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Telsy TX-900S
1st generation mobile phone encryptor

The TX-900S was a compact sophisticated duplex voice encryption device for mobile application, introduced by Telsy in Turin (Italy) in 1985 as the successor to the simplex TX-800. It was one of the first devices that were fully digital and used DSP technology for processing voice signals.
 
The TX-900S was housed in a slim-line green metal case and had only two buttons on its front panel and a single connector at the rear to which all peripherals were connected. The unit was suitable for simplex and duplex radio links, making it the ideal solution for integration into 1st generation mobile phone equipment.

Different versions of the TX-900 were available. The image on the right shows a typical TX-900S unit. It measures only 175 x 260 x 27 mm, and contains a single PCB. Due to its size, it could easily be used in a mobile environment.
  
Telsy TX-900S voice encryptor

The TX-900M was designed for mobile use in combination with full duplex mobile radios such as 1st generation car phones. The complementary TX-900F was used at the other end, where it could be integrated with a fixed telephone (PSTN) and/or mobile radio network. All necessary hardware, such as transformers and echo-cancelling circuits, are included two-fold in each unit.

The TX-900S unit shown here, was a special version that allowed secure end-to-end communication with automatic key-exchange. It could be used over ordinary telephone lines (PSTN) as well as over radio links. The unit has only two push-buttons (marked CLEAR and CRYPTO) and an 8-character alpha-numerical LED display that is used when setting the key.

The TX-900 was followed in 1986 by the popular KV-3000 that exceeded all sales expections. A smaller (simplex) variant of the TX-900 was the later TX-1020 that was used for narrow-band FM radios, such as the ones used by the police.
 
TX-900S front panel TX900S rear panel Telsy TX-900S voice encryptor 12V-DC input (center contact +) DB25 connector TX-900S top view Bottom view Slim-line design

 
Models
  • TX-900M
    Self-contained mobile unit in rugged metal housing. It connected directly to the mobile radio via a single connector.

  • TX-900
    Custom designed single PC-device, mounted inside the mobile radio.

  • TX-900F
    Fixed unit that was normally installed at the radio-telephone exchange. Depending on the application, it consisted of 1 or 2 PCBs.

  • TX-900S
    Self-contained special version, offering end-to-end protection (with personal key) between two TX-900S units, or 'radio link only' protection when used in combination with other members of the TX-900 family. The TX-900S can be connected to a Telsy remote control telephone or to a mobile radio. This version is featured on this page.

  • TX-900S-MS
    Multi-standard version of the TX-900S that was used for demonstration purposes. It could be programmed to simulate the TX-900, TX-900F, TX-900S and the older TX-800 (simplex version of the TX-900).

Connections
All connections to the TX-900S are via a single 25-way sub-D connector (DB25) at the rear. A female version of the DB25 connector is mounted on the TX-900, so a male version of this connector is required for connection to the outside. The diagram below shows the pinout of this connector. The connector on the FX-900M is wired identically.

FX-900 DB25 pinout


The connector can be used for connection to the radio as well as for connection of a handset or remote control unit. All signals that are required for such a connection are present on the DB25.
 
Interior
Considering its age, the TX-900S was a highly compact beautifully engineered device, consisting of a single PCB and a minimum of controls. All that was needed externally, was a 12V-DC power supply (e.g. the battery of a car) and a communication device (telephone with PTT line or radio).
 
The TX-900S was one of Telsy's first products with DSP technology, featuring the TMS-320, the first generation DSP by Texas Instruments (USA). As the unit is suitable for full-duplex, two identical DSP circuits are present on the board.

The image on the right shows part of the PCB inside the TX-900S. The Integrated Circuits (ICs) with the golden caps are custom chips that are built at Telsy's specification by AMI (American Microsystems Inc.) and Texas Instruments (TI). The line interface consists of two AM-9701 Subscriber Line Audio Processing Circuits.
  
Custom chips

 
TX-900S interior Custom chip Custom chips TMS-320 DSP chip TMS320 DSP chip Two identical circuits for full-duplex DB25 connector (interior) DC-DC converter

 
References
  1. Telsy S.p.A., TX-900S
    Device featured on this page kindly donated by Telsy.

  2. Telsy S.p.A., TX900 Series Preliminary Instruction Manual
    24 September 1985.

  3. Texas Instruments, TMS-320 First Generation Digital Signal Processors
    January 1987.

  4. Advanced Micro Devices (AMD), AM-7901 Datasheet
    Subscriber Line Audio Processing Circuit WORLD-CHIP. September 1985.

Further information

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