Thermo Scientific AquaSensors™ DataStick™ Modbus ...

Thermo Scientific AquaSensors 

DataStick Modbus 

Communications 

User Guide

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This publication supersedes all previous publications on this subject.

Table of Contents 

1 Introduction.................................................................................................................................. 2 

2 Hardware Setup ........................................................................................................................... 3 

3 Communications Parameter Configuration via Fieldbus........................................................... 6 

3.1 Default Communications Parameters...................................................................................... 6 

3.2 Modbus Slave Address ........................................................................................................... 6 

3.3 Modbus Baud Rate ................................................................................................................. 6 

3.4 Modbus Parity ........................................................................................................................ 7 

3.5 Restoring Default Slave Address, Baud Rate, and Parity......................................................... 8 

4 Function Codes............................................................................................................................. 9 

5 Register Numbers and Byte/Word Order ................................................................................. 10 

5.1 Variable Classes ................................................................................................................... 11 

5.1.1 Class Process Value...................................................................................................... 11 

5.1.2 Class Scale Factor......................................................................................................... 11 

5.1.3 Class Configuration Value ............................................................................................ 12 

5.1.4 Class Information ......................................................................................................... 12 

5.1.5 Class Calibration........................................................................................................... 12 

5.1.6 Class Status .................................................................................................................. 12 

6 Modbus Register Map................................................................................................................ 14 

6.1 Embedded in an AV38 ......................................................................................................... 20 

7 References................................................................................................................................... 20 

8 Limited Warranty...................................................................................................................... 21 

9 Terms and Conditions................................................................................................................ 22 

Contact Information 

To contact Thermo Scientific AquaSensors Technical Support: 

Within the United States call 1.800.225.1480 or fax 978-232-6015. 

Outside the United States call 978.232.6000 or fax 978.232.6031. 

In Europe, the Middle East and Africa, contact your local authorized dealer. 

Visit us on the web at www.thermo.com/processwater 

Modbus Adapter Part Numbers: 

• CA12B: 316 Stainless Steel Housing 

• CA22B: CPVC Housing 

• CA32B: PEEK® Housing 

• CA42B: Ryton® Housing 

Thermo Scientific AquaSensors DataStick Modbus Communications User Guide 1

1 Introduction 

The Thermo Scientific AquaSensors Modbus Adapter is a gateway between any DataStick measurement 

system and a Modbus master. It regulates external power and provides full-featured interactive 

measurement, calibration, configuration, and diagnostic features without the need for an intermediate 

analyzer or controller. 

The Adapter register map can be used for direct access to measure, calibrate, configure, and diagnose 

information, even when the sensor head is changed from one type of analytical measurement to another 

with power applied. The registers support both floating-point and integer variables. Some status 

information is passed as an ASCII string. Open protocol ASCII commands and responses are also 

supported for applications that require direct control of message traffic. 

The Modbus Adapter must be configured with the correct Baud rate, parity, and station address. This 

can be done at the factory or in the field. The configured station address, Baud rate, and parity stay with 

the Adapter even when the DataStick is removed from the system. 

Table 1: Register Map Overview 

Start End Description 

40001 40016 ASCII Command up to 32 bytes is directly passed to the DataStick. 

40017 40032 

40033 40048 ASCII Response up to 32 bytes in response to an ASCII command. 

40049 41000 

41001 41006 Operational status registers. Integer. 

41007 42000 

42001 42004 Read process values. Floating point. 

42005 42010 Write sample calibration values. Floating point. 

42011 42022 

42023 42032 Write configuration data. Floating point. 

42033 43000 

43001 43027 Read/Write Measure, Calibration and Configuration data. Integers. 

43028 44000 

44001 44005 Read Scale Factors. Integers. 

44006 45000 

45001 45020 Read/Write Sensor Information as ASCII. 

45021 45024 Read/Write Sensor Information as Integer. 

The DataStick automatically supports multiple measurement types and, if all configuration and 

calibration registers are used, all sensor heads can be automatically supported. 

Refer to the DataStick Manual for detailed information on installation, maintenance and operation. 


