Modbus Wiring; Occupancy Input - Smart temp SMT - 131 Manual

Modbus Wiring

Modbus communications unlock many additional functions within the SMT-131 thermostat. It
also permits all modes, functions and controls to be remotely supervised and adjusted from a
central location if needed. Modbus object data can be found on page 10 of this manual.
The SMT-131 uses a high powered Maxim chipset that will "theoretically" permit the connection
of up to 255 devices on a single Modbus node. RF noises and other interferences will reduce the
real world permitted connections to a maximum of 200 in most cases.
Modbus and other communication protocol rely on the quality of cable and good installation
practises to ensure reliable communications. Modbus requires the use of screen cable with the
drain grounded in a single location.
when using a common power supply or when using a modbus master that also powers the SMT-
131, ensure the R (24v active), C (24v common) Data A & B wires are not crossed.
Hint! When the SMT-131 is connected to a working Modbus Network and receiving valid data,
the decimal point in the temperature display will blink. If there is no communication or the SMT-
131 is receiving corrupted data, this decimal point will remain on steady.

Occupancy Input

When the room is detected "unoccupied" by a third party
device or the optional Smart Temp OC-3D occupancy
detection system, the SMT-131 will replace the user set point
with installer pre-set temperatures or alternatively, turn the
thermostat off. The user settings will return when the room is
again occupied.
The Digital Inputs in the SMT-131 are volt free. To initiate the
"unoccupied" function simply bridge the D+ and D- terminals
through a clean contact (switch or relay terminal).
DO NOT APPLY ANY EXTERNAL VOLTAGES TO THIS INPUT.
Modbus relies on a series connection and
will normally not tolerate "Star" wiring or
"T" branches.
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