Innotech Genesis V4 System - Manual (Page 46)

Installation Instructions for Innotech Genesis Systems
Edition 011100 3-20
3-4.2.2. RS485 Comms Connections. The RS485 Comms link between the RMI
and the other LEMs is designed to allow reliable communications over long distances in
electrically adverse signal conditions.
The following guidelines, which are common to the RMI and 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 Paragraph A-4.1 in
Appendix A 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 RMI and any other REM must not exceed 500 metres in
length.
On most units, the RS485 Comms connection block contains four terminals. The exception
is the MP module, which has three terminals and is explained in Paragraph 3-4.2.11. 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 RMI 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.
CAUTION
THE RS485 COMMS SYSTEM WILL NOT FUNCTION
PROPERLY IF THE CONNECTION POLARITIES ARE NOT
CORRECT.
ENSURE CORRECT POLARITY OF THE
CONNECTIONS THROUGHOUT THE LENGTH OF THE COMMS
CABLE.