface............................................................................... 11
7.1

Operation with multiple slave subscribers ............................................................................ 11

7.2

Automatic detection of baud rate, framing format and Modbus dialect ............................. 12

7.3

Structure of Modbus data telegrams .................................................................................... 12

7.4

Modbus communication device ............................................................................................ 13

7.5

Modbus address .................................................................................................................... 14

7.6

MODBUS control commands................................................................................................. 15

7.6.1

Control command 0x03 → Read (multiple) registers.................................................... 16

7.6.2

Control command 0x06 → Write to (only one) register ............................................... 17

7.7
8

Calculating the checksum ...................................................................................................... 18

Register overview .......................................................................................................................... 19
8.1

Meaning of the individual bits in the status bit bar (SYS_Status): ........................................ 20

8.2

Meaning of the individual bits in Sys_Settings_C (customer settings): ................................ 20

8.3

Description of the unit code: CONC_unitX: ........................................................................... 21
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9

Information on start-up and operation ......................................................................................... 21
9.1

Self-test / warm-up time ....................................................................................................... 21

9.2

Setting the zero point ............................................................................................................ 22

9.3

Setting the end point ............................................................................................................. 22

9.4

Calculating the correction value for the end point ............................................................... 22

9.5

Restoring the calibration parameters to factory settings ..................................................... 23

10

Integrated cell heater ................................................................................................................ 23

11

Mechanical dimensions ............................................................................................................. 24

12

Legal information ...................................................................................................................... 25

Page 3

1

GeneralThe SILAREX gas sensor is a “high performance”, independently measuring gas measurement cell thatworks with the tried and tested NDIR method. High resolution, measurement accuracy and long-term
stability are just some of the features of this newly developed sensor. Thanks to the integratedpressure and temperature compensation as well as its convenient interfaces, the SILAREX sensor isquick and easy to integrate into existing measuring and control systems. Based on the physicalmeasurement method of infrared absorption, SILAREX provides the best conditions for reliable,
precise and permanently stable measurements as well as selectivity. Its compact design and low
maintenance effort make it ideal for use in difficult conditions.
The present instructions refer to firmware version 2.11 and 2.12 (see Table 7).

1.1 For your safety
Meaning of warning signs
The following warning signs are used in this document to indicate the corresponding warning texts.
CAUTION!

Indicates a potential hazardous situation. If this is not avoided, injuries or damage to the product orenvironment may occur.
Also warns against improper use.
NOTE

Information on the use of the product
Before connecting and using the SILAREX, ensure that you have read and understood these
instructions fully. Please contact our Service department if you have any questions or if anything is
unclear. Warning signs indicate important information.
Store these instructions or give them to the device operator for storage if necessary; if you sell thisdevice, hand over these instructions to the purchaser. When installing and operating the device, youmust follow the statutory requirements and guidelines that relate to this product!

1.2 Intended use
The SILAREX is a gas measurement cell with independent measurement capabilities and is used todetermine gas concentrations in accordance with its specifications. It is not suitable for any othermeasurement or testing purposes and must not be used in any other way.

CAUTION!

SILAREX must not be operated in potentially explosive environments or under harsh conditions (e.g.
high, condensing humidity, heavy air flow, in aggressive atmospheres or outdoors withouta housing).

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1.3

Loss of warranty / liability / disclaimer
CAUTION!

Opening the sensor as well as manipulating or damaging the device will invalidate the warranty!
The warranty may also be invalidated if aggressive chemicals are used, contamination occurs, liquids
penetrate the device or the instructions in this description of the module and communication are notobserved!
smartGAS Mikrosensorik GmbH shall not be liable for consequential loss, property damage or
personal injury caused by improper handling or failing to observe the instructions in the module andcommunication description.

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2

Measurement cell with hose connections

The SILAREX measuring cell is made of aluminium and gold-plated. It is equipped with hoseconnections which ensure that the measurement gas passes through the measurement process. The
actual measurement cell is located between the gas inlet and gas outlet
2.1 Hose connection / hose material
Hoses must have an inside diameter of 3 mm and an outside diameter of 5 mm to connect to themeasuring cell. Make sure that the hoses are firmly connected to the hose connections.
The connection to the pressure transducer for the (indirect) internal cell pressure measurement is
produced at the gas outlet by means of a “T” hose adapter.
Please observe the direction of the gas flow, which is indicated by the labels “INLET” and “OUTLET”.
Mixing up the gas flow could result in measurement values that may show significant deviations fromthe factory calibration.
Ensure that hoses suitable for measurement are used. Certain applications generate corrosive gasesthat could cause problems with the hose material.
2.2 Gas flow
The gas flow should be constant and between 0.1 l/min and 1.0 l/min. The gas must
be dry and free from particles.
Corresponding filters can be purchased from smartGAS.
2.3 Mounting / installation site
Mount the Silarex using M3 screws with the four polyamide spacer bolts, which are mounted on theunderside of the cell. When screwing the sensor onto the mounting plate, make sure that no stress isapplied during the mounting.
When using different spacer bolts (than those mounted at the factory) or spacer sleeves, ensure
a minimum clearance of 3 mm to the mounting plate.
NOTE