2 Hardware Setup 

The DataStick consists of three parts that are assembled at the factory: 

1. Plug-in Modbus Adapter 

2. Universal Sensor Module 

3. Plug-in Sensor Head 

The following is excerpted from “Ten Ways to Bulletproof RS-485 Interfaces”, National Semiconductor 

Application Note 1057: 

The optimal configuration for the RS-485 bus is the daisy-chain connection from node 1 to 

node 2 to node 3 to node n. The bus must form a single continuous path, and the nodes in the 

middle of the bus must not be at the ends of long branches, spokes, or stubs. Connecting a 

node to the cable creates a stub, and, therefore, every node has a stub. Minimizing the stub 

length minimizes transmission-line problems. A good rule is to make the stubs as short as 

possible. 

There are several options for terminating an RS-485 bus. The first option is no termination. 

This option is feasible if the cable is short and if the data rate is low. Reflections occur, but 

they settle after about three round-trip delays. For a short-cable, the round-trip delay is 

short and, if the data rate is low, the unit interval is long. Under these conditions, the 

reflections settle out before sampling, which occurs at the middle of the bit interval. 

The most popular termination option is to connect a single resistor across the conductor pair 

at each end. The resistor value matches the cable’s differential-mode characteristic 

impedance. If the bus is terminated in this way, no reflections occur, and the signal fidelity is 

excellent. The problem with this termination option is the power dissipated in the 

termination resistors. 

If power dissipation must be minimized, an RC termination may be the solution. In place of 

the single resistor, a resistor in series with a capacitor is used. The capacitor appears as a 

short circuit during transitions, and the resistor terminates the line. Once the capacitor 

charges, it blocks the DC loop current and presents a light load to the driver. Low-pass 

effects limit the use of the RC termination to lower data-rate applications, however. 


The characteristic impedance of the cable supplied with the Modbus Communications Adapter is 120 . 

A Modbus Communications Adapter does not have an integral termination resistor, so the stub created 

by its integral cable must be kept as short as possible. This can be accomplished by locating the junction 

box as close as possible to the Modbus Communications Adapter. 

For applications involving a single Modbus Communications Adapter, a termination resistor should be 

placed across the bus at the master and also across the bus in a junction box located as close as possible 

to the Modbus Communications Adapter. 

For applications involving multiple Modbus Communications Adapters, a termination resistor should be 

placed across the bus at each end. 

The standard cable length for a Modbus Adapter is 10 feet, and it comes with wires that are stripped and 

tinned for use with terminal blocks. A water-resistant label is placed at the end of the cable that 

provides a chart of wire colors and functions for use with Modbus running on RS-485 (Part number 

CA22B) or RS-232 (Part number CA22A). Be sure the correct model is being used. 

Any Class II power supply that provides 10 to 30 volts may be used to power the DataStick 

measurement system. The power ground and the signal ground MUST be connected together at the 

power source for the Modbus Adapter to operate correctly. 

The shield wire must be connected to ensure proper electromagnetic immunity from other electrical 

equipment. 

Table 2: Modbus Wiring 

RS-485 RS-232 Wire Color 

Hardware Hardware 

10 to 30 VDC 10 to 30 VDC RED 

Signal Ground Signal Ground BLACK 

Comms (–) Comms (Rx) BLUE 

Comms (+) Comms (Tx) WHITE 

Earth Earth SHIELD 


The Modbus Adapter can be removed and replaced in the field. This may be desirable for any number 

of reasons, some of which include: 

• Diagnostics: Temporarily plug in a USB adapter for PC diagnostics 

• Feature Upgrade: Temporarily plug in a programming adapter to load new code 

• Repair: Replace a damaged DataStick assembly without rewiring 

• Change Measurement: Quick sensor swap 

The Modbus Adapter is keyed and plugs into the end of the DataStick Universal Sensor Module that is 

marked “Communications Adapter”. 

Insert the Modbus Adapter until it bottoms out. Rotate the Adapter until it engages with the connector. 

Push the adapter in gently, and then tighten the retaining ring with a 15/16-inch wrench. It is very 

important to tighten the retaining ring to ensure a reliable connection. 

Retaining ring. Rotate clockwise to 

tighten. Use a 15/16-inch wrench. 

Potted Serial Number and 

Part Number. 

O-Ring seal should be free of 

dirt when inserted into the 

DataStick. 


