Innotech Genesis II Digital Controller (v5) - Manual (Page 51)

Page 51
Genesis II Installation Instructions
© Mass Electronics Pty Ltd 2010 Chapter 3 – Electrical Installation
3-4.2 REM Comms Connections
The RS485 Comms link between the Genesis II Direct Digital Controller and REMs, or the MPCII Mid
Points Controller and REMs, is designed to allow reliable communications over long distances in
electrically adverse signal conditions.
The following guidelines, which are common to all REMs, should be followed when installing the
RS485 Comms network:
• Cables used for RS485 Comms must be shielded, single twisted pair, 120 Ohms characteristic
impedance with 36 to 45 picoFarads per metre capacitance between conductors.
• The RS485 Comms cable must be organised as a bus topology. See 5-3.3 for a description of bus
topology. To connect a module to the RS485 cable, the cable is cut at the point the module is
to be connected. Then, the two new cable ends are wired into the module. The two shields are
then terminated at terminals marked SHLD1 and SHLD2 respectively. “Stub” connections are not
allowed in the bus topology configuration.
• The cable run between the digital controller and any other REM must not exceed 600 metres in
length.
On most units, the RS485 Comms connection block contains four terminals. The four terminals on
the standard RS485 connection block are:
• S1 Screen of Cable Number 1
• + Positive Comms Line
• - Negative Comms Line
• S2 Screen of Cable Number 2
The four-terminal arrangement accommodates the use of one or two RS485 Comms cables. A
single cable connection is used when the module is located at the end of the network and two cable
connections are used when the REM is connected to a point in the cable network other than the end.
An End of Cable Jumper plug near the RS485 connection block provides the proper termination
impedance for the Comms line. If only single RS485 Comms cable is connected to the module, the
End of Cable Jumper must be installed. If two cables are fitted, the End of Cable Jumper must be
removed.
The RS485 Comms System will not function properly if the connection polarities are incorrect. Ensure correct
polarity of the connections throughout the length of the Comms Cable.
IMPORTANT
Figure 3-6 shows how to connect a two-cable Comms network to a module. If the module has only
one RS485 Comms cable connected to it, the cable screen must be connected to S1. If two Comms
cables are fitted, the screen of the second cable must be connected to the S2 terminal. Screens must
not be connected to each other.
CABLE #1 CABLE #2
S1 S2
+ _
Figure 3-6: RS485 Comms Cable Connections