Do not use the other (free) threads in the sensor for mounting purposes.

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3

Electrical connection

The SILAREX is connected using the ST1 and ST2 connectors supplied. The supply voltage andcommunication is connected using ST1.
Connector ST2 is a power output that can be connected to external peripheral devices (e.g. a gaspump) of up to 200 mA.

ST1

ST2
Figure 1: Position of the connectors

Connector ST1
Pin
Assignment
1+Vcc 12 – 24 V DC
2
NC
3
GND
4
RS485+
5
RS485-

Connector ST2 (power output)
Pin
Assignment1
5.8 V DC2
GND
max. 200 mA
Table 2: ST2 pin assignment

Table 1: ST1 pin assignment

3.1 Current consumption
The following table shows an overview of the current or power consumption. It is strongly
recommended to only use adequately dimensioned and voltage-stabilized supply voltages in order toprevent malfunctions due to voltage dips.
Appropriate cable diameters must be used for long supply lines in order to avoid excessive voltage
drops over the lines!
Supply voltage
12 V
24 V

Current consumption
300 mA
250 mA

Table 3: Voltage-dependent power consumption

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4

LED status display

Three LEDs (green/yellow/red) are located on the Silarex circuit board. These show the current
device status according to table Table 4:

Figure 2: Position of the status LEDs

LD1 (red) O
illuminates-

LD2 (yellow) O
flashes

LD3 (green) O---

flashes
illuminates

Device status
device error / contact service- warning,- heating control not at setpoint,- measured value falls below or exceeds theguaranteed range
warm-up phase (30 minutes)
normal operation

Table 4: LED status display

When the system is switched on, it also performs a brief LED check during which the LEDs light up
briefly.

5

Pressure compensation (internal cell pressure )Owing to the physical properties of the gases, their density changes depending on the pressure. Thischanges, however, depending on the altitude and weather conditions.
For this reason, a pressure transducer for measuring the internal cell pressure should already be
integrated in the Silarex. Hence, the current pressure in the cell is included in the internal calculationof the concentration value.
Automatic pressure compensation in the pressure range of 600 – 1150 mbar takes place. If the value
falls below or exceeds this range, a loss of accuracy is to be expected.

Figure 3: Position of the internal cell pressure sensor

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6

Data interfaces

6.1 RS 485 mode
The RS 485 interface is a serial interface that works in 2-conductor mode (half-duplex). Datatransmission occurs via symmetrical signals on the RS+ and RS- lines. The reference signal is GND.
The RS485 interface facilitates single master / multiple slave mode, with the sensor acting as theslave. A PC, microcontroller or something similar can be used as the master.
If the users do not have the same zero potential (GND), potential shifts might occur. To prevent thesefrom affecting the terminal devices, the interfaces should be electrically isolated from the rest of thecircuit in this case (e.g. by a fast optocoupler).
6.2 Signal profiles
The signals are transmitted differentially at the RS485 interface. RS+ routes the signal unchanged,
and RS- in its inverted form – see Figure 4. The data signal is evaluated via the difference between
the two signals [RS+] - [RS-].

Figure 4: Signal transmission and evaluation on the RS485 data bus

RS485 transmitters provide a voltage difference of at least ±2V under load. The voltage difference
can be smaller due to potential damping effects. The sensitivity of the receiver is ±200mV and canevaluate valid signals up to this value:[RS+]-[RS-]> 200mV
logical “1”[RS+]-[RS-]< -200mV
logical “0”[RS+]-[RS-]< 200mV
false interpretation of the data is possible.

6.3 Data exchange between master and Silarex (slave)
Figure 5 shows a possible scenario between master and SILAREX (= slave).
The following times refer to MODBUS ASCII and a baud rate of 2400 Bd.