3 Communications Parameter Configuration via Fieldbus 

Communications parameter configuration involves setting the slave address, Baud rate, and parity of the 

Modbus Adapter’s fieldbus port using a freely available tool such as ModScan32 from WinTECH 

Software Design (http://www.win-tech.com/html/modscan32.htm). 

NOTE: New communications settings for slave address, Baud rate, and parity do not take effect 

until power to the Modbus Adapter is cycled! 

3.1 Default Communications Parameters 

The default communications parameters are: 

• Slave address: 1 

• Baud Rate: 19.2 kbps 

• Data Bits: 8 

• Stop Bits: 1 

• Parity: Odd 

These parameters are set at the factory unless custom parameters have been requested. If the parameters 

are customized, the Modbus Adapter will be accompanied by a paper tag that indicates the custom 

parameters. 

3.2 Modbus Slave Address 

Register Address: 43022 

Default Value: 1 

Supported Slave Values: 1 to 247 

Units: N/A 

This slave address specifies the station number of the DataStick on the Modbus network. It must be a 

unique station number on the network and can be any value from 1 to 247. Read register 43022 to 

obtain the current station number. Write to the same register to change it. 

3.3 Modbus Baud Rate 


Default Value: 0 (19.2 kbps) 

Supported Values: 1.2 kbps to 230.4 kbps 

Units: kbps (kilobits per second) 

The Baud rate specifies the speed at which data is transferred between the Modbus master and the 

Modbus adapter. Read register 43023 to obtain the current Baud rate. Write to the same register to 

change it using the integer value in the following table. 


Table 3: Supported Baud Rates 

Baud Rate Integer Value 

19.2 kbps (Default) 0 

1.2 kbps 1 

2.4 kbps 2 

4.8 kbps 3 

9.6 kbps 4 

14.4 kbps 5 

19.2 kbps 6 

28.8 kbps 7 

38.4 kbps 8 

57.6 kbps 9 

76.8 kbps 10 

115.2 kbps 11 

230.4 kbps 12 

3.4 Modbus Parity 


Default Value: 0 (Odd) 

Supported Values: Odd, None and Even. 

Units: N/A 

The parity specifies the parity between the Modbus master and the Modbus Adapter. Read register 

43024 to obtain the current parity. Write to the same register to change it using the integer values in the 

following table. 

Table 4: Parity 

Parity Integer Value 

Odd (Default) 0 

None 1 

Odd 2 

Even 3 


3.5 Restoring Default Slave Address, Baud Rate, and Parity 

In the event that the slave address, Baud rate, and parity of a Modbus Communications Adapter is 

unknown, its communications settings can be restored to factory default values by following the 

procedure below: 

Pin 1 

Pin 2 

Pin 5 Pin 9 

Figure 1: The pin numbering of the DataStick-end of the Adapter. 

1. Disconnect the Adapter from the network. Attention: Determine how your control system 

will respond before disconnecting an Adapter from the network. 

2. Detach the Adapter from the DataStick. 

3. Make an electrical connection between pins 5 and 9 on the DataStick-side of the Adapter using a 

clip lead or equivalent as shown in Figure 1. Be careful to make the connection only between 

pins 5 and 9. 

4. Apply power to the Adapter and wait at least 10 seconds. 

5. Disconnect the power from the Adapter and remove the electrical connection between pins 5 and 

9. Re-attach the Adapter to the DataStick. 

6. Establish communications with the Adapter using a protocol of Modbus RTU, a slave address of 

1, a Baud rate of 19.2 kbps, and a parity of odd. 


4 Function Codes 

The Modbus Adapter supports the Remote Terminal Unit (RTU) mode of serial transmission. Further, it 

supports the Modbus Function Codes shown in Table 5. 

Table 5: Supported Modbus Function Codes 

Code Name 

03 Read Holding Registers 

06 Preset Single Register 

16 Preset Multiple Registers 

Modbus Function Codes are defined in the Modbus Application Protocol Specification, V1.1. If the 

function code contained in a query is not supported, the Modbus Adapter will respond with an Exception 

Code of 01 indicating an illegal function. 


