Rainbow Bar

Model 375
100Base-TX to Fiber Optic

Rainbow Bar

Reference Manual

0315-0293 Rev. C


Printed 10.31.01

Table of Contents

1.0 Introduction
1.1 Overview (Distance Limitation) 
1.2 The Model 375 Fiber Optic Transceiver vs. the Two-Port Repeater 
1.3 Full Duplex Considerations
1.4 Features

2.0 Installation
2.1 Cabling Considerations
2.2 Location
2.3 Connections

3.0 Operation
3.1 Status LEDs
3.2 Starting Up the Model 375
3.3 Diagnostic Checks

4.0 Model 375 Device Specifications
5.0 Help

1.0 Introduction

The Model 375 Fiber Optic Transceiver provides users with an inexpensive means to increase the distance between 100Base-TX compliant Local Area Networks (LANs) while utilizing network connections which are safe from electrical interference. It also frees the user from the problems associated with copper wiring pairs where quality is not sufficient for error-free transmission of data at 100 Mbps. The Model 375 converts 100Base-TX compliant signals to a fiber optic format and vice versa. This enables users with 100Base-TX terminal equipment to connect via fiber optic link segments that can be extended to a distance of nearly 2 km (for longer distances see section 1.3, "Full Duplex Consideratons"). The current 100Base-TX standard limits copper link segments to 100 meters without the use of repeaters. In addition to this length extension, a Model 375-equipped LAN gains the advantages of fiber optic communications by minimizing concern for ground loops, power surges, lightning strikes and electrical interference from nearby equipment.

The Model 375 twisted-pair-to-fiber fiber optic transceiver allows any two 100Base-TX compliant ports to be connected by fiber optic cable. A properly installed unit converts between electrical signals and optical signals, while assuring that collision information is preserved and translated from one segment to the other. The operation of the device is transparent to the network.

Because the 100Base-TX port of a Model 375 is compliant with IEEE 802.3, it may be attached to any other compliant 100Base-TX port. Two Model 375s can be used as a pair to provide an end-to-end 100Base-TX compliant system.

The Model 375 is offered in two versions for use with multi-mode fiber: 
Model 375SC With SC fiber connectors 
Model 375ST With ST fiber connectors 
The Model 375 is also offered in three versions for use with single-mode fiber:

Model 375SC _ SM With SC fiber connectors 
Model 375ST _ SM With ST fiber connectors 
Model 375ST _ SM_1 With ST fiber connectors and extended distance

1.1 Overview (Distance Limitation)

In order to understand the Model 375 Fiber Optic Transceiver, it is necessary to consider some of the features of the underlying 10Base and 100Base standards. Devices communicate with each other using a serial bit stream. Prior to beginning a transmission, the device initiating communication monitors the link to determine its status. If the status is active, transmission does not occur. There may be other devices that attempt transmission at approximately the same time. If more than one device begins to transmit at the same time, a collision occurs. The 10Base and 100Base standards account for this event. Each transmitting device continues to monitor the status for a period of time. If the device senses activity (a collision) during this period a recovery procedure is initiated.




Slot Time

512 BT (51.2msec)

512 BT (5.12msec)

InterFrame Gap

9.6 msec

0.96 msec

Table 1: Network Specification Parameters

The 100Base standard was developed to closely emulate the 10Base standard, simplifying the transition between them. This is seen in Table 1, which shows some standard operating parameters for 10Base and 100Base systems.

The Slot Time is the time after a device begins a transmission that it listens for a collision. The InterFrame Gap is the amount of time after the link becomes inactive that a device must wait before transmitting data on the link. As can be seen from the table, the Slot Time is defined by the units Bit Time (BT), which represents the rate of transmission.

For a 10 Megahertz 10BaseT system, the Bit Time is 100 nanoseconds. For a 100 Megahertz 100BaseT system, the Bit Time is 10 nanoseconds. Therefore, the Slot Time for a 10BaseT system is 51.2 microseconds and the Slot Time for a 100BaseT system is 5.12 microseconds.

The Slot Time is an important parameter in the design of a network. This is related to the amount of delay between the transmitting device and the furthermost receiving device. The following figure illustrates this.

