Model 277 - Multimode Fiber Optic Modem


Reference Manual

Table of Contents

1.0 Equipment Description
2.0 Specifications
3.0 Equipment Operation
4.0 Indicators
5.0 Help

1.0 Equipment Description

1.1 General
The Model 277, Multimode Fiber Optic Modem, is an asynchronous fiber optic modem. The optical interface can operate in either point-to-point or ring configurations. The interface employs two "ST" type connections. The unit operates on fiber optic cables with core diameters from 50 to 100 microns.

The electrical interface operates in either point-to-point or multidrop configurations. When configured as RS-232 or RS-422 the unit will operate point-to-point. With the RS-485 interface selected, equipment can be multidrop connected, onto low cost twisted pair wire. See Figure. This interface can be connected directly to the customers equipment or to a Metrotek Model 285 Superverter™. The versatility of the two interfaces of the Model 277 allows for extremely flexible system design.

1.2 Optical Interface
In an optical point-to-point configuration, two Model 277's are connected back to back to form a simple high speed full duplex optical link. In optical ring operation, 2 or more 277's are cascaded in a ring. A ring consists of one master modem and 2 or more slaves. See Figure below. The slaves pass the received optical data, along with the transmit data from their electrical interface, to their optical transmitters. The master does not pass the received optical data. Master/slave modes are switch selectable. Master mode is selected for both modems in an optical point-to-point configuration.

The Model 277 optical interface can form a ring of up to 10 units. This allows for a maximum optical system loss budget of 120 dB. In order to extend the optical distance between any two Model 277's, Model 277's can be inserted in the optical path to act as line extenders. Slave mode should be selected for optical line extenders. See Figure.

Note: The Model 277 uses infrared light for communications. This is not visible to the eye.

1.3 Electrical Interface
The Model 277's electrical interface is switch selectable to RS-232 or RS-422/RS-485 voltage levels. When the RS-232 interface is selected, communications to the electrical interface is full duplex, point to point. With this interface, data can be transmitted onto the optical fiber, for a point-to-point optical link or for an optical ring. Switch selection enables data to flow from the electrical interface to the optical transmitter or can be controlled by a high on RTS. On the electrical interface, a high on the data in pin causes light to be emitted.

Full duplex (4 wire) or half duplex (4 or 2 wire) may be selected when the RS-422/RS-485 interface is selected. The RS-422/RS-485 interface of the Model 277 may operate in a multidropped or point-to-point environment. If RS-422 is selected the electrical interface is point-to-point only. If the customer's equipment has a RS-485 interface it can be multi-dropped directly onto the Model 277's interface by selecting the 4 wire or 2 wire RS-485 mode. If the customer's equipment has a RS-232 interface the equipment can be converted to a multidrop, RS-485 interface, by a Metrotek Model 285. Up to 31 units of the Model 285 can be multidropped onto the same electrical interface of the Model 277. The pinout of the RS-422 or 4 wire RS-485 electrical interface conforms to EIA-530.

1.4 Flow Control
Operation in the RS-485 multidrop or the ring environment requires that all devices connected to the line have some level of intelligence, in order to establish an orderly flow of data. When RS-485 (half duplex) is selected the Model 277 controls the Receive Data line on the electrical interface. When no data (no light on fiber) is received by the optical receiver for 10 microseconds the output of the Receive Data electrical transmitter is tristated. In the RS-485 mode this enables many transceivers to communicate on the same twisted pair. When the RS-485 2 wire mode is selected, half duplex data is transmitted and received on the same twisted pair.

2.0 Specifications

2.1 Optical Interface
Optical connectors - ST™ (trademark AT & T.)
Power Out - > +15 dBuW (32 microWatts), into 62.5/125 um core/clad fiber.

