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Chapter 8: RS-485 Modbus Output
65-2660XL-XX-04 M2A-XL Transmitter
Chapter 8: RS-485 Modbus Output
Overview
This chapter describes the M2A-XL’s RS-485 Modbus output and how to configure the M2A-XL
to make use of it. It also discusses how to wire the M2A-XL into a Modbus system.
The M2A-XL 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-XL in a Modbus System
The M2A-XL is a 2-wire Modbus RTU device. When wiring the M2A-XL 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-
XL’s line driver provides for up to 128 M2A-XLs to be connected together without the need for a
repeater. Figure 11 & Figure 12 below illustrate typical M2A-XL wiring configurations. If more
than 128 M2A-XLs need to be connected together, RS-485 repeater(s) should be used such that
no more than 128 M2A-XLs reside on any given network segment.
CAUTION:
The network segment can only support 128 units if RKI M2A-XLs 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 11). The
following signals are available at the Modbus terminal strip:
Table 8: Modbus Terminal Strip Signals
Modbus Terminal
Label (RS-485
Name)
Modbus Signal Name
A
D0
B
D1
C
Common
