No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon, Inc.

The information contained herein is believed to be accurate and reliable. However, Geokon, Inc. assumes no responsibility for errors, omissions or misinterpretation. The information herein is subject to change without

notification.

Copyright ©, 2015-2016 by Geokon, Inc. (Doc Rev B. 12/6/2016)

Instruction Manual

Model 3810A Addressable Thermistor String

Warranty Statement

Geokon, Inc. warrants its products to be free of defects in materials and workmanship, under

normal use and service for a period of 13 months from date of purchase. If the unit should

malfunction, it must be returned to the factory for evaluation. Upon examination by Geokon, if

the unit is found to be defective, it will be repaired or replaced at no charge. However, the

WARRANTY is VOID if the unit shows evidence of having been tampered with or shows

evidence of being damaged as a result of excessive corrosion or current, heat, moisture or

vibration, improper specification, misapplication, misuse or other operating conditions outside

of Geokon's control. Components which wear or which are damaged by misuse are not

warranted. This includes fuses and batteries.

Geokon manufactures scientific instruments whose misuse is potentially dangerous. The

instruments are intended to be installed and used only by qualified personnel. There are no

warranties except as stated herein. There are no other warranties, expressed or implied,

including but not limited to the implied warranties of merchantability and of fitness for a

particular purpose. Geokon, Inc. is not responsible for any damages or losses caused to other

equipment, whether direct, indirect, incidental, special or consequential which the purchaser

may experience as a result of the installation or use of the product. The buyer's sole remedy

for any breach of this agreement by Geokon, Inc. or any breach of any warranty by Geokon,

Inc. shall not exceed the purchase price paid by the purchaser to Geokon, Inc. for the unit or

units, or equipment directly affected by such breach. Under no circumstances will Geokon

reimburse the claimant for loss incurred in removing and/or reinstalling equipment.

Every precaution for accuracy has been taken in the preparation of manuals and/or software,

however, Geokon, Inc. neither assumes responsibility for any omissions or errors that may

appear nor assumes liability for any damages or losses that result from the use of the

products in accordance with the information contained in the manual or software.

TABLE of CONTENTS

1. Introduction ............................................................................................................. 1

2. Installation ............................................................................................................... 1 2.1 Hardware Requirements: ..................................................................................... 2 2.2 TTL to RS-485 Converter: .................................................................................... 2 2.3 USB to RS-485 Converter: ................................................................................... 2

3. Modbus RTU Protocol ............................................................................................ 3 3.1 Introduction to Modbus: ....................................................................................... 3 3.2 Modbus RTU Overview: ....................................................................................... 3 3.3 Modbus Tables: ................................................................................................... 3 3.4 Reading 3810A Sensors: ..................................................................................... 4

4. Modbus and Campbell Scientific Dataloggers ..................................................... 5 4.1 Description: .......................................................................................................... 5 4.2 Wiring: .................................................................................................................. 5 4.3 Sample CR1000 Program: ................................................................................... 6 4.4 Sample CR6 Program: ......................................................................................... 7

5. Temperature Calculation ........................................................................................ 8 5.1 Steinhart-Hart Coefficients: .................................................................................. 8 5.2 Floating Point Resistance Readings (preferred method): .................................... 8 5.3 Raw ADC (Analog to Digital Converter) Readings: .............................................. 8

6. PC Application ........................................................................................................ 9 6.1 System Requirements: ......................................................................................... 9 6.2 Installation: ........................................................................................................... 9 6.3 Running the Application: ...................................................................................... 9

7. Reading Addressable Thermistor Strings with a Handheld PC ........................ 13

Appendix A – Specifications .................................................................................... 14 A.1 Power ................................................................................................................ 14 A.2 Communication .................................................................................................. 14 A.3 Measurements ................................................................................................... 14

LIST of FIGURES and TABLES

Figure 1 – A 3810A Addressable Thermistor String ...................................................... 1

Figure 2 – Close up of one of the Addressable Thermistor Nodes ............................... 1

Figure 3 – Geokon TTL to RS-485 Converter ............................................................... 2

Figure 4 – USB to RS-485 Converter ........................................................................... 2

Table 3 – RAM Storage ................................................................................................ 3

Table 4 – Example Query - Sensor #2, floating point resistance reading ..................... 4

Table 5 – Example Response - Sensor #2, floating point resistance reading ............... 4

Figure 5 – PC terminal program screen capture ........................................................... 4

Table 6 – Datalogger to RS-485 to TTL Converter Wiring ............................................ 5

Table 7 – 3810A Thermistor String to RS-485 to TTL Converter Wiring ....................... 5

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Figure 1 – A 3810A Addressable Thermistor String

Figure 2 – Close up of one of the Addressable Thermistor Nodes

1. Introduction

Thermistors are semiconductors, which behave as thermal resistors – that is, resistors with a high (usually negative) temperature coefficient of resistance. The thermistor beads are made from a mixture of metal oxides encased in epoxy or glass. The beads are small in size and extremely robust with a high degree of stability over a long life span. The accuracy and interchangeability of the thermistor bead used on the 3810A is +/-0.05°C, within the temperature range of 0C to 50C.

