Thermo Scientific AquaSensors™ DataStick™ Modbus ...

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
Thermo Scientific AquaSensors DataStick Modbus Communications User Guide 20