Manual 35018V1
505XT Digital Control System for Steam Turbines
Woodward
196
Chapter 6.
Communications
Modbus Communications
The 505 control can communicate with plant distributed control systems and/or CRT based operator
control panels through Modbus communication ports. There is one serial port that supports RS-232 and
RS-485 communications using ASCII or RTU MODBUS transmission protocols. There are 2 ports
available for either Modbus UDP or TCP/IP protocol which can be utilized from Ethernet port 1 or 2.
Modbus 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.
Our experience has been that not all serial interfaces on laptops or
Desktop PCs work the same. Many USB-to-Serial converters work, but
some do not.
To use a 505 Modbus port to monitor and/or operate, check the “Use Modbus” checkbox under the
Configurations menu / Communications page.
Monitor Only
The three Modbus communication ports, are defaulted as read-only. As read-only ports, the 505 can be
monitored but not controlled from an external device. By simply connecting a monitoring device,
configured to communicate through Modbus, and to the 505’s defaulted protocol settings (parity, stop bits,
etc.), this device can be used to monitor all the 505’s controlling parameters, modes, etc. without affecting
control.
Configuration of the protocol is found on the Communications page under both the Configuration and
Service menus. There are options for the serial settings, slave address number and a checkbox to enable
write commands from each individual link.
Monitor and Control
Once a Modbus port is configured within the 505’s Configuration mode, the 505 will accept RUN mode
commands from an external network master device (DCS, etc.). This allows a Modbus compatible device to
monitor and perform all 505 RUN mode parameters and commands except for the Overspeed Test enable,
On- Line/Off-Line Dynamics select, and Override Failed Speed Signal commands.
Each Modbus port is independent of the other, and can be used simultaneously. Each only must have its
own slave device address and each has its own enable write checkbox. The last command given from
any of the ports has priority or is the mode or function selected.
Modbus Communication
The 505 control supports two Modbus transmission modes. A mode defines the individual units of
information within a message and the numbering system used to transmit the data. Only one mode per
Modbus network is allowed. The supported modes are ASCII (American Standard Code for Information
Interchange), and RTU (Remote Terminal Unit). These modes are defined in the following table.