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Some non-standard I/O devices do not support digital bits, so this command fails.

If CitectSCADA cannot read these bits, it assumes the PLC is bad, and remains off line. On startup, CitectSCADA tries to read some digital inputs to verify that the PLC does exist. To handle these non-standard I/O devices, the Modbus protocol has some special options to change its operation: 1. When CitectSCADA tries to communicate with these I/O devices, you might get errors on startup or during communication. However, some of these I/O devices do not support the Modbus protocol correctly. Notes Some non-standard PLCs support the Modbus protocol. The maximum request length for the Modbus protocol is 2000 bits.

Other PLCs might also be supported contact Citect Technical Support for details. The Modbus protocol also supports the Siemens TI 405 PLC, and the Moore Industries IOExpress Data Collection Units. The Modbus protocol differs from the Modbusa protocol only in the data framing technique used: the Modbus protocol is RTU-based while the Modbusa protocol is ASCII-based. Configuring Your CitectSCADA Project You'll find additional information in the Advanced Configuration and Maintenance and Troubleshooting sections to help you resolve communication or configuration errors.Ĭopyright © 2005 Citect Pty Ltd (All rights reserved.Ībout Modbus The Modbus protocol supports serial communication with many I/O devices (including the Modicon 484, 584, 884, and 984 programmable controllers).

This process involves the following steps: 1. Modbus Driver The Modbus driver Help is designed to assist you in setting up and configuring communication with a Modbus system.