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The communications port of the SMT‐770 has 3 terminals used for communication.
"A", "B" & "T". Terminals "A+" & "B‐" are used for data communication, the terminal
"T" ( a shared terminal from the thermostat T T input)
is used as a Vss (screen ground to protect the integrity
of the communication signal).
Figure 18
A maximum of 32 SMT‐770s can be connected to any single hub.
Each SMT‐770 on the hub must have a unique network address (factory default is 7).
These settings are adjustable from the advanced installer menu. See page 19 for more
detail on setting the communications address.
A typical ModBus wiring example is given below. If using a common power supply to
power all thermostats on a network, it is highly recommended that all thermostats
power is wired in phase, i.e. "R" to "R" and "C" to "C" and NOT crossed.
Figure 19
In many cases where multiple thermostats are used in a single network or on a long
network run the two DIP switched located between the "R" & "R/Com" terminals on
the thermostat PCB should be switched on to improve network reliability.
39 | P a g e
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