Model 273 - Four Channel Fiber Optic Multiplexer


Reference Manual

Table of Contents

1.0 General Description
2.0 Specifications
3.0 Installation
4.0 Troubleshooting
5.0 Help

1.0 General Description

The Model 273 Fiber Optic Multiplexer provides the customer with four, asynchronous, RS-232 ports. Each RS-232 port has one full duplex, high speed, data channel and one full duplex control channel. The 4 data and 4 control signals are combined into a composite signal and sent out over an optical fiber. For full duplex operation, 2 multiplexers and optical fibers are required.

The maximum concurrent data rate for all eight RS-232 ports, in a full duplex pair of multiplexers, is 64 kbits per second. Jumper selection can double the maximum data rate of RS-232 port #1. In this mode port #4 is disabled, enabling port #1 to communicate at a maximum bit rate of 128 kbits per second.

As an aid to installation and system troubleshooting, the Model 273 has a loop test switch on the front panel. Valid receive data on the composite link, are indicated by the SYNC LED. Signals on the RS-232 data lines can be monitored via the 4 Transmit data LED's and the 4 Receive data LED's.

Included with the Model 273 is an external power transformer and 4 Modular Adapters. The Modular Adapters provide for flexibility in connecting the multiplexer interface to the customer's equipment interface.

2.0 Specifications

2.1 Digital Interface

Electrical: Four RS-232; signals supported at each port - TD, RD, Control in, Control out, Ground

Connectors: Four RJ-45

Data Rate: Normal speed mode - 0 to 64 kbits/sec. (sample rate - 390,625 samples/sec.) Double speed mode - 0 to 128 kbits/sec. (sample rate - 781,250 samples/sec.)

Control rate: 0 to 16 kbits/sec. (sample rate - 97,656.25 samples/sec.)

2.2 Optical Interface

Connectors: ST (trademark AT&T)

Power Out: +12 dBuW minimum (15.8 micro Watts) into 62.5/125 micron core/clad fiber

Systems Loss Budget: 12 dB

Wavelength: 850 nanometer, multimode

Composite Baud Rate: 4.1667 Megabaud

2.3 Indicators

11 LED's; Transmit Data 1-4, Receive Data 1-4, TEST, SYNC, POWER

2.4 Power Requirements

115 VAC 60 Hz (220 50 Hz optional)

2.5 Operating Temperature

0° to 50° C, 5 to 95% RH (no condensation)

2.6 Size

7" W x 1.5" H x 5.5" D

3.0 Installation

Installation consists of the following steps:

Connect the user equipment to the Model 273 Digital Interface Ports.
Connect the optical fibers to the Model 273.
Set Double/Norm speed mode jumper (optional - Factory set to Norm).
Set the front panel TEST switch to out.
Connect the AC power transformer.
A more detailed description of each step of the installation procedure, follows below.

3.1 Digital Interface

The digital (terminal) interface to the Model 273 consists of four, 8 pin, RJ45 connectors on the rear of the unit. These are labeled Port 1, 2, 3 and 4. The position of the pins on the RJ45 connector, and their respective signal names, are shown below.

Pin 1 = No Connection  
Pin 2 = No Connection  
Pin 3 = Control Into Mux CI
Pin 4 = Data Into Mux DI
Pin 5 = Data Out Of Mux DO
Pin 6 = Ground GND
Pin 7 = Control Out Of Mux CO
Pin 8 = No Connection  

Digital Interface on Rear of Model 273

For the customer's convenience, Metrotek provides eight Modular Adapters (four male and four female) and four, 25 foot long cables. These adapters enable the interface of the Model 273 to be custom-tailored to the customer's interface requirements. The Modular Adapters consist of a RJ45 connector and a DB25 connector. Male and female DB25 connectors are supplied with the Model 273 Modular Adapters. Joined to the RJ45 connectors are 8 wires terminated with the pins for the DB25 connector. The customer selects the proper signal connections and pushes the self locking pins into the rear of the DB25 connector. See the figure below for Modular Adapter wiring.

NOTE: Looking at the rear of the RJ45 Plug-A-Verter with wire contacts of the RJ45 at the bottom.