5 Register Numbers and Byte/Word Order 

According to the Modicon Compact 984 Ladder Logic Manual, 043503387, the I/O Reference 

Numbering System defines reference number 4XXXX as “an output or holding register. A 4X register 

may be used to store numerical data (binary or decimal) in state RAM or to send the data from the CPU 

to an output unit in the control system.” For this reason, the register map for the Modbus Adapter is 

defined to begin at 40,001. The Modbus Adapter is the CPU in the above statement, and the customer’s 

Modbus master is the output unit in the control system. 

As defined, some registers are read/write; some are read-only, and some are write-only. If a write-only 

register is read by the Modbus master, the register’s value will be 0. 

The defined registers are grouped into banks so that all registers in a bank can be read with a single 

Modbus read request. If the register number received in a query is outside the range of the defined 

banks, the Modbus Adapter will respond with an Exception Code of 02 indicating an illegal data 

address. The legality of some register numbers depends on the type of Sensor Head installed in the 

DataStick. For example, the Cell Constant register is legal only if a Contacting Conductivity or 

Electrodeless Conductivity Sensor Head is installed in the DataStick. DataStick commands and 

responses are defined in Section 7 of this document. If the register number received in a query is 

invalid, the Modbus Adapter will respond with a value of 0. It is the responsibility of the PLC 

programmer to know which register numbers are valid based on the Sensor Type. 

For consistency, all values of data type float are aligned at even-numbered registers. 

Values of type float are represented in 32-bit IEEE 754 floating-point format. This requires two 

registers per value. By default, the lower register number contains the upper 16 bits and the higher 

register number contains the lower 16 bits as shown in Table 6. This is known as “Big-Endian” word 

order. Floats can be made Little-Endian in their word order by changing the value of the PLC Data 

Type accordingly as shown in Table 7. The byte order can also be affected by PLC Data Type. For 

compatibility with Thermo Scientific AquaSensors products such as AquaComm and the AV38, set PLC 

Data Type to Big Endian (2). 

Table 6: Word order of values of type float 

42001 

42002 

Upper 16 bits 

Lower 16 bits 

Sensor Value (float) 

FPConvert can be used to determine hexadecimal representations of floating-point values (and vice 

versa). This utility can be obtained from http://www.61131.com. 

Table 7: Defines the valid values for PLC Data Type. 

Value Type 

0 Little Endian – ControlLogix 

1 (default) Little Endian Word Swap – SLC 

2 Big Endian 

3 Big Endian Word Swap 


The Modbus adapter responds with an exception if both registers of a float data type are not read or 

written in the same request. This will reduce the likelihood of data corruption within the Modbus 

Master. 

There exist three banks of registers that deserve further discussion. They are banks 42XXX, 43XXX, 

and 44XXX. Register bank 42XXX contains Process Value, Calibration, and Configuration variables of 

type float. Register bank 43XXX contains the same Process Value, Calibration, and Configuration 

variables, but of type integer. Register bank 44XXX contains the corresponding Scale Factors for the 

integer variables in bank 43XXX. The order of the variables in banks 43XXX and 44XXX is identical 

so that a Modbus master can access the Scale Factor for an integer Process Value simply by adding 

1,000 to the integer Process Value register number. 

5.1 Variable Classes 

The class of each variable is shown in the register map in Section 6 of this manual. Please note that 

there is not a one-to-one correspondence between variables and registers. This is because variables of 

type float each occupy two registers. 

To make Modbus master programming easier, variables of each class are grouped together in the register 

map in banks. Each bank is 1,000 registers in size, but the number of variables defined in each bank is 

restricted to fewer than 100 so they can all be read/written by a single Modbus read/write request. 

5.1.1 Class Process Value 

This class of variable represents the current value of the process that is being measured by the DataStick. 

For example, a pH sensor installed at a wastewater treatment plant might measure the pH of the treated 

water leaving the plant. The Sensor Value may be monitored continuously by the plant operator and 

may be recorded to demonstrate compliance with applicable EPA regulations. 

This class of variable originates in the DataStick and exists in two types; float and integer. The former 

is for Modbus masters that support floating-point math, and the latter is for Modbus masters that do not. 

5.1.2 Class Scale Factor 

Variables of this class are of type integer. 

5.1.2.1 Integer representation of Floating Point numbers 

Not all PLC’s support floating-point math. For this reason all values of type float are also available as 

type integer. 

