3. From the database:
DCS generally provides a unified database. In other words, once a piece of data exists in the database in the DCS system, it can be used in any situation.
The following references, such as in the configuration software, in the monitoring software, in the trend chart, in the report... and the database of the PLC system is usually not unified.
Configuration software, monitoring software and even archiving software have their own databases. Why it is often said that Siemens S7-400 is called DCS only after it reaches 414 or above, because

The PCS7 system of Siemens uses a unified database, and PCS7 requires the controller to be at least S7 414-3 or higher.

4. In terms of time scheduling:
Generally, the PLC program cannot run according to the pre-set cycle period. The PLC program is executed from the beginning to the end once and then executed from the beginning. (right now
Some new PLCs have been improved, but there are still restrictions on the number of task cycles) and DCS can set the task cycle. For example, quick tasks, etc. same
It is the sampling of the sensor. The change time of the pressure sensor is very short. We can sample with a 200ms task cycle, while the lag time of the temperature sensor is very short.
Large, we can use the task sampling period of 2s. In this way, DCS can reasonably schedule the resources of the controller
5. In terms of network structure:
Generally speaking, DCS usually uses a two-layer network structure. The first layer is a process-level network. Most DCS uses their own bus protocol, such as Yokogawa’s
Modbus, Profibus of Siemens and ABB, CANbus of ABB Control Module etc., these protocols are all based on the standard serial port transmission protocol RS232 or RS485 protocol
Basically. The field IO module, especially the analog quantity sampling data is very large, and there are many field interference factors, so the data throughput should be used
Large network standard with strong anti-interference ability. The bus structure based on the RS485 serial port asynchronous communication mode meets the requirements of on-site communication. IO sampling data
After CPU conversion, it becomes integer data or real data, which is transmitted on the operation-level network (second layer network). Therefore, the operation-level network can adopt data
A network standard with moderate throughput, fast transmission speed, and convenient connection. At the same time, because the operation-level network is generally placed in the control room, the requirements for anti-interference are relatively relatively high.
low. Therefore, standard Ethernet is the best choice. The TCP/IP protocol is a standard Ethernet protocol, and generally we use a communication rate of 100Mbit/s.
The work tasks of the PLC system are relatively simple, so the amount of data that needs to be transmitted is generally not too large, so the common PLC system is a network structure.
Structure. Either the process-level network and the operation-level network are merged together, or the process-level network is reduced to an internal link between modules. PLC will not or rarely use
Ethernet.

Bently Nevada	Allen Bradley	EMERSON
128275-01	2711P-T7C4D8	KJ3001X1-BB1
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330930-065-02-00	1756-PB75R	KJ3002X1-BE1 12P0682X052
3500/44M	1784-SD1	KJ3243X1-BA1
3300/03-01-00	1771-OMD	KJ3222X1-BA1 12P2532X122 CE4003S2B1
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3500/50 133434-01	1771-P4S1	KJ2002X1-BA1