Model 72A - Optically Isolated Short Haul Modem
Reference Manual

Table of Contents

1.0 General Description
2.0 Specifications
3.0 Installation
4.0 Operation
5.0 Troubleshooting
6.0 Connector Pin Assignments
7.0 Power
8.0 Help
- Spanish Manual -

1.0 General Description

The 70 Series represents a compatible family of Short Haul Modems (SHM) and support hardware. These devices require metallic circuits with dc continuity end-to-end for proper operation. The circuits may be customer private lines or unconditioned telephone company metallic circuits. The 70 Series are used to extend the distance between two RS-232 based devices beyond the 50 foot limitation of the RS-232 specification. The short haul modem will allow transmission over 10 miles at 1200 Baud and 2 miles at 9600 Baud. Ass SHM's comply with Bell System Specification 43401. Communications between modems can be Full Duplex (simultaneous 2-way) or Half Duplex over two twisted pairs or Simplex (one way) over one twisted pair.

Caution: The unit is NOT intended to operate on home telephone lines and may damage home telephone equipment.

2.0 Specifications

2.1 Common: To all SHM models except as noted.

Interface: Conforms to EIA RS-232c and CITT V.24 specifications. Pin 4 connected to Pin 5: Pins 6, 8 and 20 connected together. Pin 2 and 3 TRANSMIT/RECEIVE DATA switch selectable: Pin 7 Signal Ground. The Model 74 supports active signals on Pins 8 and 20 and does not support Pin 6.

Connectors: RS-232: Models 72, 72AM, 72P, 74 and 74P have male DB-25 connectors. The 725F, 72AF, 72FP, 74F and 74FP have female connectors.

Analog: Models 72, 72F, 74 and 74F have 4 screw terminals; 72P, 72FP, 74P & 75FP have a RJ-11. Models 72AM and 72AF have both 4 screw terminals and a RJ-11.

Transmission: Asynchronous full duplex-two twisted pairs, simplex-one Twisted pair, current loop +12 ma. via optical coupler conforms with Bell System Specifications 43401.

Operating Distance: The following table gives maximum distance as a function of Baud rate.

Baud Rate 19200 9600 4800 2400 1200 600 300
Distance .8 2 3 5 10 10 10

Size: 3.5"L x 2.2"W x 1"H

Environment : 0° to 50° C, 5% to 95% relative humidity

2.2 Power: 115 Volts AC 60Hz (220 Volts 50Hz optional)

3.0 Installation

3.1 Digital Interface

The digital interface for the Models 72, 72AM and 74 SHM's are a male DB-25 pin connector. The Models 72F, 72AF and 74F are equipped with a female connector. The SHM may be attached directly to a computer terminal or printer: or it can be placed in series with a standard 25 conductor cable (on the EIA end).

3.2 Analog Interface

The analog interface features either 4 screw terminals marked T+, T-, R+ and R-, or a RJ-11. (The 72A has both). The T terminal represent the transmit pair whereas the R terminals represent the receive pair.

Cables should be high quality and at least 24 gauge stranded twisted pair and color coded to facilitate installation and repair. Solid conductor wire should not be used on long distances at data rates over 1200 Baud. When installation involves electrically noisy environments, an outside shield is recommended.

The required connections for the analog interface are shown below.

The indicators on the Model 74 can be used to determine the correct connections for the 4 wire line. With both modems connected and no data flowing the TD and RD indicators should be off. If either RD indicator is on the respective tow wires R+ and R- should be reversed. If DTR is on at one modem then DCD should be on at a distant modem.

4.0 Operation

4.1 Common

All SHM's are equipped with a switch which allows the reversing of Pins 2 and 3 of the RS-232 connector. When the switch is in the DTE position, Pin 2 of the RS-232 connector is an output (transmit to host device): Pin 3 of the RS-232 connector is an input. When the switch is reversed to the DCE position, the connection are reversed, i.e., Pin 2 is an input while Pin 3 is an output. When interfacing to a CRT terminal, the switch should be set to DCE.

4.2 Model 74

These remotely controlled SHM's provide for the transmission and reception of handshake signals in addition to standard full duplex data. Normal data transmission is inhibited until DTR (Pin 20) is raised, logic 1. Raising DTR at the local modem will cause DCD (Pin 8) to cause at the remote modem. Data can now be sent from the local modem the remote modem. Similarly, raising DTR at the remote modem will raise DCD at the local modem so that data can be transmitted from the remote site to the local site. If any time DTR is dropped, at either modem, data transmission will be suspended from that modem and the distance modem will drop its DCD.

4.2.1 Model 74 Indicators

The model 74 is equipped with 5 indicators which inform operating personnel of:

ON - Power Applied
TD - Transmit Data to RS-232 Port
RD - Receive Data from RS-232 Port
DTR - Data Terminal Ready Status
DCD - Data Carrier Detect Status

4.3 Model 72A

The Model 72A SHM comes with a Loopback switch. When in the normal position data from the local 72A can be sent to the remote 72 and vice versa. When in the loopback position T+ is connected to R+ and T- to R- internally on the local 72A. In addition, the T+ wire from the remote 72 is looped back to R+ and T- to R-. This allows for the verification of the local 72, the 4 wire dedicated line and the remote 72A.

5.0 Troubleshooting

5.1 Modem Verification

Individual modem performance can be verified if one of the devices to which the modems are attached is capable of operation in the FULL DUPLEX echo mode. Most CRT terminals are capable of operation in the echo mode. In this mode the terminal will generate data from its keyboard but data will not be displayed until it is received back at its input port. A modem may now be tested by connecting it to the terminal via the RS-232 connector and then providing loopback connections as follows: Set to DCE
T+ to R+ and T- to R-
Note: For the 72A simply place the loopback switch to the loopback position.

If the modem is functioning any data entered on the keyboard should appear on the screen. This procedure can be used to test all Short Haul Modems.

5.2 Detailed Checks

The following checks can be made by personnel having access to an oscilloscope.

The T terminals of the modem will produce signals of +/-10 Volts (+/-5 Volts for Model 72A), approximately, when the twisted pairs are not connected, or, there is a break in the line. When the line is connected the voltage swing on any of the four screw terminals, T or R, should be approximately one to three volts, depending on the resistance of the line.

6.0 Connector Pin Assignments

2 BA 103 Transmit Data *
3 BB 104 Receive Data *
4 CA 105 Request to Send **
5 CB 106 Clear to Send **
6 CC 107 Data Set Ready ***
7 AB 102 Signal Ground Gnd
8 CF 109 Data Carrier Detect ***
20 CD 108.2 Data Terminal Ready ***
* These signals can be reversed using the selector switch.
** Connected together
*** Connected together

NOTE: For Model 74 a handshake is provided on Pins 20, DTR (input) and Pin 8, DCD (output) thus these signals do not comply with the above list.

7.0 Power

Model 72, 72A & 74 - Operating power is generated by the combination of a wall mounted transformer and circuitry internal to the SHM. The transformer plugs into a 120 Volt AC receptacle. A multi conductor 8 foot line cord delivers low voltage AC to the SHM. The rectifier circuits internal to the SHM convert this AC to approximately + -12 Volts DC.

8.0 Help

If any assistance is required please visit our  Technical Support Center.


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. These deliveries will be sent COD freight and import/export charges.


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 number (RMA) is necessary
and can be obtained by visiting our Technical Support Center. Reference RMA number on the outside container.

Document No. 0315-0001/Rev. E

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