RKI Instruments M2 Rig Monitor - Manual (Page 39)

M2 Rig Monitor Operator’s Manual 34
Chapter 7: RS-485 Modbus Output
Overview
This chapter describes the M2 Rig Monitor’s RS-485 Modbus output and how to configure
the Rig Monitor to make use of it. It also discusses how to wire the Rig Monitor into a
Modbus system.
The Rig Monitor provides an RS-485 serial communications interface. It is a Modbus Slave
Device, supporting 2-wire RS-485 Modbus RTU serial communications.
Wiring the M2 Rig Monitor in a
Modbus System
The Rig Monitor is a 2-wire Modbus RTU device. When wiring the Rig Monitor 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 M2
Rig Monitor’s line driver provides for up to 128 M2 Rig Monitors to be connected together
without the need for a repeater. Figure 13 & Figure 14 below illustrate typical Rig Monitor
wiring configurations. If more than 128 M2 Rig Monitors need to be connected together,
RS-485 repeater(s) should be used such that no more than 128 M2 Rig Monitors reside on
any given network segment.
CAUTION: The network segment can only support 128 units if RKI M2 Rig Monitors 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 M2 Rig Monitors produced after November 2010 have the capability to
support a 128-unit segment. M2 Rig Monitors produced before November 2010
can only support a 32-unit segment.
The MODBUS terminal strip is located on the terminal PCB (see Figure 4). The following
signals are available at the Modbus terminal strip:
Table 6: Modbus Terminal Strip Signals
Modbus Terminal Label (RS-485
Name)
Modbus Signal Name
A D0
B D1
C Common