Figure 1: 10Base/100Base System

In Figure 1, assume that Unit A (which is a conforming Data Terminal) is just starting to transmit data over the attaching link to Unit B (another conforming Data Terminal). The transmitted data travels along the link in time TA. Also assume that some time after Unit A begins to transmit, Unit B (assuming the line is not busy) also begins to transmit. The problem occurs when Unit B begins to transmit just before the data from Unit A arrives. When the Unit A data arrives at Unit B, Unit B knows immediately that a collision has occurred and can begin recovery operations. However, Unit A will not know there has been a problem until the data from Unit B arrives (TB). Therefore, Unit A has to wait at least TA + TB before it senses that no collision occurred during it's transmission. The standards recognize that some time is necessary to sense the collision, so some additional time (TC) is added. Therefore, the Slot Time is the sum of TA, TB, and TC. TA and TB represent the round-trip delay of the cabling network and are equal. If the delay characteristic of the media used is known, the maximum cable length can be determined from the time delay, TA. For example, twisted-pair CAT 5 cable has a delay of approximately 7-8 nsec/m.

In 100 MHz systems, cable distances are effected both by collision detection requirements and signal integrity. In a fiber system using 100Base-FX, signal integrity is not compromised and therefore the distance is determined only by collision detection. When using twisted-pair wiring, signal degradation limits cable distance to 100 meters. A special device called a repeater is necessary to extend the distance allowed beyond 100 meters. However, because a repeater itself adds delay, the allowable distance is only slightly increased. Table 2 lists the cabling distances from the 100Base standard using repeaters for 100Base-Tx and 100Base-Fx. The Model 375 Fiber Optic Transceiver is designed to extend the distance further than a repeater by keeping the inserted delay to a minimum.


Copper Transmission Media Network Diameter*

Media Network


From Data Terminal to Data Terminal



Two Segments between Data Terminals with one CLASS I repeater in series



Three Segments between Data Terminals with two CLASS II repeaters in series



Table 2: 100Base-T Maximum Cabling Diameter Specifications

* The diameter is equivalent to the maximum end-to-end distance.

1.2 The Model 375 Fiber Optic Transceiver vs. the
Two-Port Repeater

A two-port repeater enables a user to extend the operational distance of a 100Base-TX segment. However, the total time delay indicated by the Slot Time (see section 1.1, "Overview") is still in effect. Therefore, the distance allowed may not be extended indefinitely without arriving at either a Data Terminal port or a Bridge Port as shown in Table 2. Without a repeater, the segment length is limited to 100 meters because of degenerative effects on the signal. With the addition of a single repeater, the segment length can be effectively extended to 200 meters. However, further extension of the effective segment length is not possible.

The reason for the severe limitation on extension of the effective segment length using a repeater is due to the functions it must perform. A repeater is responsible for signal restoration of both amplitude and timing components of the incoming signal. This involves synchronizing to the signal after restoring the amplitude and regenerating the signal for further transmission. It is also responsible for detecting collisions and taking action when they are discovered. As such, each repeater added to a network performs a useful service, but requires time to perform the stated responsibilities. For the 100Base standard, there are two types of repeaters defined: CLASS I and CLASS II. CLASS I repeaters are intended to be used to switch between different transmission formats (100Base-TX, 100Base-FX and 100Base-T4). The allowed time delay for a CLASS I repeater is 168 Bit Times (1.68 microseconds). The allowed time delay for a CLASS II repeater is 92 Bit Times (0.92 microseconds). These added time delays effectively remove the ability for the extension of the effective length segment beyond 200 meters for a 100Base-TX network using repeaters.

The Model 375 is not a 100Base repeater. It does not perform timing restoration nor does it sense collisions. The Model 375 converts the 100Base-TX signal to a fiber signal which may be sent over a longer distance than is allowed by either a direct 100Base-TX connection or one using repeaters. A Model 375 adds much less time delay than a repeater because the repeater functions are not included. A Model 375 requires five Bit Times to convert the twisted-pair signal into fiber and five Bit Times to convert the fiber signal into twisted pair. Therefore, a Model 375 requires ten Bit Times to transmit from one twisted pair to the other twisted pair. If two Model 375s are used as a pair, the total round-trip added time delay is approximately 20 Bit Times. Because a Model 375 does not perform timing restoration, it should not be used with long 100Base-TX segments. However, it can be used with long fiber segments.

1.3 Full Duplex Considerations

The transmission medium in Figure 1 was shown as a single bi-directional segment. In practice, the 100Base-TX and 100Base-FX standards are really two parallel single-direction segments. This architecture can therefore accommodate full-duplex operation where collisions would not occur. The current standard does not define how this full-duplex feature would be utilized. In a full-duplex network, there is no need to consider the time limitations imposed by collision considerations. A Model 375 Fiber Optic Transceiver can take advantage of full-duplex operation to allow much longer fiber segments. The standard Model 375 may be used in full-duplex networks with fiber segments of up to 2 km. An optional fiber transmitter in the Model 375SM-1 allows the fiber segments to be extended to up to 90 km. Additional full-duplex considerations are beyond the scope of this manual.