Page 9Figure 5: Time diagram – Data exchange between master and SILAREX (slave)

The duration of a query string is 70 - 73 ms. It is therefore possible for a short pause to follow (max.
400 ms). The module response then follows. This depends on the number of bytes being read out. If
only one byte is read out, the module response is around 70 ms. When multiple bytes are read out,
the response phase is extended accordingly.
Basically, it can be said the SILAREX sensor responds to a query within 400 ms. The character string isthen sent immediately without a response pause.
CAUTION!

At higher baud rates (> 2400 Bd), significantly faster response times can be expected.

6.4 RS 485 termination (bus terminating resistor)
From a line length of approx. > 30 m, it is advisable to use a terminating resistor to preventreflections on the signal lines. The bus terminating resistor (150 Ohm) integrated on the Silarex
circuit board can be switched on by connecting jumper JP 1.

Figure 6: Position of JP1

NOTE

The cable used for wiring should be twisted and shielded (twisted pair cable).

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6.5 RS 232 mode via RS 485 interface
If only an RS 232 interface is available for data communication, the RS 485 interface can be
configured for the RS 232 mode by an external circuit. RS232 mode requires the sensor to be wired
as follows:

Figure 7: External wiring for RS 232 mode (R1/R2 approx. 2.2 kOhm)

NOTE

Please note the following for the RS 232 mode:- The transmission rate must not exceed 38400 Bd- The jumper JP 1 for the termination must not be connected

7

Modbus communication via RS 485 interface

The SILAREX sensor supports the MODBUS protocol in ASCII and RTU mode via its RS 485 interface. In
ASCII mode, in addition to the standard variant, there is a smartGAS-specific derivative that differsfrom the standard in terms of the checksum calculation.
In principle, Modbus communication functions on the basis of a query/response mechanism. The
master sends the query to one of possibly several slaves (subscribers). Each connected subscriber
therefore receives a subscriber address that is unique in the network. Only the subscriber that has
found its address in the query from the master will respond.
The type of query is determined by a control command (function code). This can, for example, be
about writing data or reading data to/from the subscriber. Depending on the control command,
there is a data portion for both the query and the response.
Each query and each response must be clearly identified by its beginning and by its end. The use ofa check field (=check word/CRC) is envisaged in the protocol to enable any possible communication
errors to be detected. The Modbus derivatives implement this in different ways.
You can obtain detailed information about the Modbus protocol at www.modbus.org

7.1 Operation with multiple slave subscribers
The RS485 data bus enables up to 32 subscribers to be integrated. The terminating resistors are usedat the start and end of the data bus (JP1). The subscribers suspended in-between are guided on theterminated data bus by means of stubs or optimally by means of a “daisy chain” (series connection
principle).

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At a transmission rate of 2.4kbps (2400 baud), the total length of the data bus, including stubs, mustbe limited to 500 m. Basically, the higher the transmission rate, the smaller the total length of thestubs.
The baud rate is determined by the slowest respective subscriber and is the same for all subscribers.

Subscriber
Designation
Address Baud rate Role
Termination
position(bus terminating resistor)
1
SILAREX sensor
222400 Bd
Slave
Yes
2
Computer---2400 Bd MasterNo
3
Temperature sensor
112400 Bd
Slave
No
4
Pressure sensor
33
2400 Bd
Slave
Yes
Table 5: Example - Data bus with multiple subscribers

As Table 5 shows, the address assigned to the subscribers does not depend on their position in the
topology. Since the MODBUS protocol is used, the master does not need an address. Only the slaves
have to be clearly addressed.

7.2 Automatic detection of baud rate, framing format and Modbus dialect
The SILAREX software is provided with automatic configuration detection. This means that the sensor
automatically detects the baud rate, the framing format as well as the MODBUS dialect used whenswitched on for the first time in the system.
The framing rates and MODBUS baud rates listed in table 6 harmonise with each another and can be
freely combined among each other.
Framing formats and MODBUS baud rates

Framing formats 
7E1
7E2
7O1
7O2
7N2
8E1
8N1
8N2
8O1

Baud rates 2400 Bd
4800 Bd
9600 Bd
9200 Bd
38400 Bd
57600 Bd
115200 Bd

Table 6: Freely combinable framing formats / baud rates

NOTE

A framing format of 8 data bits must be used for the communication via MODBUS RTU.