Power Budget - > 12 dB - on 62.5/125 um core/clad fiber
Wavelength - 850 nm, multi-mode
Optical Configuration - Switch selectable, Master/Slave

Electrical connector - DB-25, male
Interface type - Switch selectable -
    RS-232 (pinout conforms to RS-232)
    RS-422, or RS-485 (4 wire pinout conforms to EIA-530)
Bit Rate -
    RS-232 - 0 to 115.2 KBPS
    RS-4XX - 0 to 1 MBPS
Interface Mode - Switch selectable - Unit can be configured to appear as a DCE or a DTE

2.3 General Specifications
Indicators - 4 LED's -
    LTD - Local Transmit Data
    FTD - Fiber Transmit Data
    RD - Receive Data
    COL - Collision
Power - 115 VAC, +/- 10%, (220 VAC, 50 Hz optional)
Environment - 0° to 50° Celsius, 5% to 95% RH non-condensing.
Size - 5" W x 5.3" L x 1.7" H

3.0 Equipment Operation

3.1 Switch Options
The Model 277 is a very versatile piece of equipment. It has many switch selectable options than can be used to tailor it to the user's communications requirements. Two of the switches are located on the front panel. Another 10 can be accessed from the bottom of the unit. The following tables list the switch selections and their functions.

Front Panel Switch Option
Switch Name Function Factory Settings
DCE/DTE DCE - Pin 2 is an input into the unit and Pin 3 is an output.
DTE - Pin 2 is an output and Pin 3 is the input into the unit.
Selects electrical interface. 232 is RS-232 and
422 is RS-422 or RS-485.

Bottom Dip Switches
DIP No. Function Factory Settings
1 ON - Connects pin 8 to pin 20 at the electrical interface. ON
2 ON - Connects pin 6 to pin 20 at the electrical interface. ON
3 ON - Connects pin 4 to pin 5 at the electrical interface. ON
4 ON - Connects pin 19 to pin 13 at the electrical interface. OFF
5 ON - Terminates 4XX data line receiver with 220 ohm resistor.
OFF - 4XX receiver not terminated.
6 ON - Local data transmitted onto fiber controlled by elect. data in only.
OFF - Local data transmitted controlled by data in AND control sig. in.
7 ON - 4 Wire 422 See 4XX table OFF
8 ON - 2 Wire 485 See 4XX table OFF
9 ON - 4 Wire 485 See 4XX table OFF
10 ON - Optical master - For host in ring, or point-to-point modems.
OFF - Optical slave - For slaves in ring, or repeaters.

3.2 Connections
After the switches have been set up properly, the connections to the Model 277 I/O ports must be made. There are 3 groups of connections. These are the fiber optic interface, the electrical interface, and the power interface. A minimum of two Model 277's are required for communications.

The connections to the fibers are through two ST connectors on the rear of the Model 277. These are marked T for optical transmit and R for optical receive. The fibers, between the two communicating Model 277's, must be cross connected. That is the optical transmitters must be connected to the optical receivers. The Model 277's optical power budget is specified as 12 dB, using 62.5 um core fiber. These units may use fibers with other core diameters. The table below specifies the loss budget for the different fibers.

Max Loss
50 8.6
62.5 12
85 14.3
100 16.2
Max. Optical Loss Budget

The connections to the electrical interface are through the DB-25 connector located on the rear of the Model 277. The signals on these pins are shown in the tables below. These signals and their functions and directions vary with the switch settings. The minimum requirement for proper connection is for a TD, RD and GND. The tables below show the signal names and their sources for the various settings of the DCE/DTE switch and the 232/4XX switch.

The Model 277 is powered from an external transformer supplied with the unit. The power transformer is plugged into 115 VAC and converts the 115 VAC to an unregulated DC voltage to be used by the unit. The connection to the transformer, on the Model 277, is located on the rear of the unit. A 220 VAC, 50 Hz transformer is available as an ordering option.

