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Data encryptor with HF-modem and FEC

The TST-4043 was a small military-grade data encryption/decryption unit with built-in HF radio modem and advanced Forward Error Correction (FEC), developed and built around 1984 by Tele Security Timmann (TST) in Pöcking (Germany). It was intended for use by Military and Diplomatic Services world-wide. The initial version was housed in metal case with dangerously sharp edges.

A later version, shown here, was housed in a typical modular case that was also used for a number of other TST products (e.g. TST-7700).

The unit is powered directly from the (220V) AC mains and allows a teleprinter, using the Baudot standard, or a computer, using the 7/8-bit ASCII standard, to be connected directly to an HF radio by means of the built-in AFSK modem [2].

The device was mainly intended for sending messages over relatively slow and noisy HF radio channels, such as the short-wave bands [2].
TST-4043 front panel

For many years HF radio was used for communication with embassies. The TST-4043 contains three basic functions that ensure a reliable and secure connection: (1) A high-end AFSK radio modem, (2) advanced suppression of errors, using Forward Error Correction (FEC) and (3) a built-in high-end data encryption device. The device supports data transfer rates between 75 and 300 baud and the FEC improves the reliability by a factor of 1000 or more on bad (noisy) channels.

The TST-4043 does not have any controls on its front panel. Instead it is fully controlled from the terminal connected to it (i.e. the teleprinter or the ASCII computer terminal). Cryptographic keys are loaded via the same terminal and do not require the use of a dedicated key loading device.

The 25-way D-type connector at the front, holds all signals that are necessary for the connection of a terminal. It has a full RS-232 interface for the connection of a computer or an ASCII terminal. 5-Bit teleprinters can be connected with either 20 mA or 40 mA, single current or double current.

TST-4043 military-grade data encryptor TST-4043 front panel TST-4043 front panel TST-4043 rear panel

The interior of the TST-4043 can easily be accessed by either removing the front panel or the rear panel, by releasing the bolts in the four corners. As nothing is attached to the front panel, it is probably the easiest to get access via the rear, after which three units become visible.

Looking inside the device from the rear, we see (from right to left) the mains power supply, the encryption board and the interface board.

The encryption board is built around a National Semiconductor NSC 800 processor, the military variant of the Zilog Z-80. The board contains (volatile) static RAM on which the cryptographic keys are kept by means of a Lithium battery.

At the bottom right are eight DIP-switches that can be used to alter the configurations of the device (the settings are currently unknown).
MSC 800 processor on the crypo board of the TST 4043

The other PCB contains the HF modem and the line interface transformers. The text has been removed from the AFSK modem chips, something that was frequently done in those years to protect the design. At the top right is a temperature controlled 2.4576 MHz oscillator. The encryption/decryption board was also available separately for OEM applications as the TST-9669.

PCBs inside the TST-4043 Crypto board (left) and HF modem (right) Crypto Board MSC 800 processor on the crypo board of the TST 4043 Lithium battery for holding the cryptographic keys DIP-switches Modem board Temperature controlled oscillator

  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence. Retrieved July 2013.

  2. TST Tele Security Timmann, TST 4043
    Sales Brochure (German). 5 pages. Retrieved July 2013 from [1].
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