Model 3810A thermistor strings are made by attaching cable to the thermistor circuit boards at customer specified intervals. Then, this section of the string is encapsulated in a protective thermoplastic molding, creating a Thermistor Node. Another section of cable and a PVC termination assembly extends 8”/200mm from the last thermistor location.

The cable consists of 2 twisted pairs, each wrapped with aluminized Mylar foil, and a single drain wire. There are four functional conductors; power, ground, and the two differential RS-485 lines for communication. Because of the bused topology, single point failures do not affect the string as a whole. That said, a break anywhere in the cable will render all sensors below the break inoperable. A short-circuit between any of the four functional conductors will render all sensors in the string inoperable. The sensors are polled individually via industry standard Modbus RTU (Remote Terminal Unit) commands.

2. Installation

The thermistor string needs to be uncoiled and placed in the medium in which it will be installed. For installations in which the string will be in contact with soil, rock, etc, care should be taken to prevent damage to the cable and node. The PVC termination assembly at the end of the string is manufactured with a 1/4 - 20 x ½” deep threaded hole in it. This allows for a weight or string/rope to be attached for assisting with the installation. The maximum pulling force that can safely be applied to the string is 50 lbs.

2

2.1 Hardware Requirements:

Communications: RS-485, half-duplex Data Rate: 9600 baud Power: 5V to 16V DC @ 1mA / Node 2.2 TTL to RS-485 Converter:

In order to use 3810A strings with devices that don’t have an RS-485 interface, a TTL to RS-485 converter may work. For this setup to work, the logger must have a TTL UART available. Figure 3 shows Geokon TTL to RS-485 Converter.

Figure 3 – Geokon TTL to RS-485 Converter

3810A String RS-485 to TTL Converter Description

Red 12V (OUT) Thermistor String Power White 485+ Communication RS-485+ Green 485- Communication RS-485- Black GND Ground Shield Shield Analog Ground

Table 1 – TTL to RS485 Wiring Chart

2.3 USB to RS-485 Converter:

If a PC is to be interfaced to 3810A strings, a USB to RS-485 converter will be needed. Geokon can provide a small, convenient USB dongle that has screw terminals for connecting to 3810A strings.

Figure 4 – USB to RS-485 Converter

Table 2 – USB to RS-485 Wiring Chart

Thermistor String Cable Conductor Color

USB to RS-485 Connector Label

Description

Red +5V 5 volt power to the string Green 485- Communication RS-485- White 485+ Communication RS-485+ Black GND Ground

3

3. Modbus RTU Protocol

3.1 Introduction to Modbus:

Model 3810A addressable thermistors use the industry-standard Modbus Remote Terminal Unit (RTU) protocol to communicate with data-loggers. As the name suggests, Modbus was designed to work on what is known as a bused network, meaning that every device receives every message that passes across the network. The Modbus standard does not specify a physical layer (connection type), but it will work with any interface that can communicate asynchronously with multiple devices (e.g., RS-485, RS-422, optical, radio, etc). Model 3810A strings use RS-485 as the electrical interface because of its prevalence, simplicity, and success as a robust, industrial physical layer.

3.2 Modbus RTU Overview:

The Modbus RTU protocol uses packets (messages made up of multiple sections) to communicate and transfer data between devices on the network. The general format of these packets is as follows:

1. Modbus Address (1 byte) – the address of the specific device on the bus 2. Function Code (1 byte) – the action to be carried out by the slave device 3. Data (multi-byte) – the payload of the function code being sent 4. CRC (2 bytes) – cyclic redundancy check; a 16-bit data integrity check calculated over

the other byes in the packet

3.3 Modbus Tables:

The most recent sensor reading is stored in a table. The reading is presented in different formats in different sections of the table. The register location and size of these variables are described in Table 3.

Storage Variable Type HEX

(Modbus Msg.) Decimal Description

RAM

ADC uint16 0x0100 256 Raw 16-bit

thermistor reading

IC Temp. uint16 0x0101 257 Low-quality 10-bit

temp. reading

Resistance float32 0x0102 258 Decimal thermistor

reading (Ohms) 0x0103 259

Table 3 – RAM Storage

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3.4 Reading 3810A Sensors:

While Modbus RTU supports roughly 20 different function codes, the simple functionality of a bused temperature sensor, such as the Geokon 3810A, eliminates the need for all but one of them. Specifically, the Read Holding Registers function code (0x03) is all that’s needed to get readings from sensors. The Read Holding Registers function will read some number of 16-bit registers (2 bytes each), starting at the Data Address that was sent in the payload. The readings can be retrieved in two different formats; one as a raw 16-bit ADC (analog to digital conversion) result, or, as a 32-bit floating point (decimal) number that is equal to the measured resistance of the onboard thermistor in Ohms. For ease of use and program simplicity, the 32-bit floating point reading is recommended. An example of this query and response is shown in the Table and Table .