Integer data types include a corresponding Scale Factor that defines the relationship between the integer 

type of the value and the float type. 

The Modbus Adapter calculates the integer type of the value from the float type by multiplying the float 

type by the scale factor. For example, for a Sensor Type of pH, the Scale Factor for the Sensor Value is 

100. If the float Sensor Value is 7.04, then the integer Sensor Value is 7.04 x 100 = 704. 


5.1.2.2 Scale Factors 

Most Scale Factors are fixed, e.g., the Cell Constant Scale Factor is fixed at 1000, but some depend on 

the type of Sensor Head installed in the DataStick. The Sensor Value Scale Factor is one of these. For 

example, when the installed Sensor Head is pH, the Sensor Value Scale Factor is 100, but when the 

installed Sensor Head is ORP, the Sensor Value Scale Factor is 1. 

5.1.3 Class Configuration Value 

This class of variable represents a value that configures some aspect of the operation of the DataStick, 

e.g., Sensor Units, or the Modbus Adapter. Variables of this class are static, non-volatile, and are stored 

in the DataStick or the Modbus Adapter. 

This class of variable exists in two types: integer and float. 

5.1.4 Class Information 

This class of variable is used for informational purposes only. Variables of this class are static, nonvolatile, 

and are stored in the DataStick, e.g., Main Serial Number. 

This class of variable exists in two types: integer and ASCII. All of them have been grouped into a 

single register bank. 

5.1.5 Class Calibration 

This class of variable instructs the DataStick to perform the action of calibrating itself. All calibrations 

involve a multi-step procedure, and one of the steps involves writing a calibration variable. 

5.1.6 Class Status 

All variables of class Status have been grouped into a single register bank. 

5.1.6.1 Originating in the DataStick 

This class of variable is used for indicating the status of the DataStick, e.g., Calibration Status. 

However, variables of this class are not static. 

Variables of this class are of type integer. 

5.1.6.2 Originating in the Modbus Adapter 

There is a unique Status variable that originates in the Modbus Adapter itself called Communications 

Status. The values that this variable can take on are defined in Table 8. On power-up, when 

communication has started but before the Modbus Adapter has read the Sensor and Temperature Values 

from the DataStick, Communications Status is Okay; while the DataStick is absent, Communications 

Status is “Communications error”; and, while the Modbus Adapter is communicating with the 

DataStick, Communications Status is “Data is valid”. 

This variable is of type integer. 


Table 8: Communications Status variable values and their meaning 

Value Meaning 

0 Okay 

1 Communications error 

2 Data is valid 


6 Modbus Register Map 

Table 9: Modbus Adapter Register Map Definition 

Register 

Number 

Description Data 

Type 

Access DataStick 

Command 

Example Variable 

Class 

40001 Command01 ASCII[2] W see Example column "GS" N/A 

40002 Command02 ASCII[2] W see Example column "NS" N/A 

40003 Command03 ASCII[2] W see Example column "R" N/A 

40004 Command04 ASCII[2] W see Example column N/A 













40017 Reserved01 
















40033 Response01 ASCII[2] R N/A "7." N/A 

40034 Response02 ASCII[2] R N/A "04" N/A 

40035 Response03 ASCII[2] R N/A N/A 






Register 

Number 


Type 


Command 


Class 

























40064 

… 

Reserved32 

41001 Communications 

Status 

integer R N/A Status 

41002 Calibration Status integer R CALSTATUS 1 Status 

41003 Sensor Memory 

Status 

integer R GSTATUS 1 Status 

41004 Configuration 

Memory Status 


41005 Calibration Memory 

Status 


41006 

… 

Run Status integer R GSTATUS 1 Status 

42001 Sensor Value float R GSNSR 7.04 Process 

Value 

42002 " 

42003 Temperature Value float R GTEMP 25.1 Process 

Value 


Register 

Number 


Type 

42004 " 

42005 Cal Sensor 1-Point 

Sample Value 

42006 " 


Sample Value 

42008 " 

42009 Cal Temperature 1- 

Point Sample Value 


Command 


Class 

float W CALS1PS 7.02 Calibration 

float W CALS2PS 10.04 Calibration 

float W CALST1PS 25.3 Calibration 

42010 

42011 

42012 

42013 

42014 

42015 

42016 

42017 

42018 

42019 

42020 

42021 

42022 

" 

