APPENDICES
53
OI-6000K-X-X-X-NXP-X OPS_GUIDE_REV 1.0
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.
Summary of Contents for OI-6000K NXP Series
Page 1: ......
Page 2: ......
Page 67: ...APPENDICES OI 6000K X X X NXP X OPS_GUIDE_REV 1 0 62 Notes ...