Thermo Scientific AquaSensors™ DataStick™ Modbus ...

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
Thermo Scientific AquaSensors DataStick Modbus Communications User Guide 4