42023 DO Salinity float R/W GSALT/SSALT 232.1 Configuration 

42024 " 

42025 DO Pressure float R/W GPRESS/SPRESS 764.2 Configuration 

42026 " 

42027 Conductivity 

Reference 

float R/W GCRTEMP/SCRTEMP 25.1 Configuration 

Temperature 

42028 " 


Compensation 

Slope 

float R/W GCCSLOPE/SCCSLOPE 2.01 Configuration 

42030 " 

42031 Cell Constant float R/W GCELL/SCELL 1.001 Configuration 

42032 " 

42033 

42034 

42035 

42036 

42037 

42038 

42039 

42040 


Register 

Number 

42041 

42042 

42043 

42044 

… 


Type 


Command 


Class 

43001 Sensor Value integer R GSNSR 704 Process 

Value 

43002 Temperature Value integer R GTEMP 251 Process 

Value 


Sample Value 

integer W CALS1PS 702 Calibration 


Sample Value 

integer W CALS2PS 0 1004 Calibration 

43005 Cal Temperature 1- 


integer W CALST1PS 253 Calibration 


Buffer 

integer W CALS1PB N/A Calibration 


Buffer 

integer W CALS2PB 0 Calibration 

43008 Cal Sensor 2-Point integer R/W N/A 0 Calibration 

Sample Point 

43009 Cal Sensor in Air integer W CALSAIR N/A Calibration 

43010 Cal Sensor Zero integer W CALSZERO N/A Calibration 

43011 Cal Abort integer W CALABORT N/A Calibration 

43012 DO Salinity integer R/W GSALT/SSALT 2321 Configuration 

43013 DO Pressure integer R/W GPRESS/SPRESS 7642 Configuration 

43014 Conductivity Ref integer R/W GCRTEMP/SCRTEMP 251 Configuration 

Temperature 


Compensation 

Slope 

integer R/W GCCSLOPE/SCCSLOPE 201 Configuration 

43016 Cell Constant integer R/W GCELL/SCELL 1001 Configuration 

43017 Sensor Units integer R/W GSUNITS/SSUNITS 0 Configuration 

43018 Sensor Filter integer R/W GSFIL/SSFIL 1 Configuration 

43019 Temperature Units integer R/W GTUNITS/STUNITS 0 Configuration 

43020 Temperature Filter integer R/W GTFIL/STFIL 1 Configuration 

43021 pH Buffer Type integer R/W GPHBUF/SPHBUF 0 Configuration 

43022 Slave Address integer R/W N/A 1 Configuration 

43023 Baud Rate integer R/W N/A 6 Configuration 

43024 Parity integer R/W N/A 2 Configuration 

43025 PLC Data Type integer R/W N/A 1 Configuration 

43026 Timeout integer R/W N/A 50 Configuration 

43027 Factor integer R/W N/A 0 Configuration 


Register 

Number 

… 


Type 


Command 


Class 

44001 Sensor Value S.F. integer R N/A 100 Scale Factor 

44002 Temperature Value 

S.F. 

integer R N/A 10 Scale Factor 


Sample Value S.F. 



Sample Value S.F. 


44005 

44006 

44007 

44008 

44009 

44010 

44011 

Cal Temperature 1- 


S.F. 


44012 DO Salinity S.F. integer R N/A 10 Scale Factor 

44013 DO Pressure S.F. integer R N/A 10 Scale Factor 


Reference 

Temperature S.F. 



Compensation 

Slope S.F. 


