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1.5.2
Use of Signal Ground (SG)
The signal ground (SG) provides a common mode signal reference for the communicating devices. Each
device must connect its SG to the cable shield drain wire. Failure to use the SG connection may cause
communication errors. If the environment is known to be noisy, an additional wire may be used for the
signal ground. The shield can then be grounded at one end only (to prevent ground loops) as a signal ground.
1.5.3
Device to Device Connection
Communication cables for RS-485 should be laid out in a daisy chain. Long stubs (T connection) should be
avoided because they create discontinuities and degrade signals. DO NOT connect devices in STAR
configuration unless using the LNL-8000 Star Multiplexer. STAR connection creates long stubs and causes
difficulty in cable termination.
1.5.4
Cable Termination
RS-485 communications is designed for higher data transmission speeds and also simplifies installation by
allowing each device to be multi-dropped from a single communication line or bus. With the increase data
speeds and transmitting and receiving the data over a single communications line, there is higher risk of
external noise. External noise could be in the form of line impedance, line ringing, or RF interference. When
using the specified communications cabling the risk of noise is all but eliminated. To ensure that the data is
sent and received without error, some End-of-Line termination of the RS-485 bus may be required.
•
RS-485 Cable termination from Host to Controller The device used to convert RS-232
communication to RS-485 determines the termination necessary for this segment of the RS-485
communication bus. These communications devices, pre-bias the RS-485 signal, which marks the state
of the signal being sent and allows the line to flow for reliable communications. This is true for most
devices that are used for Host to ISC communications, but any device that has been approved by Lenel
will indicate how termination should be configured for proper operation in its documentation. Refer to
the specific device diagrams being used in the following sections of this hardware manual.
•
RS-485 Cable termination from controller to down stream modules (LNL-500X, 1100, 1200, 1300,
1320, 4000, 8000) Termination of this section of the RS-485 bus always remains the same. Each end of
the RS-485 bus must be terminated using the on-board jumpers provided with each piece of OnGuard
hardware. Please refer to the termination drawings for each component being installed in this hardware
manual.
•
RS-485 Cable termination from LNL-500X to Third-party hardware devices Termination may be
different for each RS-485 hardware device that is connected to the LNL-500X interface gateway
module. Please refer to the gateway model being used for the hardware installation application.
1.6
RS-232 Interfaces
A number of products provide RS-232C interface for communication. This interface is intended for short
distance communication because its high impedance is more susceptible to noise. Cable length is generally
limited to 50 feet (15.24 m.) If required, this distance may be extended to a few hundred feet by using low
capacitance shielded cables.
Hardware Installation Guide
revision 7 —
23
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