AMERICAN FIBERTEK MRM-1485 (01) PDF DOCUMENT

The AMERICAN FIBERTEK MRM-1485 operates as half of a transmitter/receiver pair for transmitting baseband NTSC, PAL, RS170, or RS343 video signals and bi-directional RS485 data. It is designed to work with the MTM-1485 or RTM-1485 video transmitter over a single multimode fiber optic cable.

The unit performs two main conversions:

1. It converts an FM modulated optical fiber input (from the transmitter) into a single video output and a single RS485 output using a 1300 nm wavelength detector.

2. It converts an electrical RS485 input signal into an optical RS485 output, sending it back on the same fiber using an 850 nm wavelength source.

The unit can be configured for two-wire half duplex or four-wire full duplex electrical interfaces and is designed for an optical loss budget of 0 to 12 dB on 50 µm or 62.5 µm multimode fiber.

The installation of the AMERICAN FIBERTEK MRM-1485 should be performed by a qualified service person and must conform to all local codes.

Mount the unit to a secure surface using #8 (3mm) hardware in the four available places. Ensure there is enough room to accommodate the required minimum bend radius of the fiber optic cable being used.

The AMERICAN FIBERTEK MRM-1485 requires a +12VDC power source for proper operation. The DC input is diode protected. In the USA and Canada, an American Fibertek PS-12 is supplied with the unit. The negative side of the power input is directly connected to ground. ANSI/NFPA 70 Class 2 wiring is recommended.

Power Connection:

Power is supplied to the unit via a two-pin terminal connector. Follow the label on the unit for the correct orientation of +12 volt DC and ground.

Input/Output Connections:

Fiber Optic: The fiber optic connection is made via an ST connector located on the right side of the unit.

Video Output: The video output connection is made via a BNC connector on the right side of the unit. The 75Ω video output can be looped through other equipment, but it must be terminated with 75Ω for proper operation. For best performance, use the shortest practical length of coax cable.

Data Input/Output: Data connections are made via a terminal block on the right side of the unit. Follow the label on the unit for the proper orientation of RS485 data input and output. Note that RS485 two-wire applications use the four-wire data out (+) and data out (-) terminals.

To access the internal switch and jumper locations for data configuration, you must open the unit:

1. Remove the screw on the bottom of the module.

2. Remove the two screws holding the end panel (the side with the video and data connections) to the chassis.

3. Slide the PCB assembly out of the chassis.

The AMERICAN FIBERTEK MRM-1485 is shipped from the factory in the RS485 4-wire configuration with internal 120 ohm data input and output terminations switched on. To change these settings, you must open the unit and modify the internal switches and jumpers.

Switch #5: 2 Wire / 4 Wire Set

• 4 Wire (Full Duplex): Remove the 2-pin shunts across headers J4 and J5. Set Switch #5 to OFF. (Factory Default)

• 2 Wire (Half Duplex): Jumper the 2-pin headers J4 and J5 with the 2-pin shunts. Set Switch #5 to ON.

Switch #1: Input Data Termination

• 120 Ohm Input Termination: Set Switch #1 to ON. (Factory Default)

• No Input Termination: Set Switch #1 to OFF.

Switch #2: Output Data Termination

• 120 Ohm Output Termination: Set Switch #2 to ON. (Factory Default)

• No Output Termination: Set Switch #2 to OFF.

Switches #3 & #4: Output Data Bias

• No Output Bias: Set Switch #3 and Switch #4 to OFF. (Factory Default)

• Output Bias On: Set Switch #3 and Switch #4 to ON. Note that bias requires termination, so Switch #2 must also be in the ON position.

The AMERICAN FIBERTEK MRM-1485 has three LED status indicators to aid in installation and troubleshooting: OLI, DATA TX, and DATA RX.

OLI (Optical Level Indicator)

This bi-color LED monitors the incoming optical signal power and the unit’s DC power status.

DATA TX LED

This red LED monitors the RS485 input data from the electrical interface that is being transmitted out onto the fiber. The LED will typically cycle on and off as data is transmitted.

DATA RX LED

This red LED monitors the RS485 data being received from the fiber and sent out to the electrical interface. The LED will typically cycle on and off as data is received.

It is very important that copper RS485 communication lines are terminated with resistors across the data wire pair to prevent signal reflections that can interfere with data transmission. The best place to locate these termination resistors is at the furthest electrical devices at the ends of the wire pair. For instance, if several RS485 devices are connected in a daisy chain, the two devices at the very ends of the chain would need to be terminated.

The AMERICAN FIBERTEK MRM-1485 has built-in, switchable termination resistors:

• A termination resistor for RS485 **input** data can be enabled by setting internal switch #1 to ON.

• A termination resistor for RS485 **output** data can be enabled by setting internal switch #2 to ON.

RS485 systems can sometimes be sensitive to noise on the data wires. To improve noise immunity, some equipment manufacturers introduce a small voltage bias to the data lines. The AMERICAN FIBERTEK MRM-1485 allows you to apply this offset bias if needed.

To apply offset bias, set internal switch #3 AND switch #4 to the ON position. These switches must be used as a set; they should be either both on or both off.

Important: If you use offset bias, you MUST also have the termination resistor enabled. Ensure that switch #2 (output termination) is in the ON position. Using offset bias without termination will cause communications to fail. In most cases, bias is not required, and these switches should remain in the OFF position.