Issues with communication interfaces between DCS and multiple Siemens PLCs
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Multiple sets of equipment in the company are equipped with Siemens S7-200 PLCs, and it is necessary to transmit the data from these PLCs to the DCS, which is equipped with MODBUS communication protocol cards. Question: The original DP plug from Siemens is supposed to be used for Profibus communication – can it be used for Modbus communication as well? If that doesn’t work, what kind of plug should I buy? Are the connections between the PLCs in series or in parallel?
Since all the PLCs are Siemens S7-200, the DP protocol can be used; a simple daisy-chain configuration can be established, and then an interface card that converts DP to 485 can be added to the DCS.
The DCS is equipped with the appropriate communication card; several PLCs are connected in series to this DCS communication card, and it’s sufficient for those PLCs to have different communication addresses. At the same time, the data storage capacity of the DCS communication card needs to be taken into consideration
First, the DCS needs to be configured with a DP card and set as the master station; then each 200 unit is set as a slave station with a different address, and these are connected to the DCS’s DP card in a series configuration. Note that the terminal resistors must be set to ON at both ends. I have implemented one-to-many communication between ProLog and Siemens PLCs, but the large volume of data can overwhelm the ProLog controller, so it is necessary to control the number of communication bytes. :)
First, confirm whether the complete set of equipment is equipped with the older model 200 or the newer Smart version. If it’s the Smart series, the CPU has an RS485 port that is a 9-pin serial port; in such cases, a corresponding 9-pin male serial connector needs to be purchased. The wiring scheme involves connecting the 3+ pins together, the 8- pins together, and the ground pin for shielding. MODBUS-RTU communication relies on serial connection – all the positive pins are connected together, and all the negative pins as well; if the distance between devices is large, terminal resistors may be necessary