Modbus Protocol - GE Mark VIe System Manual

4.3 Modbus Protocol

Modbus is a Request-Response protocol typically used for Supervisory Control and Data Acquisition (SCADA) interface,
either as the Master for data acquisition by the SCADA or a Slave to transmit data to the SCADA. The application data unit
(ADU) layer offers three methods of implementation:
• TCP/IP
• RTU
• ASCII
The Request-Response protocol is straight forward to implement; however, it has an inherent latency in the Request-Response
cycle between the two computers instead of the Event Driven messages (exception reporting) available with OPC and GSM.
Additionally, it does not support individual high-resolution time stamps for each alarm and event, but rather a single time
stamp for a set of data transmitted from the Slave to the Master upon request.
In the Mark control systems, Modbus is supported in both the controllers and WorkstationST. For the Mark VIeS Safety
controller, only Ethernet Modbus and the Modbus Read command from the Modbus Master is supported (Serial Modbus and
the Write command is not supported).
Note Modbus protocol does not provide security against unauthorized commands or interception of data. A separate Modbus
gateway should be considered rather than a shared resource.
4.3.1 WorkstationST Modbus
Modbus in the WorkstationST has full support, as follows:
• One or multiple Modbus masters running simultaneously
• One or multiple Modbus slaves running simultaneously
• Ethernet or serial communication running simultaneously
• Each Modbus page is configured with ToolboxST, enabling virtually any variable in the system to be mapped to a page
106 GEH-6721_Vol_I_BP GEH-6721_Vol_I Mark VIe and Mark VIeS Control Systems Volume I
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