Typical Modular Adapter Connections
RJ45 Model 273 DB25 to DCE DB25 to DTE
Pin No. Pin No. Pin No.
3 5 (8) 4 (20)
4 3 2
5 2 3
6 7 7
7 4 (20) 5 (8)

After determining the signal requirements of the equipment to be connected to the Model 273, and wiring the Modular Adapters, the customer may connect the equipment to the digital I/O ports of the two Model 273's. Ports 1, 2, 2, and 4 communicate with Port 1, 2, 3, and 4 respective of the other Model 273.

3.2 Fiber Optic Interface
Connection between a pair of Model 273's is accomplished through 2 fiber optic cables. These cables are connected to the female ST connectors on the rear of the multiplexers. The connectors come with removable plastic caps. The connector labeled T is the output of the fiber optic transmitter and the one labeled R is the input to the fiber optic receiver. The fibers must be cross connected between the 2 multiplexers. That is, the transmitter of one Model 273 is connected to the receiver of the other Model 273. This is done with both fiber optic cables.

The optical system loss budget of 12 dB was give for fiber optic cables with a core diameter of 62.5 microns and a cladding diameter of 125 microns. The system loss budget is the maximum optical loss the Model 273 will work reliably with. This loss is the total of all the losses in the system, including the fiber, the connectors and any splices. The multiplexers will work with other fibers, but the loss budget will be different. Below are the loss budgets for the different fibers.

Core Max Loss
(microns) (dB)
50 8.6
62.5 12
85 14.3
100 16.2

3.3 Double Speed Mode

Port 1 can be made to communicate data at double the bit rate by disabling Port 4. This is called the "Double Mode." The double rate sampling mode is activated by changing a jumper. In this mode the control signal of Port 1 is still sampled at the same rate, and although the data line of port 4 is disabled, the control line of Port 4 is still available for general use.

To change the Model 273 to Double Mode the unit must be opened. To do this:
  1. Disconnect the power transformer.
  2. Remove 2 screws from the bottom of the unit.
  3. Pull off the top shell of the case.
  4. Locate jumper JP2.
  5. Move the jumper to pins 1 and 2 of JP2.
  6. Put top shell back on unit.
  7. Reinstall screws.
3.4 Test Switch

Set the front panel Test switch to the out position. This is the normal operating mode.

3.5 Power Supply

The Model 273 is supplied with an external AC power supply. The supply sent with your equipment is either for 115 Volts or 220 Volts (determined at time of purchase). Check the label on the power supply to make sure that the power supply voltage matches your AC power line voltage.

4.0 Troubleshooting

As an aid to installation and system troubleshooting, the Model 273 ha a loopback test switch on the front panel. When the Test switch is pushed in, the multiplexer sends out a signal to the remote multiplexer. Upon receiving this test signal the remote multiplexer turns on its Test LED and in addition sends a test signal back to the originating multiplexer. When the originating multiplexer receives this signal it also turns on its Test LED. The successfully completed test verifies correct operation of both composite links of the two multiplexers. This test does not interfere with the operation of the data and control lines.

The sampling rate of the Model 273 causes some limitations when looping the received signals directly back to the transmit input at the interface. Some errors may occur if the data or control signals are double sampled. This occurs when the data or control line are sent from one Model 273 and received by the other Model 273 and looped back directly through the RS-232 interface to the originating Model 273. This only occurs with data at or above 60 Kbits/sec. (Normal Rate) and above 120 Kbits/sec. (Double Rate). The control pins should not be looped at a data rate above 14 Kbits/sec.

Valid composite link, receive data , is indicated Sync LED. When this LED turns on, the composite link from the remote multiplexer's optical transmitter, to this unit's optical receiver, is operating correctly. The signals on the RS-232 data lines can be monitored via the 4 Transmit Data LED's and the 4 Receive Data LED's located on the front panel. These LED's turn on when the signal at the interface is at high. The on time of the TD and RD LED's is stretched in order for noisy signals on these lines to be more visible to the human eye.

5.0 Help

If any assistance is required call Metrotek Customer Service at (800)-835-3298 or (631)-423-3232.
Technical Support E-Mail:


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 wharehouse 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 telephone (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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