1.4 Features

The Model 375 Fiber Optic Transceiver has the following features: 
Translates signal activity information between twisted-pair and fiber, including collision information 
Eight diagnostic LEDs provide information about device operation and network status 
Compatible with IEEE 802.3 specification for 100Base-TX 
Maintains the full-duplex nature of the 100Base-TX architecture 
Allows the extension of the distance between DTE ports from 100 meters to approximately 400 meters in collision-sensing systems and extends the distance to 500 meters for full-duplex systems SC or ST fiber connectors 
Can detect a Near-End Fault and transmit the detection to the other end, which will then indicate a Far-End Fault. 
Easy to install and operate


2.0 Installation

2.1 Cabling Considerations

Section 1.0, "Introduction," gave useful background in determining the cabling lengths in a network system. When using a Model 375, it is important to remember that 400 meters of fiber (approximately two microseconds of delay in one direction) is allowed in a collision-sensing system with no repeaters. If repeaters are added to the network, they will detract from the total cable length allowed. Table 3 shows the pertinent quantities involved in determining the cable length in a network.


Time delay

Total Network Time Delay

5.12 microseconds

Category 5 Cable Delay

7.5 nanoseconds (app.)/m

Multimode Fiber Delay

5 nanoseconds (app.)/m

Collision Sensing Time

1.12 microseconds (app.)

CLASS I Repeater Delay

0.92 microseconds

CLASS II Repeater Delay

1.68 microseconds

Model 375 Fiber Optic Transceiver Delay

50 nanoseconds

Table 3: Pertinent Factors in 100Base-TX Network Design

The total time allowed in a network is 512 Bit Times (5.12 microseconds). You must allow for the time required to sense a collision (sensing time) so the amount of time remaining for your total network delay is approximately 400 Bit Times (4 microseconds). The following examples may be helpful in designing your network connection.

Example1: Two data terminals and one interconnecting fiber segment using a pair of Model 375s.


Figure 2 - Configuration for EXAMPLE 1

In this example, a Model 375 Fiber Optic Transceiver is connected with a short (1 meter) 100Base-TX crossover cable to each data terminal. A fiber cable pair is connected to these same Model 375 units to complete the circuit. The 100Base-TX cable delay totals 15 (7.5 x 2) nanoseconds. The pair of Model 375s add a total delay of 100 (50 x 2) nanoseconds. Both the cable and the transceivers are in the round-trip portion of the circuit and must be added twice. Adding in the required collision-sensing delay of 1.12 microseconds, the total time delay excluding the fiber cable is 1.35 (1.12 + 0.1 + 0.1 + 0.015 + 0.015) microseconds. As the total delay allowed is 5.12 microseconds, the maximum fiber cable delay is 3.77 microseconds. This quantity represents the round-trip delay. Therefore, the one-way delay is 1.885 microseconds, which is equivalent to approximately 377 meters of fiber. For this example, the total network diameter is 379 meters.

Example 2.
Two data terminals and one CLASS II repeater using a pair of Model 375s.

Figure 3 - Configuration for Example 2


A 100Base-TX repeater is added to the first example. This repeater is connected with an additional 1-meter CAT 5 cable. The total delay excluding the fiber segment is 1.35 microseconds (from above) plus 15 nanoseconds for the additional CAT 5 cable delay plus the repeater delay. The repeater delay in the standard already accounts for round-trip considerations. The repeater delay is therefore, 0.95 microseconds. The total delay excluding the fiber segment is 2.315 microseconds. Subtracting from the maximum system delay, the total remaining delay is 2.805 (5.12 - 2.315) microseconds. This delay represents a fiber cable of 280.5 meters. For this example, the total network diameter is 283.5 meters.

Example 3. Two data terminals and two CLASS II repeaters using a pair of Model 375's.

Figure 4 - Configuration for Example 3


Another 100Base-TX repeater is added to Example 2. Assuming the same length CAT 5 cable is added along with the repeater, the additional delay excluding the fiber length is 0.965 (0.95 + 0.015) microseconds. This would be subtracted from the 2.805 microseconds of remaining delay above to yield 1.84 microseconds. This delay represents a fiber segment of 184 meters. For this example, the total network length is 188 meters. This example shows the combination of repeaters and Model 375s is actually worse than repeaters alone. This would not be an example of a system where a Model 375 would provide additional network diameter. It would, however, allow the user to benefit from a fiber optic connection as described in section 1.0, "Introduction."

