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PSI-MOS-RS485W2/FO 850 T Serial to Fiber Converter
2-wire RS485 to Redundant Fiber

  • PSI-MOS-RS485W2/FO 850 T Serial to Fiber Converter
  • Two ST type fiber connectors
  • Extend serial data up to 2.6 miles
  • Immune to EMI, RFI and transient surges
  • Point-to-point or star configuration
  • Terminal Block for 2-wire RS485 Connections


For applications where redundancy is required, the PSI-MOS-RS485W2/FO 850 T can be used to transmit data from RS485 2-wire devices out over two fiber optic cables. By transmitting serial data over optical fiber, these serial to fiber converters provide an economical path to extend the reach of RS485 devices. The serial data input is transmitted out both fiber ports to different locations or back-up systems.

All popular protocols with 10/11-bit UART data format and NRZ data coding are supported, such as:

  • S-BUS
  • DH-485, and more…

Expandable Serial to Fiber Network infrastructures

Up to ten (10) Serial to Fiber Converters can be grouped together using the TBUS DIN Rail bus system for voltage and data. This allows the serial converter to operate in additional network topology environments:

as a star coupler, taking the serial data input signal and distributing it to up to 20 Fiber optic output ports
linear structure can be used to network several RS485 devices
linear and star structures can be cascaded to create complex tree structures
Long Distance Serial Data Transmission over Fiber

With the FO 850 T Serial to Fiber Converter you can extend your serial data transmission up to 4.2km (2.6 miles). Therefore, any two pieces of asynchronous serial equipment, located miles apart, can communicate at half duplex over fiber optic cable at rates up to 500 kbps.

EMI, RFI and Transient Surge Immunity

Another advantage of the FO 850 T fiber optic transmission system is the electrically isolated connection of devices. Electromagnetic interference (EMI) is a common phenomenon in typical environments like industrial plants, warehouses and factory floors. This interference can cause corruption of data over RS485 or copper-based Ethernet links. Data transmitted over fiber optic cable however is completely immune to this type of noise, thus preventing the negative effects of voltage equalization currents and electromagnetic interference on the data cables. A Serial to Fiber Media Converter therefore enables you to inter-connect your serial devices over fiber ensuring optimal data transmission, increased availability of the system, and improved network design flexibility for point-to-point connections and star structures.

Flexible Fiber Optic Connections

The FO 850 T operates at 850 nm wavelength, using a separate LED emitter and photo-detector on ST type connectors. Almost any multimode glass fiber size can be used including 50/125 m, 62.5/125 m, and 200/230 m.

Power Budget Considerations

Calculating the power budget is critically important with planning the fiber optic link. The optical power budget is the amount of light required to transmit data successfully over distance through a fiber-optic connection. The amount of light energy available within the setup will dictate the length of the fiber optic cable run between serial media converters within the network. Optical power budgets are critical to help businesses avoid signal distortion. To learn how to calculate optical power budget read our technical note. Transmit and receive dBm can be found in the Hardware specifications.

High Quality Features and Support

The FO 850 E are also equipped with comprehensive diagnostic functions to increase system availability, simplify start-up and permanently monitor the optical transmission quality. The integrated optical diagnostics allow permanent monitoring of the FO paths during installation and operation. The floating switch contact is activated when the signal level on the fiber optic paths reaches a critical level. This early alarm generation enables critical system states to be detected before they result in failure. These cost and time saving features, along with free worldwide technical support, make the FO 850 T serial to fiber converter the smart choice for IT professionals.

  • Supply voltage and data signals routed through via DIN rail connectors
  • Connections can be plugged in using a COMBICON screw terminal block
  • Automatic data rate detection or fixed data rate setting via DIP switches
  • Redundant power supply possible by means of optional system power supply unit
  • High-quality electrical isolation between all interfaces (RS-485, fiber optic ports, power supply, DIN rail connector)
  • Approved for use in zone 2
  • Integrated optical diagnostics for continuous monitoring of fiber optic paths
  • Intrinsically safe fiber optic interface (Ex op is) for direct connection to devices in zone 1
  • Floating switch contact for leading alarm generation in relation to critical fiber optic paths
  • Suitable for data rates up to 500 kbps


PSI-MOS-RS485W2/FO 850 T Serial to Fiber Media Converter Applications

  • near heavy electrical equipment
  • in environments with electrical (EMI) or radio (RFI) interference
  • in environments with transient surges
  • in industrial plants, warehouses and factory floors
  • enabling asynchronous serial equipment to communicate at half duplex, with rates up to 500 kbps, over optical fiber

Point-to-point connections between serial devices over fiber
Configure the point-to-point connection redundantly to increase availability by using two PSI-MOS-RS485W2/FO 850 T Serial to Fiber Converters. Easily convert a data link from a single copper cable out to two fiber optic cables.

RS485 Redundant Point to Point Network Diagram

Redundant Star Topology Network

You can network RS-485 2-wire devices within a star structure. Depending on the number of star lines required, several PSI-MOS-RS485W2/FO 850 T Serial to Fiber Converters are connected to TBUS DIN Rail bus systems for voltage and data. This makes up to 20 fiber optic ports available. Cross-wiring for serial data and the supply voltage is provided automatically by the DIN rail connector.

RS485 Redundant Star Network Diagram

Linear Networks
The fiber optic linear structure can be used to network several RS-485 2-wire devices. FO 850 E serial to fiber converters are used at the beginning and end of the fiber optic line. FO 850 T serial to fiber converters with two fiber optic ports are used along the line. You can use up to ten PSI-MOS-RS2485/FO devices along the line in a linear structure.

RS485 Linear Network Diagram

Tree Networks
Linear and star structures can be cascaded to create complex tree structures, in a redundant configuration. The number of devices that can be cascaded is only limited by the timing response (timeout) of the bus system used due to the bit retiming of the serial to fiber optic converter.

Redundant Tree Network Diagram

Block Diagram

PSI-MOS-RS485W2/FO 850 T Block Diagram


1.091,00  (1.352,84  inc tax)
Perle Systems, 1 Year
PSI-MOS-RS485W2/FO 850 T - RS485 2-wire to fiber Converter: Terminal block serial to 2x duplex fiber multimode 850nm (ST) [ 4.2km, 2.6 miles ] – 24VDC EU wall power adapter included

1.069,00  (1.325,56  inc tax)
Perle Systems, 1 Year
PSI-MOS-RS485W2/FO 850 T - RS485 2-wire to fiber Converter: Terminal block serial to 2x duplex fiber multimode 850nm (ST) [ 4.2km, 2.6 miles ]