RKI Instruments Digester Gas Monitor - Manual (Page 74)

68 • Overview Digester Gas Monitor Operator’s Manual
Chapter 9: RS-485 Modbus Output
Overview
This chapter describes the Digester Gas Monitor’s RS-485 Modbus output and how to
configure the Digester Gas Monitor to make use of it. It also discusses how to wire the
Digester Gas Monitor into a Modbus system.
The Digester Gas Monitor provides an RS-485 serial communications interface. It is a
Modbus Slave Device, supporting 2-wire RS-485 Modbus RTU serial communications.
Wiring the Digester Gas Monitor in a
Modbus System
The Digester Gas Monitor is a 2-wire Modbus RTU device. When wiring the Digester Gas
Monitor into a Modbus system, adhere to standard Modbus wiring practices per the
Modbus Over Serial Line Specification and Implementation Guide V1.0 or later. This
document can be found online at www.modbus.org/specs.php.
The Modbus protocol supports a maximum of 247 unique slave addresses (1-247). The
Digester Gas Monitor’s line driver provides for up to 32 Digester Gas Monitors (or some
combination of Digester Gas Monitors and other Modbus slave devices such as the RKI
Instruments M2) to be connected together without the need for a repeater. Figure 21 &
Figure 22 below illustrate typical Digester Gas Monitor wiring configurations. If more than
32 Digester Gas Monitors (or Digester Gas Monitors and other slave devices) need to be
connected together, an RS-485 repeater(s) should be used such that no more than 32
Digester Gas Monitors reside on any given network segment.
The MODBUS terminals are located on the controller terminal strip (see Figure 3). The
following signals are available at the Modbus terminals:
Table 11: Modbus Terminal Signals
Modbus Terminal Label
(RS-485 Name)
Modbus Signal Name
A D0
B D1
C Common