B&B Electronics FOSTC Datasheet

© 2002 by B&B Electronics. All rights reserved.

www.bb-elec.com [e-mail] [e-mail]

International Office: 707 Dayton Road PO Box 1040 Ottawa, IL 61350 USA 815-433-5100 Fax [phone]

European Office: Westlink Commercial Park Oranmore Co. Galway Ireland [phone] Fax [phone]

PRODUCT INFORMATION B&B ELECTRONICS

TX

RX

FOSTC

RX

TX

FOSTC

RS-232
RS-422

or RS-485

Device

Duplex

Multimode

Fiber

or System

RS-232
RS-422

or RS-485

Device

or System

SW1:6 = OFF SW1:6 = OFF

Point to Point

TX

RX

FOSTC

RX

TX

FOSTC

RS-232
RS-422

or RS-485

Device

Multimode

Fiber

or System

RS-232
RS-422

or RS-485

Device

or System

RX

TX

FOSTC

RS-232
RS-422

or RS-485

Device

or System

RX

TX

FOSTC

RS-232
RS-422

or RS-485

Device

or System

SW1:6 = ON

SW1:6 = ON

SW1:6 = ON

SW1:6 = ON

Multi-Drop Ring

FOSTC-0812-1/6

Model FOSTC

RS-232, 422 or 485 Signals Up To

2.5 Miles with Fiber Optic Modem

Description

Fiber optic cabling has inherent resistance to EMI/RFI and transient immunity, making it ideal for industrial and utility
data communication applications.

The FOSTC was designed to provide the most versatile connection possible between any asynchronous serial
equipment using Fiber Optic cable. The FOSTC can be used for point-to-point communications between serial devices,
or in a multi-drop fiber ring configuration, allowing multiple serial devices to communicate with each other.

It allows any two pieces of asynchronous serial equipment to communicate full or half-duplex over two fibers at typical
distances up to 2.5 miles (4 km). To extend the distance of the fiber link beyond 2.5 miles, use B&B model FOSTDRP
Fiber Optic Repeater.

Features/Applications

Point-to-point or multi-drop ring configuration
RS-232, RS-422, or RS-485 operation
Use as a converter from RS-232 to RS-422/485
RS-422/485 data rates up to 500 kbps
RS-485 Automatic Send Data driver control
Inherent EMI/RFI and transient immunity.
Eliminate ground loops
Extend serial signals up to 2.5 miles
Uses popular ST type fiber connectors
Standard DB25 female (DCE) for serial connections
12VDC powered (separate supply required)

Figure 1: Typical Setups

Fiber Optic Connections

The FOSTC uses a separate LED emitter and photo-detector operating at 820 nm wavelength. Connections to the
emitter and detector are on ST type connectors. Almost any multimode glass fiber size can be used including 50/125

m, 62.5/125 m, 100/140 m, and 200 m. One fiber is required for each connection between a transmitter and
receiver. In a point-to-point configuration, two fibers are required between the two modems, one for data in each
direction. A multi-drop ring configuration requires one fiber between TX and RX around the loop. See Figure 1 for
typical point-to-point and multi-drop configurations.

The most important consideration in planning the fiber optic link is the “power budget” of the fiber modem. This value
represents the amount of loss in dB that can be present in the link between the two modems before the units fail to
perform properly. This value includes line attenuation as well as connector loss. For the FOSTC the typical connector-

© 2002 by B&B Electronics. All rights reserved.

www.bb-elec.com [e-mail] [e-mail]

International Office: 707 Dayton Road PO Box 1040 Ottawa, IL 61350 USA 815-433-5100 Fax [phone]

European Office: Westlink Commercial Park Oranmore Co. Galway Ireland [phone] Fax [phone]

PRODUCT INFORMATION B&B ELECTRONICS

Figure 2: RS-232 Connection Diagrams

DB9 DTE Device

TD

RD
RTS
CTS
DTR
DSR
DCD

Signal GND

3
2
7
8
4
6
1
5

DB9 DCE Device

TD

RD
RTS
CTS
DTR
DSR
DCD

Signal GND

3
2
7
8
4
6
1
5

DB25 DTE Device FOSTC DB25

TD

RD
RTS
CTS
DTR
DSR
DCD

Signal GND

2
3
4
5
20
6
8
7

2
3
4
5
20
6
8
7

or

DB25 DCE Device

TD

RD
RTS
CTS
DTR
DSR
DCD

Signal GND

3 (RD)
2 (TD)
4
5
20
6
8
7

or

FOSTC DB25

FOSTC DB25 FOSTC DB25

2
3
4
5

6
8
7

2
3
4
5
20
6
8
7

3 (RD)
2 (TD)
4
5
20
6
8
7

FOSTC-0812-2/6

to-connector power budget is 12.1 dB. Because 62.5/125 m cable typically has a line attenuation of 3 dB per Km at
820 nm, the 12.1 dB power budget translates into 2.5 miles. This assumes no extra connectors or splices in the link.
Each extra connection would typically add 0.5 dB of loss, reducing the possible distance by 166 m (547 ft.). The actual
loss should be measured before assuming distances.

RS-232 Connections

Connection of the FOSTC is simple and straightforward. The DB25 female serial connector is used for connecting to
either RS-232, RS-422 or RS-485. The RS-232 signals are pinned as a DCE device (input on Pin 2 and output on Pin

3). A straight through cable can be used from your DB25 port on any DTE device such as a PC or terminal. A standard
9 to 25-pin adapter can be used in cases where the serial port on the DTE device is a DB9. A null modem cable or
adapter that swaps pins 2 and 3 is needed for connecting to modems or other DCE devices. See Figure 2 for
connection diagrams to 9 pin and 25 pin DTE and DCE devices. Because RS-422 and RS-485 signals are also
available on the same connector, take special care not to hook any external signals to these pins. This is not a problem
for most serial devices, but a custom cable must be made that does not connect to the extra pins on the DB25
connector if your device has power or special non-standard outputs.

RS-422 & RS-485 Connections

The RS-422/485 driver and receiver are connected to 4 pins on the DB25 connector. Signal ground is on Pin 7. When
connecting to a four-wire RS-422/485 device or system, connect the output of your device to pins 16 (B or +) and 17 (A
or +). Connect the input to your device to pins 14 (B or +) and 15 (A or -). For two-wire RS-485 systems, the driver and
receiver of the FOSTC must be connected together by tying pins 14 and 16 together and 15 and 17 together. This
allows the FOSTC to communicate half-duplex over the same pair. Refer to Figure 3 for connection diagrams to your
RS-422 or RS-485 equipment.

If termination is needed, a spot on the PCBD of the FOSTC labeled Rt allows you to solder in a termination resistor
across the RD(A) and RD(B) lines. Removing R8 and R16 and replacing them with through-hole components can also
change the off-state bias resistor values. Before making modifications to the FOSTC, be sure to consult B&B
Electronics’ free RS-422/485 Application Note or other sources of information to see if termination is necessary. The
Application Note is available from our Web site, or call and we will be happy to send you one at no charge.