7.3 Structure of Modbus data telegrams
The following two tables show the basic structure of an ASCII data telegram and a RTU datatelegram. The tables show that the address, control command and data portion are based on thesame source data for both telegram types:
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Dialect
ModbusASCIIStart
1 character’:’

Address
2 characterse.g.: “A0“

Controlcomm.
2 characterse.g.: “03“Data
0 to 2x252 characterse.g.: “00050002”

LRC
2 characterse.g.: “A6“End
2 characters
CR, LF

Communication:

0x3A

0x41, 0x30

0x30, 0x33

0x30, 0x30, 0x30, 0x35
0x30, 0x30, 0x30, 0x32

0x41, 0x36

0x0D, 0x0A

CRC
2 bytese.g.: 0xA4,
0xD3
0xA4, 0xD3

End

Dialect
Modbus
RTU

Start

Address
1 bytee.g.: 0xA0

Controlcomm.
1 bytee.g.: 0x03

Data
0 to 1x252 bytes
e.g.: 0x00, 0x05, 0x00, 0x02

Communication:

Pause 3.5
characters

0xA0

0x03

0x00, 0x05, 0x00, 0x02

Pause
3.5 characters

In ASCII mode, each 8-bit byte is therefore sent as two ASCII characters. One byte corresponds to two
nibbles. One nibble consists of 4 bits and represents precisely one hexadecimal character. As can beseen in the telegram examples, the result of the byte containing the information “0xA6” is the two
ASCII characters “0x41” = ‘A’ and “0x36” = ‘6’.
In ASCII mode, 7 data bits are sufficient for transporting the characters via the interface. The ASCII
mode has a historical advantage. All Modbus data telegrams can be “read” with an ASCII terminal;
plain text appears on the screen.
In RTU mode, however, each 8-bit byte is handed over unchanged. This inevitably means that UART
frames with 8 data bits need to be used in RTU mode. The advantage of the RTU mode lies in the
more effective utilisation of the interface because only around half of the data volume needs to betransmitted compared to the ASCII mode.

7.4 Modbus communication device
Figure Figure 8 shows the state diagram of the transmission and receiving devices in principle,
regardless of whether it is master or slave:

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Receive •‘:’ (0x3A)/
Empty receive buffer

Wait(ready to send or
receive)

RequestReceive ‘:’ (0x3A)
Receive

Receive ‘LF’ (0x0A) / checkreceived telegram (LRC, parity,
slave address, etc.)

Receive characters/
Set characters in thereceive bufferStartsending

Send’:’ (0x3A)

Receive CR(0x0D)
Send all characters

Send

Send ’CR’, ‘LF‘
(0x0D, 0x0A)

Sending end

Receive •‘:’ (0x3A)/
Empty receive buffer

Wait for end oftelegram

Figure 8: State diagram of a Modbus subscriber (ASCII operating mode)

If an incomplete query is sent to the SILAREX sensor, the sensor does not return a response. The
module behaves the same when at least one register in the register area being queried does notexist. Error-free telegrams are processed, others are discarded.
7.5 Modbus address
With the SILAREX sensor, the as-delivered device address (Modbus address) corresponds to the last
two numbers of the serial number on the type plate.

NOTE

Example for calculating the Modbus address:
Device address = #35 decimal → 0x23 hex
If the serial number ends with “00”, the address is always #100 decimal = 0x64 hexadecimal.
The address “0” must never be used!
Figure 9 is a flow diagram that shows how unknown Modbus module addresses can be determined.
Any register (e.g. serial number) can be queried via all module addresses (1–247) with a timeout of
one second. If a module is queried with the correct address, it reacts by sending a response. The
module address is included in this response. Thus, at the end of the search cycle, module responses
can be used to analyse which module addresses are presently connected to the bus system. Whenthe serial numbers are queried, it is then possible to conclude which address is assigned to which
module.

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The permitted address range for SILAREX is between 1 and 247. According to the Modbus
specification, the addresses 248–255 are reserved. Address 0 stands for broadcast and must not be
used!
START

Connect module

Timeout := 1s:

Query telegram for reading out the serial number
Addr.
03
0086
0004
LRC
CRLF

Addr.: = 1 to 247 STEP 1
Calculate LRC

Send query

Addr. = 248?

Ye
s

STOP

no
Timeout?

Yesno
Valid response

no

Ye
s
Output serial no.
and address

Figure 9: Flow diagram for determining module addresses

7.6 MODBUS control commands
Two command codes (function codes) are sufficient for communication with the SILAREX sensor.
These are the following commands:• 0x03 – Read (multiple) holding registers and• 0x06 – Write (exactly one) register
One register is 16 bits wide and thus consists of 2 bytes:
15
High order byte – Hi

Register…

0
Low order byte – Lo

All the SILAREX data that the user can access is shown on registers that are each 16 bits wide.