Device

Address Function

Code Data

Address # of

Registers CRC

HEX16 02 03 0102 0002 6404 DEC10 2 3 258 2 25,604

Table 4 – Example Query - Sensor #2, floating point resistance reading

Device

Address Function

Code Byte

Count Lower 16-bits

Upper 16-bits

CRC

HEX16 02 03 04 C87C 4628 04F5 DEC10 2 3 4 floating point result 1,269

Table 5 – Example Response - Sensor #2, floating point resistance reading

Figure 5 – PC terminal program screen capture

Table shows the IEEE-754 floating point response as two parts—each one comprised of two bytes. Because of how this information is stored in memory, the two parts are received in reverse-order; the complete floating point number in HEX is 0x4628C87C (10,802.12 Ohms).

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4. Modbus and Campbell Scientific Dataloggers

4.1 Description:

Model 3810A sensor strings can be easily read using a CR1000, CR800, or CR6 Datalogger. CRBasic is the programming Language used with Campbell Scientific CRBasic Dataloggers. Campbell Scientific’s LoggerNet software is typically used when programming in CRBasic.

4.2 Wiring:

CR1000/CR800/CR6RS-485 to TTL

Converter Description

12V V IN (12V) Converter Power C1 TX Transmit C2 RX Receive G GND Ground

AG Shield Analog Ground

Table 6 – Datalogger to RS-485 to TTL Converter Wiring

3810A String RS-485 to TTL

Converter Description

Red V OUT Thermistor String Power White 485+ Communication RS-485+ Green 485- Communication RS-485- Black GND Ground Shield Shield Analog Ground

Table 7 – 3810A Thermistor String to RS-485 to TTL Converter Wiring

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4.3 Sample CR1000 Program:

The following Sample program reads 1 string with 5 addressable thermistors. The String in this example communicates with the CR1000 through the Control Ports C1 and C2, which are setup as COM1:

'Constants used in Steinhart-Hart equation to calculate sensor temperature for '10k thermistor Const A = 1.128706256E-3 Const B = 2.342327483E-4 Const C = 0.8707279757E-7 Public ErrorCode 'Result of ModBusMaster communications attempt Public Resistance As Float 'Resistance of thermistor must be stored as 'Type Float Public Celsius(5) 'Calculated Celsius for 5 sensors in string Public Count 'Counter to increment through temperature sensors 'Define Data Tables DataTable (Test, 1,-1) Sample (5,Celsius(),IEEE4) 'Sample Celsius for 5 sensors in string EndTable 'Main Program BeginProg 'Open COM port with TTL communications at 9600 baud rate SerialOpen (Com1,9600,16,0,50) 'Read all 5 sensors in string every 4 seconds Scan (4,Sec,0,0) 'Loop to read each thermistor For Count = 1 To 5 'Reset temporary storage for next reading Resistance = 0 'Use Modbus command to retrieve resistance from thermistor string ModBusMaster (ErrorCode,Com1,9600,Count,3,Resistance,&H103,1,1,50,0) 'Calculate thermistor temperature from ohms to Celsius using Steinhart-hart 'equation Celsius(Count) = 1/(A+B* LN(Resistance) + C * LN(Resistance)^3)-273.15 Next 'Call Table to store Data CallTable Test NextScan EndProg

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4.4 Sample CR6 Program:

The following Sample program reads 1 string with 5 addressable thermistors. The String in this example communicates with the CR6 through the Control Ports C1 and C2, which are setup as ComC1:

'Constants used in Steinhart-Hart equation to calculate sensor temperature for '10k thermistor Const A = 1.128706256E-3 Const B = 2.342327483E-4 Const C = 0.8707279757E-7 Public ErrorCode 'Result of ModBusMaster communications attempt Public Res As Float 'Resistance of thermistor must be stored as 'Type Float Public Celsius(5) 'Calculated Celsius for 5 sensors in string Public Count 'Counter to increment through temperature sensors 'Define Data Tables DataTable (Test, 1,-1) Sample (5,Celsius(),IEEE4) 'Sample Celsius for 5 sensors in string EndTable 'Main Program BeginProg 'Open COM port with TTL communications at 9600 baud rate SerialOpen (ComC1,9600,16,0,50,3) 'Read all 5 sensors in string every 4 seconds Scan (4,Sec,0,0) 'Loop to read each thermistor For Count = 1 To 5 'Reset temporary storage for next reading Res = 0 'Use Modbus command to retrieve resistance from thermistor string ModbusMaster (ErrorCode,ComC1,9600,Count,3,Res,&H103,1,1,50,0) 'Calculate thermistor temperature from ohms to Celsius using Steinhart-hart 'equation Celsius(Count) = 1/(A+B* LN(Res) + C * LN(Res)^3)-273.15 Next 'Call Table to store Data CallTable Test NextScan EndProg

