When two data equipment units are to be connected together and the distance separating them is beyond the maximum specified for their common interface you may resort to extended low capacity cable. When the distance goes beyond the capability of this extended cable or you want the ability to increase the data rate at some future time then you can use signal boosting devices at the terminating ends in order to accomplish the transmission. These signal boosting devices come under a variety of different names. They are referred to as short haul modems, line drivers, or limited distance modems. In this book we will refer to these devices as short haul modems. Examples of some short haul modems manufactured by Telebyte are shown in Figure 11.

Model 221

Model 225

Model 78x

Figure 11: Examples of Telebyte’s Short Haul Modems

Short haul modems differ in many attributes. It is often a challenge to pick the correct one for your application. Such modems differ with respect to: media driven, data rate supported at a specific distance, ability to serve asynchronous or synchronous traffic and being full duplex or half duplex. Some transfer data only. Others transfer data and control signals.

Along with this short haul modems differ in the way they are powered. Some short haul modems receive their power from a wall transformer while others receive power from a host computer. Still others utilize power stealing and receive power from the interface signals themselves. The modems may also differ with respect to isolation. Finally, short haul modems have different types of indicators and diagnostic capabilities.

I hope to lead you through the variety of choices available for short haul modems. Most importantly, I hope to provide you with a STEP BY STEP METHODOLOGY for choosing the modem appropriate to your application and avoiding confusion. In any case if you still get confused then Telebyte’s excellent customer service staff will help you. They are only an "800" telephone call away.


As a first step when presented with the need for a short haul modem take a look at the application! You need to answer a few simple questions:

Is the data to be transmitted asynchronous or synchronous (async or sync)? The answer allows you to choose either async or sync modems. Async modems cannot handle synchronous data but some sync modems can operate in an async mode, but they generally cost more. However, many people with a mixed async/sync traffic environment on their premises prefer to use only one type of modem and this leads them to using synchronous modems for all traffic even at a cost penalty.

Take a second look at the application! Do you need only one modem at each location or at one of the locations do you need multiple modems? The application may be just connecting one data equipment unit to only one other unit or it might be transferring data from one equipment unit to multiple sites. If you just need one modem at each location this leads you to the immediate choice of stand-alone modems. On the other hand if at one of these locations you need multiple modems to handle multiple simultaneous transmissions you can use rack mounted modems. This may occur if a multi-user computer system is at one location. These offer more features and are more convenient than having an aggregate of individual stand-alone modems grouped together. Furthermore, you usually are able to use one AC power source for all modems in the rack.


Take a look at the environment in which communication is to take place! Is it an ordinary office on a single floor? Is it an entire office building where wires may have to be run along side of other electrical conductors through an elevator shaft? Is it a factory where heavy electrical equipment is operating, such as welding equipment, stamping machinery and the like?

If it is an ordinary office then it is probably benign relative to electrical interference such as EMI (Electro-Magnetic Interference) and ground loops. A benign environment does not really drive the choice of the modem. However, if it is one of the environments, where you have interference problems, there is an impact on modem choice. Interference influences the type of communication taking place.

Let’s consider this non-benign situation where there is a possibility of EMI, common mode interference etc. In a non-benign environment you must consider the wiring of the building, the media at the outset. If the existing wiring is old you should consider replacing it. If you can afford it consider fiber optic cable. This cable allows the greatest accommodation to growth in traffic and provides the best protection against EMI. This is especially true in a heavy industrial environment. The use of fiber optic cable leads you immediately to choose short haul modems with fiber optic interfaces. If replacement with fiber optic cable is not possible make sure that you have twisted pair cable with shielding in this non-benign environment. But, make sure that the shielding is properly grounded. Use an optically isolated modem. This eliminates ground loops. Since Telebyte’s Series 70 OPTICALLY ISOLATED short haul modems use differential current to drive the wire they can also protect against EMI and common mode interference.