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7.6.1 Control command 0x03 → Read (multiple) registers
This control command allows you to read values from the SILAREX sensor. It is essentialthat only registers defined in these instructions can be read. Therefore, thismust be checked especially when multiple registers are queried.
RequestField

Response
(hex)

Field

(hex)

Meaning of the data(according to ASCII table)

Control command

03

Control command03

Hi start address

00

Number of bytes

08

Lo start address80

Hi register value (128)

53

‘S’

Number of Hi registers

00

Lo register value (128)

58

’X’

Number of Lo registers

04

Hi register value (129)

20

’3 ’

Lo register value (129)

53

’0’

Hi register value (130)

46

’0’

Lo register value (130)

36

’0’

Hi register value (131)

20

’0’

Lo register value (131)

20

’3’

Example 1: Reading out the 4 registers for “Device Type”

In this example, four registers of the SILAREX sensor were read starting from register start address
0x0080 (decimal 128). The response consisted of a payload of 8 bytes that can be resolved with theaid of the ASCII table. Example: Response HEX 53 → to ASCII table → letter S
The response is now “SX300003”. Thus, it is a SILAREX sensor (SX)
for (3) measurement channels and the derivative number (00003).
RequestField
Control command
Hi start address
Lo start address
Number of Hi registers

(hex)
03
00
0E
00

Number of Lo registers

01

Response
Field
Control command
Number of bytes
Hi register value (14)Lo register value (14)Meaning of the data
(hex)
03
02
01
C8

456

Example 2: Reading out the “Conc” register (for displaying the gas concentration)

In this example, one register was read starting from the register start address 0x0E (decimal 14). The
two data bytes were transmitted combined as a hexadecimal value. If this value (01C8) is converted
to a decimal number, the result is a concentration value of 456.
RequestField
Control command
Hi start address
Lo start address
Number of Hi registers
Number of Lo registers

(hex)
03
00
23
00
01

Response
Field
Control command
Number of bytes
Hi register value (35)
Lo register value (35)

Meaning of the data
(hex)
03
02
00
03

3, means ppm x 1

Example 3: Reading out the “Unit” registerIn this example, one register was read starting from the register start address 0x0023 (decimal 35).
The two data bytes were transmitted combined as a hexadecimal value. If this value (0x0003) isconverted to a decimal number, the result is “3”. This stands for the unit ppm with the scaling x 1.

Page 16

Combined with the data from examples 1 and 2, the SILAREX sensor that was read has therefore
measured a gas concentration of 456 ppm.

7.6.2 Control command 0x06 → Write to (only one) register
This command enables a new value to be systematically written to an addressed register. However, it
is only possible to write to those registers intended for this purpose.
RequestField
Control command
Hi register address
Lo register address
Hi register value (192)
Lo register value (192)

Response
(hex)
06
00
C0
00
A0

Field
Control command
Hi register address
Lo register address
Hi register value (192)
Lo register value (192)

Meaning of the data
(hex)
06
00
C0
00
A0

The new addressof the module (160)

Example 4: Writing to the “Modbus_address” registerIn this example, a new Modbus address A0 (hex) = 160 dec. was assigned to the SILAREX sensor.
Once this communication sequence is complete, the device is only responsive at this new address!
NOTE

The address 0 as well as addresses >247 must not be assigned!
RequestField
Control command
Hi register address
Lo register address
Hi register value (32)
Lo register value (32)

(hex)
06
00
20
00
01

Response
Field
Control command
Hi register address
Lo register address
Hi register value (71)
Lo register value

Meaning of the data
(hex)
06
00
20
00
01

The zero point
has been reset

Example 5: Writing to the IR_zero1 register (setting the zero point for measurement channel 1)

In this example, the zero point of the SILAREX sensor has been reset. This was done by writing thevalue 1 to register 0x0020 (decimal 32). The device subsequently internally calculated and saved thecurrent correction value for the zero point. Reading out the same register then shows the value ofthe correction.
NOTE

The zero point must only be set when zero gas and then a stable concentration value are applied.
RequestField
Control command
Hi register address
Lo register address
Hi register value (84)
Lo register value

(hex)
06
00
24
27
10

Response
Field
Control command
Hi register address
Lo register address
Hi register value
Lo register value

