68 • VOC Pro Transmitter
Appendix B: Modbus Communications
RKI controllers have the capability of accepting Modbus sensor inputs for data
communications with VOC Pro 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. RKI controllers currently allow a maximum of 64 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 the VOC Pro, 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. The VOC Pro requires 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 controller 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.