Engineering Documentation

Chapter 2 – Network Electrical Systems
RS-485 MS/TP Communications
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Using Cimetrics Routers on an APOGEE BACnet MS/TP
Network
NOTE
Cimetrics routers may only be used for non-smoke control applications.
Although Cimetrics BACnet routers are not the preferred solution, they may be used
on an APOGEE BACnet MS/TP network. In order for Cimetrics routers to work
properly, they must be wired as shown in the following illustration.
● Only one router is allowed per isolated network section and it must be an end
device.
– Limiting each isolated network section to one router and using 1.5-pair cable
with the reference connection near the router minimizes the voltage difference
between the two ground references.
– The limitation of one router per network section is due to the type of
environment in which the controllers are normally installed. Very few APOGEE
network installations can be considered electrically quiet. For example, a
small-sized office environment may be electrically quiet.
● The Cimetrics router’s RS-485 circuitry is not earth grounded unless the paint on
the chassis is removed and the chassis is then connected to earth.
– In addition, an internal “Z” jumper must be removed to help ensure that the
RS-485 circuit is isolated from earth.
– In order to keep the Cimetrics router an isolated device, do not tie the chassis
to earth.
● If Polycool devices are used on the network, do not enable the line termination
feature.
● 1.5-pair cable is highly recommended. Using 1 TSP cable reduces noise immunity.
● The following must be done if single-pair cable is used:
– The network must be terminated with 120 ohm resistors (550-975P100).
– Do not tie the shield to the third terminal on the network plug. Instead, use a
wire-nut to bypass the shield and make a continuous shield connection as
shown in the following figure.
Fig. 12: Using Cimetrics Routers on an APOGEE BACnet MS/TP Network.