Meaning of the data
(hex)
06
00
24
27
10

Correction value has
been set to 10000

Example 6: Writing to the CONC_gain1 register (end point correction comparison)In this example, a new end point correction was set for the SILAREX sensor. A value of 2710 (hex) =10000 (decimal). This is also the delivery condition. A value of 11000 would mean, for example, thatthe concentration value displayed is 10% higher than what is internally measured. This register
therefore enables deviations of the SILAREX sensor in the concentration display to be corrected.
Page 17

NOTE

The end point must only be set in this way when a suitable test gas and then a stable concentration
value are applied!
Before the end point is set, the zero point must be set correctly.
7.7 Calculating the checksum
The calculation of the checksum (LRC) specifically for the ASCII (smartGAS) operating mode will now
be explained based on an example. How the calculation of the LRC checksum in ASCII standard and
CRC checksum in RTU works is described thoroughly in the documentation of the Modbus standard.
It is helpful here to have a conversion table for ASCII values in hexadecimal and decimal format asfollows:
ASCII

’0’

’1’

’2’

’3’

’4’

’5’

’6’

’7’

’8’

’9’

‘A’

‘B’

‘C’

‘D’

‘E’

‘F’

Hex

30

31

32

33

34

35

36

37

38

39

41

42

43

44

45

46

Dec.

48

49

50

51

52

53

54

55

56

57

65

66

67

68

69

70

The checksum is calculated using the address, the control command and the associated data after
the conversion to ASCII has occurred. By way of example, we generate a query for reading out theConc register from the SILAREX sensor with the address 14 (decimal) = 0E (hex.)
Therefore, in hexadecimal format, the resulting byte string is 0x23, 0x03, 0x00, 0x0A, 0x00, 0x01.
After the ASCII conversion, the result is the data string 2303000A0001. The data string is now
converted and the checksum is formed:
ASCII
Dec.

Address
0
E
↓
↓
48
69

Command
0
3
↓
↓
48
51

Start register
0
0
0
↓
↓
↓
48
48
48

A
↓
65

Number of registers
0
0
0
1
↓
↓
↓
↓
48
48
48
49

Sum
--618

Sum = 618
Checksum = 255 - 618 + 1 = - 362
Modulo sum (256) = - 362 + 256 + 256 = 150 (dec.) → 96 (ASCII hex.)
Putting the starting character at the beginning and the calculated checksum and end code at the end
would mean that the following data string would be sent: 2303000A000196<CR><LF>
The checksum is included each time data is sent and is then recalculated by the recipient again. If thedata set is corrupted or adulterated, the checksum calculated by the recipient would deviate fromthat which was sent. The data set would then be unusable.

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8

Register overview
Address
0x05
0x08
0x09
0x0A
0x0B
0x0E

Name
T_int
T_amb
P_amb
P_kue
SYS_Status
CONC_1

R/W
R/--R/--R/--R/--R/--R/---

0x11

CONC_2

R/---

0x14

CONC_3

R/---

0x20
0x22
0x23

IR_zero1
CONC_fs1
CONC_unit1

R/W
R/W
R/W

0x24
0x3C 0x3F
0x40
0x42
0x43

CONC_gain1
gas_name1

R/W
R/---

IR_zero2
CONC_fs2
CONC_unit2

R/W
R/W
R/W

0x44
0x5C 0x5F
0x60
0x62
0x63

CONC_gain2
gas_name2

R/W
R/---

IR_zero3
CONC_fs3
CONC_unit3

R/W
R/W
R/W

0x64
0x7C 0x7F
0x80 0x83
0x84 0x85
0x86 0x89
0x90 0x97
0x99
0xC0

CONC_gain3
gas_name3

R/W
R/---

Device TypeR/---

Software version

R/---

Serial No.

R/---

Item No.

