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65-2619RK-CH4-4 M2 Transmitter Operator’s Manual
Chapter 8: RS-485 Modbus Output
Overview
This chapter describes the M2’s RS-485 Modbus output and how to configure the M2 to
make use of it. It also discusses how to wire the M2 into a Modbus system.
The M2 provides an RS-485 serial communications interface. It is a Modbus Slave Device,
supporting 2-wire RS-485 Modbus RTU serial communications.
Wiring the M2 in a Modbus System
The M2 is a 2-wire Modbus RTU device. When wiring the M2 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
M2’s line driver provides for up to 128 M2s to be connected together without the need for
a repeater. Figure 10 & Figure 11 below illustrate typical M2 wiring configurations. If more
than 128 M2s need to be connected together, RS-485 repeater(s) should be used such that
no more than 128 M2s reside on any given network segment.
CAUTION:
The network segment can only support 128 units if RKI M2s are being used. The
use of any other instruments will lower the number of units that may reside on any
given network segment.
NOTE:
Only M2s produced after November 2010 have the capability to support a 128-
unit segment. M2s produced before November 2010 can only support a 32-unit
segment.
The MODBUS terminal strip is located on the terminal PCB (see Figure 2). The following
signals are available at the Modbus terminal strip:
Recommended Modbus Wiring
The recommended Modbus wiring for the M2 is illustrated in Figure 8 below. In this
configuration, 5 wires are used for wiring the M2 into a Modbus system. Figure 8 also
illustrates typical alarm device wiring.
Table 8: Modbus Terminal Strip Signals
Modbus Terminal Label (RS-485
Name)
Modbus Signal Name
A
D0
B
D1
C
Common
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