65-2662XL-04 M2A-XL Transmitter
Chapter 8: RS-485 Modbus Output
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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 13 & Figure 14 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.
NOTE:
Only M2A-XLs produced after November 2010 have the capability to support a 128-
unit segment. M2A-XLs 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 13). 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
