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

psi mos rs485w2 fo850t lg
  • 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

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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

Hardware Specs

PSI-MOS-RS485W2/FO 850 T Technical Specifications

Ambient conditions
Ambient temperature (operation) -20 °C … 60 °C
Ambient temperature (storage/transport) -40 °C … 85 °C
Permissible humidity (operation) 30 % … 95 % (non-condensing)
Altitude 5000 m (For restrictions see manufacturer’s declaration)
Degree of protection IP20
Noise immunity EN 61000-6-2:2005
Standards and Regulations
Electromagnetic compatibility Conformance with EMC Directive 2014/30/EU
Type of test Vibration resistance in acc. with EN 60068-2-6/IEC 60068-2-6
Test result 5g, 10-150 Hz, 2.5 h, in XYZ direction
Type of test Shock in acc. with EN 60068-2-27/IEC 60068-2-27
Test result 15g, 11 ms period, half-sine shock pulse
Shock 15g in all directions in acc. with IEC 60068-2-27
Noise emission EN 55011
Noise immunity EN 61000-6-2:2005
Free from substances that could impair the application of coating according to P-VW 3.10.7 57 65 0 VW-AUDI-Seat central standard
Connection in acc. with standard CUL
Standards/regulations EN 61000-4-2
EN 61000-4-3
EN 61000-4-4
EN 61000-4-5
EN 61000-4-6
Vibration (operation) In acc. with IEC 60068-2-6: 5g, 150 Hz
Conformance CE-compliant
ATEX II 3 G Ex nA nC IIC T4 Gc X
II (2) G [Ex op is Gb] IIC (PTB 06 ATEX 2042 U
II (2) D [Ex op is Db] IIIC (PTB 06 ATEX 2042 U)
UL, USA/Canada Class I, Zone 2, AEx nc IIC T5
Class I, zone 2, Ex nC nL IIC T5 X
Class I, Div. 2, Groups A, B, C, D
Optical interface FO
Number of FO ports 2
Transmit capacity, minimum -4 dBm (200/230 µm)
-17.6 dBm (50/125 µm)
-14 dBm (62,5/125 µm)
Minimum receiver sensitivity -32.5 dBm (50/125 µm)
-32.5 dBm (62,5/125 µm)
-32.1 dBm (200/230 µm)
Overrange receiver -3 dBm (200/230 µm)
Wavelength 850 nm
Transmission length incl. 3 dB system reserve 2800 m (with F-K 200/230 8 dB/km with quick mounting connector)
4200 m (with F-G 50/125 2.5 dB/km)
3300 m (with F-G 62,5/125 3.0 dB/km)
Transmission medium PCF fiber
Multi-mode fiberglass
Transmission protocol Protocol-transparent to the RS-485 interface
Connection method B-FOC (duplex ST®)
Bit delay ≤ 1 bit
Bit distortion, input ± 35 % (permitted)
Bit distortion, output < 6.25 %
Electrical isolation VCC // RS-485
Test voltage data interface/power supply 1.5 kVrms (50 Hz, 1 min.)
Electromagnetic compatibility Conformance with EMC Directive 2014/30/EU
Noise emission EN 55011
Net weight 210.08 g
Housing material PA 6.6-FR
Color green
MTBF 159 Years (Telcordia standard, 25°C temperature, 21% operating cycle (5 days a week, 8 hours a day))
24 Years (Telcordia standard, 40°C temperature, 34.25% operating cycle (5 days a week, 12 hours a day))
Conformance CE-compliant
ATEX II 3 G Ex nA nC IIC T4 Gc X (Please follow the special installation instructions in the documentation!)
II (2) G [Ex op is Gb] IIC (PTB 06 ATEX 2042 U) (Please follow the special installation instructions in the documentation!)
II (2) D [Ex op is Db] IIIC (PTB 06 ATEX 2042 U) (Please follow the special installation instructions in the documentation!)
UL, USA/Canada Class I, Zone 2, AEx nc IIC T5
Class I, zone 2, Ex nC nL IIC T5 X
Class I, Div. 2, Groups A, B, C, D
Digital outputs
Output name Relay output
Output description Alarm output
Number of outputs 1
Maximum switching voltage 60 V DC
42 V AC
Limiting continuous current 0.46 A
Power supply
Nominal supply voltage 24 V DC (With UL approval)
Supply voltage range 18 V DC … 30 V DC
Max. current consumption 130 mA
Typical current consumption 120 mA (24 V DC)
Connection method COMBICON plug-in screw terminal block
Serial interface
Interface 1 RS-485 interface, 2-wire
Operating mode Semi-duplex
Connection method Pluggable screw connection
File format/coding UART (11/10 bit switchable; NRZ), slip-tolerant
Data direction switching Automatic control
Transmission medium Copper
Transmission length ≤ 1200 m (depending on the data rate, with shielded, twisted data cable)
Termination resistor 390 Ω
220 Ω
390 Ω (Can be connected)
Conductor cross section solid min. 0.2 mm²
Conductor cross section solid max. 2.5 mm²
Conductor cross section flexible min. 0.2 mm²
Conductor cross section flexible max. 2.5 mm²
Conductor cross section AWG min. 24
Conductor cross section AWG max. 14
Serial transmission speed 4.8/ 9.6/ 19.2/ 38.4/ 57.6/ 75/ 93.75/ 115.2/ 136/ 187.5/ 375/ 500 kbps
Width 35 mm
Height 99 mm
Depth 105 mm
PSI-MOS-RS485W2/FO 850 T
Environmental Product Compliance
China RoHS Environmentally Friendly Use Period = 50
Reach and RoHS Compliant Reach and RoHS Compliant
cUL Listed
cULus Listed
UL Listed
cUL Recognized
cULus Recognized
UL Recognized
Commercial data
Packing unit 1
Weight per piece 249.0 g
Country of origin Germany
Warranty 1 Year
eCl@ss 4.0 27230207
eCl@ss 4.1 27230207
eCl@ss 5.0 27230207
eCl@ss 5.1 27230207
eCl@ss 6.0 27230207
eCl@ss 7.0 27230207
eCl@ss 8.0 19179290
eCl@ss 9.0 19179290
ETIM 2.0 EC001423
ETIM 3.0 EC001423
ETIM 4.0 EC001423
ETIM 5.0 EC000310
ETIM 6.0 EC000310
UNSPSC 6.01 30211506
UNSPSC 7.0901 39121008
UNSPSC 11 39121008
UNSPSC 12.01 39121008
UNSPSC 13.2 43222604

Application Diagrams

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

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