RKI Instruments M2A - Manual (Page 56)

51 M2A Transmitter Operator’s Manual
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 29 & Figure 30 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 9). The following signals
are available at the Modbus terminal strip:
Ground Jumper
The M2A includes a 2-pin ground header (see Figure 9) 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 28, the ground jumper should be installed. If the M2A is wired in a Modbus system as
shown in Figure 27, the ground jumper should be removed.
Table 10: Modbus Terminal Strip Signals
Modbus Terminal Label (RS-485
Name)
Modbus Signal Name
A D0
B D1
C Common