65-2643RK0504SS M2A Transmitter Operator’s Manual
39
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 12 & Figure 13
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.
NOTE:
Only M2As produced after November 2010 have the capability to support a 128-unit segment. M2As
produced before November 2010 can only support a 32-unit segment.
The MODBUS terminal strip is located on the terminal PCB (see Figure 4 on page 12). The following signals are
available at the Modbus terminal strip:
Ground Jumper
The M2A includes a 2-pin ground header (see Figure 4 on page 12) 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 11, the ground jumper should
be installed. If the M2A is wired in a Modbus system as shown in Figure 10, 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