Take another look at the environment in which the communication is to take place! If the communication is between two buildings then there is the potential for damage to equipment from lightning strikes. Even with buried wire a lightning strike one mile away may generate ground currents which can severely damage equipment. Choose a modem with built-in lightning protection or if you cannot find the appropriate modem obtain one within the constraints of the choices outlined above and use external lightning protection. Such protection devices will be discussed in Chapter 6.


Next carefully look at your computer application in order to determine the maximum speed at which the data has to be transferred in order to run efficiently. Also look at the distance between the communicating equipment. Various vendors have tables indicating the speed versus distance capability of their modem products. Look at the interface required to the data equipment. See if it is the standard DB25 pin connector associated with an EIA-232C serial port or if it is a 9 pin DB9 connector used in PC AT or 386 com ports. Under the constraints of the considerations of the above paragraphs pick out one of Telebyte’s modem products.

Short haul modems may be either AC powered or host powered. AC powered modems require a wall transformer, the use of an outlet and are usually more expensive. However, They always have the correct power level for the internal components. They provide better performance for a wider mix of equipment and can support LED indicators.

Host powered modems are divided into two types, Control Signal Powered types and Data Powered types. The Control Signal Powered modem derives power from the control signal on the interface pins CTS/RTS or DSR/DCD/DTR. The Data Powered modem derives power from the data pins, TD and RD. Host powered modems are less expensive than AC powered modems and do not need a wall outlet. Data powered modems are typically more expensive than Control Signal Powered Modems since they require additional circuitry to generate the DC voltages from the Transmit Data signal. Furthermore, they only provide reliable communication with data equipment if the voltage levels are known to be well within operating range required by the modems.

Host powered modems cannot afford to use any power for LEDs therefore they are not utilized. If these visual aids are necessary AC powered units should be used.

Finally, look at the application and the need to transport either data only or data and handshake signals. Some short haul modems will only transmit data. Others will transmit both data and one or more control signals such as CTS, RTS etc. The control signals change at a rate far below the data rate. Their transmission allows handshakes to be effected. Usually, their transmission is executed by some type of "side channel" which of course adds to complexity and cost.

Short Haul Modem Selection Guide

Protocol Wire
or Fiber
Full Duplex &
of Wires
of LED's/


72A ASYNC W No 19.2 4 2 AC SA
201 ASYNC W No 19.2 4 --- TD SA
203 ASYNC W No 19.2 2 --- TD SA
209F ASYNC W No 19.2 4 --- TD SA
214A ASYNC W No 38.4 2 1 AC SA
224 ASYNC W Yes 19.2 4 LCD AC SA
225 ASYNC W Yes 19.2 4 LCD TD SA
227 ASYNC W No 115.2 4 LCD TD SA
271 ASYNC F No 56 2 Fibers --- AC SA
272 ASYNC F No 2500 2 Fibers --- AC SA
274 SYNC/ASYNC F Yes 256 1 Fiber 6 AC SA
276 ASYNC F No 1000 2 Fibers --- AC SA
277 ASYNC F No 1000 2 Fibers 4 AC SA
78X T1/E1 F No 2 M 2 Fibers 7 AC SA
8277 ASYNC F No 1 M 2 Fibers 4 DC DIN

Pick the Modem That's Best for Your Application
1. All standalone Short Haul Modems except Models 209 are available with male or female RS-232 connectors.

Please specify M or F when ordering.
Model 209F is a DB9 female connector to match the PC com port of a PC

2. Power Source: TD = Transmit Data, PS = Power Stealing (takes power from pins 4,5,6,8, or 20), DC = Host Powered, AC = Self Powered (comes with a wall mounted transformer.)

3. Physical: Standalone (SA) or Rack Mount (RM)



We have said that a short haul modem is essentially a signal boosting device for connecting data equipment over long premises distances. There are a number of devices similar to the short haul modem but specifically oriented to the needs of the Personal Computer (PC). Basically, they allow the PC to operate remotely from its attached, external components. These are usually referred to as Printer, Mouse, Video Display or Keyboard Extenders. They are not widely available but they are available. They are principally used on the trading room or factory floor where the PC user may be quite remote from the PC itself.

Please make a selection.