What should be noted regarding communication between the on-site PLC and the DCS in the control room?
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Some of the complete sets of equipment on site have their own PLCs; to display and operate them from the DCS in the control room, the following is required: 1. The communication language needs to be standardized, 2. Are there any considerations regarding the communication interfaces? 3. To enable operation from the DCS, does anything need to be designed in the PLCs? ? ?
First of all, it is not recommended to use communication methods for cross-system operations. If operation is necessary, communication at the monitoring level, such as OPC, is generally not used. MODBUS and DP are recommended. Additionally, on DCS, it is necessary to carry out the relevant underlying configuration as well as develop the human-machine interface.
It is not recommended to operate a PLC through a DCS; monitoring it on the DCS is acceptable, but performing operations this way is unsafe. PLCs have many logical functions and require fast response times, while the data transmitted to the DCS via communication may sometimes be out of sync.
When communicating between a DCS and a PLC, the following points need to be taken into account and standardized: (1) The communication protocols supported by both sides must be identical; for example, both should support the MODBUS RTU protocol. (2) The relevant parameter settings must be consistent, including the baud rate, parity bits, data bits, etc. (3) The station addresses must be the same; the DCS acts as the master station while the PLC acts as the slave station. If the station address of the PLC is 2, then it should also be 2 in the DCS, and this address cannot be used by any other PLC – it cannot repeat.
I’m using a DCS from Zhejiang University’s Control Systems Department; all that’s needed is a communication card to enable communication with the DCS. However, it’s not recommended to use it to control PLCs, as the response time is too slow – around 7.8 seconds. It’s best for monitoring
PLC communication with DCS typically occurs as a module of the DCS or by communicating with the DCS’s communication module; the communication protocols used are DP or 485, with DP being faster and supporting both reading and writing operations.