Can utility systems with PLCs be connected to a DCS for monitoring and control?
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This post was last edited by mmkingzm on 2018-2-3 at 15:24. The boiler systems, refrigeration systems, air compressors, nitrogen generators, and water purification equipment in the factory are equipped with PLCs, and it is possible to use Modbus communication for monitoring and control purposes. There are a few questions: 1. Can the bandwidth and speed of Modbus communication meet the requirements (30*AI, 50*DI, 20*DO)? 2. Are there any relevant design specifications? 3. What are the key points to note during implementation? :P
It’s possible; there are two methods. One is to directly modify the PLC program to send the variables that need to be monitored to the DCS. This method requires the original program of the single-station PLC. There is also a type of gauge that allows you to directly access the variable you want to monitor, connected to the DCS via hard wires; this requires more channels and cables
It is best to use communication devices from the same brand; if both sides are Siemens, the stability will be higher. Communicating between PLCs of different brands can be troublesome. It is also best not to participate in cascade control through communication.
Modbus communication is not very reliable; it’s okay for monitoring, but not suitable for control.
There are no major issues with monitoring Modbus, but control is uncertain
What are the reasons for the unreliable Modbus communication control?
The PLC handles the control itself; for interlocking, communication-based methods are not very reliable. It is possible to achieve integration with the DCS through hardwiring, while other monitoring functions can be handled via communication.
The purpose of communication is to enable monitoring and control from the control room, with the goal of achieving unattended operation wherever possible
1. It is not recommended to use Modbus communication for control; for starting and stopping, hard wiring is advised, while communication can be used for modifying parameters. Additionally, it is necessary to consider the distance between these individual units and the central control unit; different distances require different communication baud rates. 2. Two tasks need to be carried out. First, it is necessary to check on the PLC side whether Modbus communication is supported; if not, the appropriate communication card must be installed. Then, a communication program needs to be written. It is recommended to use continuous variables when writing such programs – for example, multiple boolean values can be stored in a 16-bit or 32-bit value, which helps to reduce the number of requests and allows more data to be read at once. 3. Verify that the DCS supports Modbus communication; if not, devices such as serial server must be added, and programs must be written to enable communication.
1. OP, your mention of MODBUS communication is also a bit vague. Traditionally, RS485 communication is used most often; this type of communication is limited by the number of bytes that can be transmitted, and calculations are required. The recommended distance for its use is around 1000 meters. It is mainly used for monitoring purposes, but not for control； Another protocol for MODBUS is TCP/IP, which refers to the network cables we commonly use; fiber optics can be used for communication over long distances. In China, most PLCs still use RS485 as their standard communication protocol. However, Siemens and other foreign companies are already promoting the use of TCP/IP cards abroad, and foreigners working on projects in China also use such cards. They offer high bandwidth, enable large amounts of data transmission, and are relatively stable, making them suitable for control purposes ； Speaking of DCS, Honeywell’s PKS actually employs this type of control structure: the controllers are equipped with firewalls, and communication with the outside world takes place through serial port servers. Other DCS manufacturers also offer TCP/IP communication cards, which are now becoming increasingly common and widely used ； 2. There are no standard requirements; traditionally RS485 is used, but TCP/IP is more common these days ； 3. Pay attention to the transmission distance during implementation; fiber optic conversion can be used for this purpose. Another factor to consider is the amount of data transmitted, as the volume of communication can affect the load on the DCS controller. The amount of data you’ve provided is fine. For important interlock signals and start/stop signals, a hardwired connection is the most reliable option; some of the signals that can be transmitted use the TCP/IP protocol, for your reference.