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5. Temperature Calculation

Thermistors can be read using a Geokon Micro 1000, Micro 800, or 8600 Datalogger, or any PC that has a USB port and Geokon’s free 3810A Utility program.

5.1 Steinhart-Hart Coefficients:

1.128706256 10

2.342327483 10

0.8707279757 10

5.2 Floating Point Resistance Readings (preferred method):

0 0102

273.15

5.3 Raw ADC (Analog to Digital Converter) Readings:

Resistance can be found by:

0 0100

Ω 10,000,

1

And temperature is:

273.15

Now, using an example ADC reading from address 0x0100 of 0x7984 (31,108):

Ω 10,000,

,1 ≅ , . Ω

, . , .

273.15 ≅ . °

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6. PC Application

A simple PC test application, Geokon 3810 A Master, is available for download at Geokon’s website by navigating to http://www.geokon.com/Software and clicking on “3810A Addressable Thermistor String Software”. Or the program may be downloaded directly by clicking on the following link:

http://www.geokon.com/content/software/3810A_Setup.zip

6.1 System Requirements:

Operating System: Windows 7 & Windows 10, 32 or 64-Bit

Disk Space: 2MB (excluding logged data)

Hardware: 1 USB Port, USB to RS-485 Converter, and a 3810A string

6.2 Installation:

Download the installer from the link and extract it. To do this, right-click and select “Extract All…” Now run the installer by double left-clicking on the icon. Follow the directions as the installer prompts you and click “Finish” when completed.

6.3 Running the Application:

NOTE: Before running the application, make sure you have a USB to RS-485 converter connecting the 3810A string to the PC.

To run the program, double left-click the icon that was created on the desktop:

Select the COM Port that your hardware is connected to:

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Enter an address range for the attached string. If the number of sensors and their addresses

are unknown, simply leave the range as “1-255”. You can then look at the responses from the

attempted readings and determine the correct address range.

If you want to sample the sensors continuously on an interval, check the “Continuous”

checkbox and enter the desired sample period into the field labelled “Interval”. The line

“Sample interval must be at least 5”, shown here, is automatically recalculated for different

address ranges (number of sensors).

11

Click on the start button to begin querying the range of sensors entered. This will start

populating the “Output” tab with responses from the entered address range.

Click on the “Output” tab to view live readings as they are taken. The window can be resized

(by clicking and dragging the border of the window) to show each sensor-column, or you can

use the scroll-bar on the bottom.

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If you want to save your collected data as a comma-delimited .txt file, which can be readily

imported into Excel for analysis, simply click the “Save Output” button in the upper-left corner

of the window. This will open a Windows “Save As” dialog, and you can then select a suitable

directory for your data and save it. The filename is a date-stamp in the format D_M_YYYY.txt.

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7. Reading Addressable Thermistor Strings with a Handheld PC

The Addressable Thermistor String Reader is made up of two components: The Readout Unit, consisting of a Windows Mobile handheld PC running the

Addressable Thermistor String Reader Application. The USB to RS-485 Converter (see figure 4) which plugs into the mini USB OTG (Host

& Client) connector on the hand-held device. This converter connects to the addressable Thermistor “string” to be measured.

For information regarding the use of a handheld PC with Addressable Thermistor strings please refer to the Addressable Thermistor String Reader manual available for download on the Geokon website or by clicking the following link: http://www.geokon.com/content/manuals/Addressable_Thermistor_String_Reader.pdf

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Appendix A – Specifications

A.1 Power

Power Supply: 5VDC to 15VDC

Current Per Sensor: 1mA (max), 0.55mA (typical)

Operating Temperature Range: -40°C to 70°C

A.2 Communication

Interface: RS-485, Half-duplex

Baud Rate: 9,600 bits/second

Query Response Time: 0.060 Seconds (max)

A.3 Measurements

ADC Resolution: 16-bit

System Resolution (0°C to 50°C): 0.0025°C (non-linear; largest step)

System Resolution (-40°C to 70°C): 0.0085°C (non-linear; largest step)

System Accuracy (0°C to 50°C): ±0.085°C

Thermistor Type: 10KΩ NTC

Thermistor Interchangeability: +/-0.05°C