65-2646XL-05-04 M2A Transmitter Operator’s Manual
Chapter 8: RS-485 Modbus Output
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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 13 & Figure 14 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:
Table 8: Modbus Terminal Strip Signals
Modbus Terminal
Label (RS-485
Name)
Modbus Signal Name
A
D0
B
D1
C
Common