R/---

Sys_Settings_C
R/W
Modbus_Address R/W

Function/description
Measured value for internal temperature (x0.1°C)
Measured value for ambient temperature (x0.1°C)
Measured value for ambient pressure in hPa (=mbar)
Measured value for internal cell pressure in hPa (=mbar)
Status bit bar, see page 20 for details
Measured value for gas concentration in channel 1 inppm, vol% or %LEL
(Please note unit code CONC_unit1!)
Measured value for gas concentration in channel 2 inppm, vol% or %LEL
(Please note unit code CONC_unit2!)
Measured value for gas concentration in channel 3 inppm, vol% or %LEL
(Please note unit code CONC_unit3!)
Setting value for zero adjustment of channel 1
Full Scale Channel 1
Unit code for gas concentrationof channel 1, see page 21 for details
Concentration calculation of gain for channel 1
Gas name of channel 1 in ASCII form
Setting value for zero adjustment of channel 2
Full Scale Channel 2
Unit code for gas concentrationof channel 2, see page 21 for details
Concentration calculation of gain for channel 2
Gas name of channel 2 in ASCII form
Setting value for zero adjustment of channel 3
Full Scale Channel 3
Unit code for gas concentrationof channel 3, see page 21 for details
Concentration calculation of gain for channel 3
Gas name of channel 3 in ASCII form
Device type, max. 8-character text,
distributed over 4 registers (e.g.: SX300003)
Software version, distributed over 2 registers
Serial no., a max. 8 characters long,
distributed over 4 registers
Article number in ASCII format
System settings, explanation see page 20
Sensor Modbus address

Table 7: Modbus register table

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R= (Read) register can only be read
W= (Write) register can be written
NOTE

All other registers not described here must not be changed under any circumstances.
8.1 Meaning of the individual bits in the status bit bar (SYS_Status):
Faults and error messages can be identified with the aid of the SYS_Status register according to thefollowing table.
Bit
Name
Value → Message
00
DET_ERR
1 → IR detector disturbed
01
TMP_ERR
1 → Temperature sensor disturbed
02
PRS_ERR
1 → Internal cell pressure sensor disturbed
03
STR_ERR
1 → IR emitter disturbed
04
EEP_ERR
1 → EEPROM error
05---without function (reserved)
06---without function (reserved)
07
WDG_WRN
1 → after watchdog reset08
Amb_SEN_ERR
1 → Ambient pressure sensor disturbed
09
WARMUP
1 → Silarex is in the warm-up phase
10
HEATER_NOT_IN_RANGE
1 → Control deviation of the cuvette heating is >2K (only ifheating is active)
11
OUT_OF_LIMIT_UNDERRUN 1 → Measured value falls below or exceeds the guaranteed
range(-20% of the measurement range end value)
12
OUT_OF_LIMIT_OVERRUN 1 → Measured value exceeds the guaranteed range (+40% of
the measurement range end value)
13
OUT_OF_RANGE
1 → Measured value calculation outside of the displayable
number range (-32768…32767) – Measured value output islimited to the displayable range.
15…14---without function (reserved)
Table 8: Allocation of the error messages in the status bit bar SYS_Status

NOTE

The value 0 always stands for the (error-free) normal state.
8.2 Meaning of the individual bits in Sys_Settings_C (customer settings):
The following configuration can be adapted by the customer:
Bit
Name
Value → Message
00
CONC_LIMITS_OFF 1 → Limitation of the measured value to the specification of thesensor is deactivated. The measured values outside of the guaranteed
specification range are not necessarily within the guaranteed
accuracy.
PFILTER_SEL
Setting of the downstream low-pass filter
02...01
00 → No downstream low-pass filtering
01 → Average value filter moderate (T90 = 7.2s)
10 → Average value filter strong (T90 = 60s)
11 → Adaptive low pass (fast with signal change)
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03

1 → Pressure changes in the system are compensated when themeasured values are calculated---Without function (reserved)
RESTORE_FACTORY 1 → Restoration of the factory settings_DEFAULT
for zero and end point of all measuring channels (CONC_1, CONC_2
CONC_3). The bit is self-resetting.
PRESS_COMP

14…04
15

Table 9: Assignment of the settings in the configuration Sys_Settings_C

8.3 Description of the unit code: CONC_unitX:
For each of the three possible measurement channels there is a unit code (e.g. CONC_unit1 register
0x23 for measurement channel 1), which describes the measuring range and the factor by which themeasured value given in register CONC_X must be multiplied.
Table 7 shows the allocation of the value read out in the register CONC_unitX to the aforementioned
factors
Register value →Unit0→ unassigned, for special applications
1→ ppm x 0.01
2
→ ppm x 0.1
3
→ ppm
4
→ Vol.% x 0.001
5→ Vol.% x 0.01
6→ Vol.% x 0.1
7→ LEL x 0.01%
8→ LEL x 0.1%
Table 10: Allocation of register value Conc_unitX to the measuring unit / multiplier

NOTE

Partial quantities of <1 vol.% are mostly specified as a ppm value. The following table shows therelationship of vol.% to ppm:
Vol.%
ppm
100
1,000,000
10
100,000
1
10,000
0.1
1000
0.01
100
0.001
10
0.0001
1
Table 11: Relationship of vol.% to ppm

9

Information on start-up and operation

9.1 Self-test / warm-up time
After the SILAREX sensor is switched on, an internal self-test takes place and the yellow LED flashes
briefly. Then a function check of the status LEDs takes place. These light up in the following order:
Red → Yellow → Green

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The SILAREX sensor is then in the warm-up phase and the green LED flashes. The sensor now delivers
measured values and system errors are evaluated.

