Smart Temp SMT - 131 User Manual - page 12
Page 12
or guest’s needs by text information displayed on the door station LCD or the door station
backlight colours.
Connection between the SMT-131 and the hotel door station is cable fitted with RJ12 plugs.
Cables can be provided at additional cost at up to 15 metres maximum. Alternatively custom
cables can be produced. They are direct wired (not cross over).
When fitting the door station, the SMT-131 DIP Sw8 must be ON. The outside door station will
flash all LCD segments if DIP Sw8 is in the wrong position.
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 13 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 150 ~ 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.
The Modbus data is provided in Addendum 1 at the back of this manual. (page 16).
Modbus relies on a series connection and
will normally not tolerate “Star” wiring or
“T” branches.