OTIS GEN II OI-6000, Operation Manual

Page: 62 / 72

APPENDICES
56 OI-6000-X-X-X-X-O OPS_GUIDE_REV 3.1
APPENDIX B: MODBUS COMMUNICATIONS
Certain Otis Monitors have the capability of accepting Modbus sensor inputs for data communications with OI-6000 series
detectors. Modbus is a communication protocol that uses an RS-485 serial connection, and can accept a number of different
devices.
Based on the type of circuit used, there is a limit on how many devices that can be connected to a Modbus sensor network.
Otis Monitors currently allow a maximum of 32 devices on a single network. The data is transferred along the Modbus network
at a specified Modbus baud, or rate of speed. Though small, networks that have a high number of devices connected will incur
a small, proportional delay in the communication transfer of data.
WIRING CONFIGURATIONS
A daisy chain is a wiring scheme in which multiple devices are wired together in a sequence, or in a ring. Daisy chains may be
used for power, analog signals, digital data, or a combination thereof. For the purposes of Otis devices, the term daisy chain
refers to multiple devices connected in a series to form a single long line of devices, connected via the wiring patterns embedded
within each device.
Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of
canceling out electromagnetic interference (EMI) from external sources as well as “crosstalk” between neighboring pairs. In
electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system
creates an undesired effect in another circuit or channel. Otis products require twisted pairs for all wired Modbus connections.
Twisted pair cables are often shielded in an attempt to further prevent EMI. Electromagnetic shielding provides an electric
conductive barrier to attenuate electromagnetic waves external to the shield and provides a conduction path by which induced
currents can be circulated and return to the source, via ground reference connection. These cables are referred to as shielded
twisted pairs (STP) and are recommended for operation areas with high noise levels.
PROPER CONNECTION
The distance of the Modbus connection from the gas detection device to the monitor cannot exceed 4,000 feet. In the instance
of daisy-chained devices, this applies to the last sensor connected on the line. Connection distances of 100 feet, or less, require
22 to 24 gauge wire. Connection distances that range more than 100 feet require 18 to 20 gauge wire.
For more information on properly wiring a daisy chain network of devices for Modbus, consult the following diagram.