Electrical Interface Pinouts

RS-232, Switch in DCE position
Pin # Signal Function Signal Generated by Customer Signal Generated by Model 277
2 TD X  
3 RD   X
4 RTS X  
1,7 GND    

RS-232, Switch in DTE position
Pin # Signal Function Signal Generated by Customer Signal Generated by Model 277
3 RD X  
2 TD   X
5 CTS X  
1,7 GND    

RS-422/RS-485, 4 wire, Switch in DCE position
Pin # Signal Function Signal Generated by Customer Equipment Signal Generated by Model 277
2 TD+ X  
14 TD- X  
3 RD+   X
16 RD-   X
4 RTS+ X  
19 RTS- X  
1,7 GND    

RS-422/RS-485, 4 wire, Switch in DTE position
Pin # Signal Function Signal Generated by Customer Equipment Signal Generated by Model 277
3 RD+ X  
16 RD- X  
2 TD+   X
14 TD-   X
5 CTS+ X  
13 CTS- X  
1,7 GND    

RS-485, 2 wire, Switch in DCE position
Pin # Signal Function Signal Generated by Customer Equipment Signal Generated by Model 277
2 TD+, RD+ TD+ RD+
14 TD-, RD- TD- RD-
4 RTS+ X  
19 RTS- X  
1, 7 GND    

RS-485, 2 wire, Switch in DTE position
Pin # Signal Function Signal Generated by Customer Equipment Signal Generated by Model 277
3 TD+, RD+ TD+ RD+
16 TD-, RD- TD- RD-
5 CTS+ X  
13 CTS- X  
1, 7 GND    

Note: When connecting to the Metrotek, Model 285, polarities of the TD and RD pins must be reversed.

4.0 Indicators

On the front panel of the Model 277 are four LED indicators. These indicators can be used to see the status of the Model 277 and the user's equipment. Below is a list of the LED's and their functions.

LTD - Local Transmit Data - This LED is used to monitor the data coming into the electrical interface from the user's equipment. For RS-232, a positive voltage on the data entering the Model 277 turns this LED on when either one of the following conditions is met: a) with the DCE/DTE switch in the DCE position pin 2, TD will turn on the LED or b) with the switch in the DTE position pin 3, RD will turn on the LED.

For RS-4XX, and the DCE/DTE switch in the DCE position, a voltage more positive on pin 2, TD+, than on pin 14, TD-, turns this LED on. With the switch in the DTE position the lines monitor are pin 3, RD+, and pin 16, RD-.

Note: In RS-485 2-wire mode when configuring the Model 277 in an optical ring, the master cannot be enabled by the "TD" signal.

FTD - Fiber Transmit Data - This LED turns on when light is transmitted from the optical transmitter. In the slave mode this is the logical "OR" of light coming into the optical receiver, and a high on the data coming from the user at the electrical interface (LTD.) In the master mode this is the same as LTD.

FRD - Fiber Receive Data - This LED is used to monitor the data into the optical receiver. The LED turns on when there is light on the fiber connected to the receiver.

COL - Collision - This LED is used to monitor collisions on the fiber. This indicates a system problem. This LED turns on when the Model 277 is being used in an optical ring and more than one slave is trying to transmit to the master unit. In the master mode this LED is disabled.

5.0 Help

If any assistance is required call Metrotek Customer Service at Phone: (727) 547-8307 or Fax: (727) 547-0687.
Technical Support E-Mail: E-MAIL

Document No. 0315-0207
Rev. A


Metrotek warrants the equipment to be free from defects in material and workmanship, under normal and proper use and in its unmodified condition, for 12 months, starting on the date it is delivered for use. Metrotek's sole obligation under this warranty shall be to furnish parts and labor for the repair or replacement of products found by Metrotek to be defective in material or workmanship during the warranty period. Warranty repairs will be performed at the point of manufacture. Equipment approved for return for warranty service shall be returned F.O.B. Metrotek office and will be redelivered by Metrotek freight prepaid, except for non- continental U.S.A. locations. These deliveries will be sent COD freight and import/export charges.


Out of warranty equipment may be returned to the FL customer service facility prepaid as described above. Return shipping charges will be billed to the customer. The repaired unit will have a 90 day warranty. In those cases where "NO TROUBLE" is found, a reduced charge will be billed to cover handling, testing and packaging.

Whether in or out of warranty a Return Material Authorization number (RMA) is necessary and can be obtained by calling customer service department Phone: (727) 547-8307 or Fax: (727) 547-0687. Reference RMA number on the outside container.

Product Page Application Note Specifications

For more information, contact:
Phone: (727) 547-8307
Fax: (727) 547-0687


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