Due to the easiness of setup and low cost, serial devices are used around the world of industrial systems. However, the longer the copper cable, the more data corruption due to the electromagnetic (EMI) and radio frequency (RFI) interference. The fiber connection provides the benefits of noisy immunity and distance. Therefore, serial over fiber is the best solution to overcome these problems, and this text will give a brief introduction of serial-to-fiber media converters.
What Is a Serial-to-Fiber Media Converter?
Serial-to-fiber media converters, sometimes also called fiber optic modem (FOM), is a device which provides electrical to optical conversion of electronic communication and data signals for transmission using tactical fiber optic cable assemblies. The converter simultaneously receives incoming optical signals and converts them back to the original electronic signal allowing for full duplex transmission. Together with the tactical fiber optic cables, the converter provides a rugged, secure, and easy deployable optical link. The serial-to-fiber media converter is available in both single and multi-channel configurations and supports both point-to-point and multi-point configurations.
Three Common Serial Interfaces
In order to get one step closer to understanding the serial-to-fiber media converters, common serial interfaces must be explained before. RS-232, RS-422, and RS-485 are the most popular serial interfaces in the industrial application. Each interface provides unique benefits for device communication. RS-232 is the most common serial interface and ships as a standard component on most Windows-compatible desktop computers. RS-422 is the serial connection used on Apple computers. RS-485 is a superset of RS-422 and expands on the capabilities. For your easy reference, a quick comparison chart listed below demonstrates the key differences of these three commonly used serial interfaces for industrial applications.
| Specifications | RS-232 | RS-422 | RS-485 |
| Speed | Full-duplex | Full-duplex | Half-duplex |
| Distance | 15M@9600bps | 1.2KM@9600bps | 1.2KM@9600bps |
| Pins | TxD, RxD, RTS, CTS, DTR, DSR, DCD, GND | TxA, TxB, RxA, RxB, GND | DataA, DataB, GND |
| Cable Cost | High | Medium | Low |
| Topology | Point-to-point | Point-to-point | Multidrop |
| Software Compatibility | High | Medium | Lowest |
| Troubleshooting | Easy | Hard | Hard |
How Does It Work?
In terms of serial-to-fiber media converters, there are two kinds of connection mode: pair and ring. The working principles of them are different.
Pair Connection Mode
The Pair Connection simply extends the point-to-point transmission distance of the serial connection. Two serial-to-fiber converters can be connected over a fiber cable between a computer and a serial device. These two locations can be up to 12 miles (20 km) apart. Beware the flow control signals for RS-232 cannot be transmitted over fiber. The DTR/DSR and RTS/CTS needs to be shortened for RS-232 application.
Ring Connection Mode
If multiple serial devices need to be connected, the Ring Connection mode provides a cost-effective solution. The serial-to-fiber converters can inter-connect to the neighboring converters and form a closed fiber-to-serial ring. Data packets are transmitted by one converter to the other and so on until the signal returns to the converter that sent the original signal. When using the Ring Connection mode, the total length of the fiber connection is up to 62 miles (100 km). The only drawback is the failure of one fiber connection will cause the entire system to fail.
Applications
Serial to fiber converters can provide transmission distance up to 2 km over multi-mode fiber and up to 60 km over single-mode fiber. Now in the market, serial to fiber converters are available in several types depending upon the protocol selected, including RS-232 to Fiber Converter, RS-485 to Fiber Converter, RS-422 to Fiber Converter, RS-485/422 to Fiber Converter, and RS-485/422/432 to Fiber Converter. The applications of RS-232 and RS-485 converters are described as follows.
RS-232 Application
This serial to fiber Converter can be connected with RS485/RS422/RS232 port of computer or other devices, solve the problem of traditional RS485/RS422/RS232 communication conflict between distance and rate. RS-232 fiber converters can operate as asynchronous devices, support speeds up to 921,600 baud, and support a wide variety of hardware flow control signals to enable seamless connectivity with most serial devices. In this example, a pair of RS-232 converters provides the serial connection between a PC and Terminal Server allowing access to multiple data devices via fiber.
RS-485 Application
In this example application a pair of RS-485 converters provides the multi-drop connection between the Host equipment and the connected multi-drop devices via fiber.