44016 

44017 

44018 

44019 

44020 

44021 

44022 

44023 

44024 

44025 

44026 

44027 

… 

Cell Constant S.F. integer R N/A 1000 Scale Factor 

45001 Main Serial 

Number01 

ASCII[2] R/W GMSNO/SMSNO "00" Information 


Number02 

ASCII[2] R/W " "00" Information 

45003 Main Serial ASCII[2] R/W " "03" Information 


Register 

Number 


Type 


Command 


Class 

45004 

Number03 

Main Serial 

Number04 

ASCII[2] R/W " "79" Information 


Number05 

ASCII[2] R/W " Information 


Number06 



Number07 



Number08 



Number09 



Number10 



Number11 



Number12 



Number13 



Number14 



Number15 



Number16 


45017 DataStick Code 

Version01 

ASCII[2] R GCVSN "D2" Information 


Version02 

ASCII[2] R " ".5" Information 


Version03 

ASCII[2] R " "1" Information 


Version04 

ASCII[2] R " Information 

45021 Node Address integer R/W GADDR/SADDR 321 Information 

45022 Sensor Type integer R GSTYPE 1 Information 

45023 Calibration Type integer R CALSTATUS 4 Information 

45024 Comms Adapter 

Code Version 

… 

6001 1 DataStick Slave 

Address 

integer R N/A 0x0303 Information 

Integer R/W GDSA/SDSA 2 AV38 

Configuration 

1 See Section 6.1, “Embedded in an AV38,” for more information about register number 6001. 


Register Map Notes: 

1. If no DataStick is connected to the Modbus Adapter, Sensor Value (float) and Temperature Value 

(float) will read “Not a Number” (NaN). NaN = FFFF FFFFH. 

2. The Main Serial Number must be read/written with a single read/write command. 

3. DataStick Code Version must be read with a single read command. 

4. Some values provided by the DataStick can exceed a register’s ability to represent them. In such 

cases, the value is limited to 32,767 or -32,768 as appropriate. For example, if the Sensor Type is 

pH and the Sensor Units are mV, the Sensor Value can exceed 400.00 mV. At a resolution of 0.01 

mV the corresponding integer value would be 40,000 which would exceed the register’s ability to 

represent it. In this case the resolution is decreased to 0.1 mV, and the integer value becomes 4,000 

which is representable. 

5. When calibrating sensors, use the primary unit of measure. 

6.1 Embedded in an AV38 

When the Modbus option is present in the AV38 Local Display, register 6001 is available and can be 

used to change the address of the DataStick with which the AV38 is communicating. In this way, a PLC 

on the host-side of the AV38 can access a DataStick that’s connected to the other side of the AV38. 

For example, to communicate with the DataStick whose station address is 3, the PLC would write 3 to 

register number 6001 in the AV38. Then, when the PLC reads registers 2001:2002, the Sensor Value of 

the DataStick whose station address is 3 would be returned. The range of acceptable values is 1-247. 

In the case of the AquaChlor Free Chlorine Measurement System, the range of acceptable values is 1- 

2. This is because one channel on the AV38 is dedicated to the Chlorine DataStick and the other is 

dedicated to the pH DataStick and no other DataSticks are on the network. 

7 References 

1. Modicon Compact 984 Ladder Logic Manual, 043503387, 

http://www.telemecanique.com/C1256FDB0057F465/all/C1256EDE0038AFB2C12570C200338EE 

2/$File/43503387_eng.pdf 

2. IEEE Standard for Binary Floating-Point Arithmetic, IEEE Std 754-1985, 

http://shop.ieee.org/ieeestore/Product.aspx?product_no=SH10116 

3. Modbus Application Protocol Specification, V1.1, http://www.modbus.org 

4. DataStick Measurement System Instruction Manual, Catalog Number MAN011DS 

5. TIA-485-A (March 1998) Electrical Characteristics of Generators and Receivers for Use in Balanced 

Digital Multipoint Systems, http://www.tiaonline.org 

6. Ten Ways to Bulletproof RS-485 Interfaces, National Semiconductor, Application Note 1057, John 

Goldie, October 1996 

7. FAILSAFE Biasing of Differential Buses, National Semiconductor, Application Note 847, John 

Goldie, July 1992 


8 Limited Warranty 

WARRANTY/REPLACEMENT PLAN 

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In the event that a defect is discovered during the warranty period, Thermo Fisher Scientific agrees, at 

its option, to repair or replace the defective product. Any product repaired or replaced under this 

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limited to, chemical reagents and salt bridges. 

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Thermo Fisher Scientific reserves the right to change or modify this warranty at any time. 


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© 2009 Thermo Fisher Scientific Inc. All rights reserved. 

258481-001 Rev. A 01-09

Thermo Scientific AquaSensors™ DataStick™ Modbus ...

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