65-2661RK-03-04 M2A Transmitter Operator’s Manual
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Chapter 8: RS-485 Modbus Output
Overview
This chapter describes the M2A’s RS-485 Modbus output and how to configure the M2A
to make use of it. It also discusses how to wire the M2A into a Modbus system.
The M2A provides an RS-485 serial communications interface. It is a Modbus Slave
Device, supporting 2-wire RS-485 Modbus RTU serial communications.
Wiring the M2A in a Modbus System
The M2A is a 2-wire Modbus RTU device. When wiring the M2A 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
M2A’s line driver provides for up to 128 M2As to be connected together without the need
for a repeater. Figure 11 & Figure 12 below illustrate typical M2A wiring configurations. If
more than 128 M2As need to be connected together, RS-485 repeater(s) should be used
such that no more than 128 M2As reside on any given network segment.
CAUTION:
The network segment can only support 128 units if RKI M2As are being used. The
use of any other instruments will lower the number of units that may reside on any
given network segment.
The MODBUS terminal strip is located on the terminal PCB (see Figure 3 on page 7). The
following signals are available at the Modbus terminal strip:
Ground Jumper
The M2A includes a 2-pin ground header (see Figure 3 on page 7) that is used when the
M2A is used in a Modbus system. Every M2A is supplied with a ground jumper (a jumper
block) installed onto this header. If the M2A isn’t wired into a Modbus system or if it is
wired into one as shown in Figure 10, the ground jumper should be installed. If the M2A
is wired in a Modbus system as shown in Figure 9, the ground jumper should be removed.
Table 8: Modbus Terminal Strip Signals
Modbus Terminal Label (RS-485
Name)
Modbus Signal Name
A
D0
B
D1
C
Common
