Regarding RS485 to optical fiber communication
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I have a piece of equipment whose local control cabinet is equipped with a Siemens S7200 series PLC. I want to use the PLC’s built-in RS485 interface to establish communication between the PLC and the RS485 interface of the DCS in the control room, via optical fiber. What tasks need to be carried out? The PLC model is CPU226; the distance between the local cabinet and the control room is no more than 3 kilometers, the optical cable will be laid underground, and the communication protocol to be used is MODBUS RTU. I hope that true experts (those with experience) can provide detailed explanations, including the model of the conversion equipment, fiber optic specifications, and a complete list of all devices that need to be purchased.
Siemens itself produces a small device for converting 485 signals to optical fiber communication; I’ve forgotten its name – I was there on site but too lazy to go and check it.
1. First, you need to determine whether to use multi-mode fiber or single-mode fiber; if the distance is 3 kilometers and the signal strength is low, it is recommended to use single-mode fiber. 2. The optical fiber has been confirmed; corresponding optical fiber jumpers, also known as pigtails, are required to connect to the optoelectronic converter. 3. Converters that convert RS485 signals to single-mode fiber signals are available in large quantities on the market. This needs to be used in pairs. 4. The interface type of the fiber optic jumper must match that of the optoelectronic converter. 5. Fiber optic cables also require fiber splicing, which can only be done with specialized equipment; such equipment is available in large quantities on the market, usually in computer stores. That’s about it.
As a little extra information, regarding converters, there are two points that need to be confirmed: 1) Medium conversion, that is, the conversion between optical signals and electrical signals; 2) Protocol conversion.
Siemens has an interface conversion module that converts RS485 to fiber optic signals: the Optical Link Module (OLM). It’s a simple module that is also very easy to use. Product information can be found on their website: http://www.ad.siemens.com.cn/products/pi/net/profibus/profibus_4.asp. Profibus field buses mostly use RS485 for serial communication, but sometimes the cables can be converted to optical cables for communication; OLM is one such device, and its topology usually includes point-to-point, star, bus, or redundant fiber optic ring topologies. However, it should be noted that OLMs of different models cannot be connected using optical fibers.