NOTE

During the self-test of approx. 3 seconds, correct measurement values are not output.
Each time after switching on, the sensor requires a warm-up phase of 30 minutes until all
temperature-dependent elements have reached their operating point.
Only then are all specifications (measurement accuracy etc. according to data sheet) reached.
For SILAREX sensors with a cell heater, it may take longer to reach the system temperature. Thisdepends on the surrounding environmental conditions. If the control deviation is too great (>±2K),
the status HEATER_NOT_IN_RANGE is output in the status bar and the yellow LED flashes.

9.2 Setting the zero point
It is advisable to set the zero point• after reinstallation of the sensor or measuring system• at regular intervals (must be adapted to the application)• after repairs/maintenance work on the sensor or measuring system
Before the zero point is adjusted, the sensor must be in operation for at least 30 minutes and a zerogas (e.g. N2 – 100 vol.%) must flow through the sensor until the indicator for the gas concentration(register Conc_1, 2 or 3) has reached a stable value.
If the aforementioned requirements are met, the value 1 is written in the register IR_zero1 (0x20)
and the zero point is thus reset. In the case of multi-channel systems, the process for IR_zero2 (0x40)
and IR_zero3 (0x60) must be repeated.

9.3 Setting the end point
Setting the end point (also called final value or span calibration) requires the use of a test gas, whichshould correspond as accurately as possible to the final value of the measuring range of the sensor to
be calibrated.
The same preconditions apply here as when setting the zero point: the sensor must be in operation
for at least 30 minutes and test gas must flow through it until a stable value has been set in theregister Conc_1, 2 or 3.
If all requirements have been met (which has to be tested by multiple querying of the register
Conc_1, 2 or 3), the correction value for the respective measurement channel is written in theregister Conc_gain 1, 2 or 3.
9.4 Calculating the correction value for the end point
Let us assume that a sensor indicates a concentration of only 978 ppm (referred to as “Conc_old”
here) in channel 1 when a test gas is applied which has the value 1003 ppm (referred to as“Conc_cal” here).
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The reading of the Conc_gain1 register yields the value 9985 (referred to as “Gain_old” here).
The new calculation of the correction value for the Conc_gain1 register then takes place as follows:
Conc_gain1_new = Conc_cal x Gain_old / Conc_old
Conc_gain1_new = 1003 x 9985 / 978 = 10240
The new value of 10240 is now written in the register Conc_gain1 (0x24) and the process iscompleted!
NOTE

Setting the end point only makes sense if the zero point has previously been set correctly.

9.5 Restoring the calibration parameters to factory settings
When bit 15 is set in Sys_Settings_C (page 20) the factory setting of the calibration parameters forzero and end point of all measuring channels (CONC_1, CONC_2 and CONC_3) can be restored.

10 Integrated cell heater
The SILAREX sensor is equipped with a cell heater. This stabilises the cell to 42°C. Depending on theambient conditions, it is possible that the heating output is not sufficient to reach the specified
temperature on the sensor. The sensor can then be installed, for example, in a heat-insulating
housing. Please contact smartGAS if you have any questions about this.

NOTE

The current temperature on the sensor can be read out via the register and 0x05 (T_int).

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11 Mechanical dimensions(all measurements in mm)

Table of the dimensions (all other dimensions are approximately the same for all device versions)
Cell length →125 mm

155 mm

305 mm

A→
B→
C→

110
110
155

140
140
185

290
290
335

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12 Legal information
The figures and drawings used in this description may differ from the originals; they are provided
solely for illustrative purposes.
All information – including technical specifications – is subject to change without notice.
All pictures and graphics in this manual: © 2020 smartGAS Mikrosensorik GmbH, Heilbronn.

smartGAS Heilbronn, Hünderstrasse 1© smartGAS Mikrosensorik GmbHsmartGAS Mikrosensorik GmbH |Huenderstr. 1 | 74080 Heilbronn | Germany
Phone: +49 7131/797553-0 | fax: +49 7131/797553-10 | www.smartgas.eu|mail@smartgas.eu

Edition 02/06_2020
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