2.2 Location

The Model 375 should be located close enough to an AC power source to permit the use of the supplied power module. In addition, it should be placed in an area that is well ventilated and away from electrically noisy equipment. Unshielded twisted-pair connections should be routed so that the effects of interference from other power or data connections are minimized.

2.3 Connections

2.3.1 Power

The power module supplied with the Model 375 provides 12 Volts DC at 500 mA to the device through the power connector located next to the RJ-45 connector. The module plugs into a standard 115-Volt, 60 Hz AC power source. An optional 220-Volt 50 Hz power module is available. There is no ON/OFF switch on the device. When the power is connected to the device, the device is ON.

2.3.2 Fiber Optic Connector

The Model 375SC is available with duplex SC-Type connectors or with two ST-Type connectors as the Model 375ST, depending on the interface to the fiber optic link segment. In addition, the Model 375 may be ordered as the Model 375SMSC or 375SMST for full-duplex installations requiring single-mode fibers. Do not remove the covers on the fiber connectors until you are ready to connect the fiber cables. The fiber optic cables must be terminated with the correct connectors. It is important when dealing with fiber optic cables to ensure that the TX on one end of the link is connected to the RX at the other end of the link. Some duplex fiber optic cables are coded to help monitor the direction of data travel. If the fibers are not coded, special attention must be paid to ensure a proper connection.


2.3.3 Twisted-Pair Connector

The Model 375 provides an RJ-45 connector to interface to the twisted-pair link. It is important to note that THE MODEL 375 DOES NOT HAVE AN INTERNAL CROSSOVER. This means that on the MODEL 375, Pins 1 and 2 are outputs, and Pins 3 and 6 are inputs. Therefore, if connecting to a DTE, a crossover cable will probably be necessary. However, if connected to a repeater, a straight-through cable will probably be necessary, as most repeaters have an internal crossover. The user should check the cable specifications for the device to be attached to the other end of the twisted-pair link to ensure proper operation. The following chart and figure describes the Model 375 pin requirements.

i The Model 375 is supplied with a crossover cable.

Model 375 RJ-45 Connector Pin Definitions

RJ-45 Pin Number

M375 connection from repeater with crossover

M375 connection from DTE or repeater without crossover


Transmit +

Receive +


Transmit -

Receive -


Receive +

Transmit +








Receive -

Transmit -







Table 4: Twisted-Pair Connector Pin Definitions

  Definitions: A straight-through cable uses a "straight connection" on both ends of the cable attaching a Model 375 to a TX-compatible unit. A crossover cable provides a "crossover connection" between the ends of the cable.

3.0 Operation

3.1 Status LEDs

Fiber TD and RD: These LEDs are lit when data is being transmitted or received by the Model 375 at the fiber optic interface.

Fiber Link Monitor: This LED is lit when the input power at the RX port exceeds a certain minimum level. If the input power falls below the minimum level, the LED goes off indicating that there is no valid link to the RX fiber port. The Fiber Link Monitor LED also goes off when its Far End Fault LED is ON indicating that the other end is not ready to receive.

Twisted-pair TD and RD: These LEDs are lit when data is being transmitted or received by the Model 375 at the 100BaseT interface.

Twisted-pair Link Monitor: This LED is lit when the input power at the RX port exceeds a certain minimum level. If the input power falls below the minimum level, the LED goes off indicating that there is no valid link to the RX twisted-pair port.

Near End
Fault: The Near End Fault LED is illuminated when there is an error detected at the local unit. An error condition could be a result of loss of the 100BaseT signal or a loss of signal from the fiber optic interface. When the Near End Fault LED is turned on it will transmit a Far End Fault condition to the remote end.

Far End Fault: This LED will be lit when a Far End Fault has been detected from the remote end. This indicates that the remote end has lost either communication from the 100BaseT at the remote end or the fiber optic signal from the local end is not being received. If communication from the remote end is lost, a Far End Fault condition cannot occur.

3.2 Starting Up the Model 375

Attach the power connector to the media converter and plug the power adapter into the wall outlet to turn the device on. Connect the twisted-pair cable and fiber optic cables. Set the Data Terminals to an operational condition. Check the Model 375 status LEDs for proper operation. Both link lights should be on, indicating a good link from the Model 375 to the computer, and between the Model 375s. If one or both of these lights are off, check the twisted-pair and/or fiber cable connections.

