Manual 35086
ProTech-GII with Math Functions
Woodward
156
Chapter 9.
Modbus Communications
Introduction
The ProTech-GII can communicate with plant-distributed control systems and/or CRT based operator
control panels through three Modbus communication ports (one port per module). Each of the three
modules (A, B, & C) has a serial port for Modbus communications. These ports support RS-232 or RS-
485 communications using a standard Remote Terminal Unit (RTU) Modbus transmission protocol. The
Modbus interface utilizes a master/slave protocol. This protocol determines how a communication
network’s master and slave devices establish and break contact, how a sender is identified, how
messages are exchanged, and how errors are detected.
Each module Modbus port is fully isolated from those of the other modules and provides all module-based
information (Input/Output channel state information, alarm and trip relay information, first-out indication,
etc.). However, it can also be used to sense the following information from the other two modules:
Sensed Speed – other two modules
Acceleration – other two modules
Alarm Latch State – other two modules
Trip Latch State – other two modules
Note:
Modbus-based write commands (for test purposes) can only be given to the module via the
module Modbus communication port.
Table 9-1. Modbus Communication Port Specifications
Number of Ports: 1
Comm Type: RS-232/RS-485, user selectable
Termination Resistor: RS-485 on board, terminal block selectable
Isolation: 500 Vac from output to chassis and output to all other circuits
Signal Cable Length: Must be limited to 1500 ft / 305 m RS-485 (low capacitance 16 AWG / 1.3 mm²),
50 ft / 15 m RS-232
Monitor Only
Each Modbus communication port is designed to continually output all Boolean Read and Analog Read
information and can be configured to accept or ignore “write” commands, depending on the specific
application requirements. Ignoring “write” commands allows the ProTech-GII to be monitored but not
controlled from an external device.
If the ProTech-GII module “Enable Write Commands” Modbus setting is configured “No”, the respective
ProTech-GII module will not accept “write” commands from an external master device (DCS, etc.). For
security purposes, the option to ignore “write” commands can only be enabled or disabled with “Config
Level Password” permissions.
Monitor and Control
If the ProTech-GII module “Enable Write Commands” Modbus setting is configured to “Yes”, the
respective ProTech-GII module will accept “write” commands from an external master device (DCS, etc.).
This allows a Modbus-compatible device to monitor all read registers and issue “Reset” and “Start/Abort
Test Routines” commands only. Modbus communication ports are independent of each other and can be
used simultaneously.
To ensure that a Modbus-based command to trigger a module test is valid, an Initiate Test command
followed by a Confirm Test command must be received to initiate a test routine. The Confirm Test
Released