3.3 Diagnostic Checks

3.3.1 Fiber Connection

When the fiber optic link is connected properly, the Fiber Link Monitor LED should be lit, except when the Far End Fault LED is on. If the Fiber Link Monitor LED is not lit, check to see that the RX on one end of the link is connected to the TX on the other end, and vice versa. Ensure that the connecting Model 375 unit is on. If the link is still not operating, check the continuity of the fiber, and ensure that the fiber connectors are clean. In the case where the Far End Fault LED is on, check the connecting Model 375's Fiber Link Monitor LED or fiber and 100BaseT connections. The Far End Fault LED indicates a good connection from the connecting Model 375's fiber TX to another Model 375's fiber Rx, and it indicates that there is a problem at the connecting Model 375's end.

3.3.2 Twisted-Pair Connection

When the twisted-pair link is connected properly, the Twisted-pair Link Monitor LED should be lit. If it is not, check to see that the twisted-pair cable is such that the transmit pins (1,2) for the Model 375 are connected to the receive circuitry on the DTE port. Then check that the receive pins (3,6) for the Model 375 are connected to the transmit circuitry on the DTE port. In other words, check the specifications of the DTE to determine if a crossover cable is necessary. Ensure the DTE equipment is operational.

3.3.3 Normal Operation

When all connections are made to the fiber optic interface and the twisted-pair interface, the Fiber Link and the Twisted-Pair Link LEDs are on. The Far End and Near End Fault LEDs will be off and the TD and RD LEDs will blink as data is being transmitted and received.

4.0 Model 375 Device Specifications

Loss Budget:

Model 375SC(or ST)

14 dBm avg.

Model 375SC(or ST)-SM

20 dBm avg.

Model 375ST-SM-1

37 dBm avg.


50 nsec (Typical)

Calculated Max. Fiber Cable Length:

Model 375SC(or ST)

2 km

Calculated using Multi-mode fiber with an attenuation of 1.0 dBm/km avg. and a bandwidth-length product of 5 GHz-km.

Model 375SC(or ST)-SM

30 km

Calculated using Single-mode fiber with an attenuation of 0.35 dBm/km avg. and a bandwidth-length product of 83 Ghz-km.

Model 375ST-SM-1

90 km

Calculated using Single-mode fiber with an attenuation of 0.35 dBm/km avg. and a bandwidth-length product of 212 Ghz-km.

Twisted Pair Output

100Base-TX Compliant


2.62"W x 4.18"L x 1.25H


66.6mm x106.3mm x31.7mm


2.5 oz.

Power Supply

12V @ 500 ma (Max.)


Fiber Link TD (green)


Fiber Link RD (green)


Fiber Link Valid (green)


Twisted Pair TD (green)


Twisted Pair RD (green)


Twisted Pair Valid  (green)


Near End Fault (red)


Far End Fault (red)

Fiber Optic Connectors

ST or SC, ordering option

Twisted Pair Connector

Comply with 100Base-TX


5.0 Help

If you require assistance, please call Telebyte Customer Service at

(631) 423-3232, fax us at (631) 385-8184 or e-mail us at support@telebyteusa.com


TELEBYTE 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. TELEBYTE's sole obligation under this warranty shall be to furnish parts and labor for the repair or replacement of products found by TELEBYTE 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. TELEBYTE factory and will be redelivered by TELEBYTE freight prepaid, except for non-continental U.S.A. locations. Non-continental deliveries will be sent COD freight plus import/export charges.

The above warranty is in lieu of all other warranties, expressed or implied, statutory or otherwise, including any implied warranty of merchantability or fitness for a particular purpose. TELEBYTE shall not be liable for any damages sustained by reseller or any other party arising from or relating to any equipment failure, including, but not limited to consequential damages nor shall TELEBYTE have any liability for delays in replacement or repair of equipment.

Out of warranty equipment may be returned to the Greenlawn, NY 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 (RMA) number is necessary and can be obtained by calling (631) 423-3232 or 1-(800) 835-3298, by faxing (631) 385-8184/7060 or by e-mailing us at support@telebyteusa.com.  You can also visit us on the web at www.telebyteusa.com/rma.htm.  Reference the RMA number on the outside container.

Document No. 0315-0293 Rev. C

Model Product Info
375ST Product Page  Application Note  Specifications
375ST-220 Product Page  Application Note  Specifications
375SC Product Page  Application Note  Specifications
375SC-220 Product Page  Application Note  Specifications
For more information, contact:
Phone: (727) 547-8307
Fax: (727) 547-0687

E-mail: fiberoptics@metrotek.com

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