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TST-3550
Portable electronic cipher system

The 3550 was a portable electronic cipher machine, developed by Tele Security Timmann (TST) in Tützing (Germany) around 1983. The device was intended for sending secure text-based messages via telephone lines and radio links, and was initially based on a a Texas Instruments (TI) Silent 700. In the mid-1980s, when the Silent 700 range was discontinued, the design was changed to use a Tandy TRS-80 Model 102. For the user this was a transparent change.

The 3550 was typically transported inside a slimline flight case, together with a printer, an acoustic modem and a battery charger. The device is powered by the computer's internal rechargeable 6V battery (4 x 1.5V AA-size), plus a separate 1.5V backup battery for the memory.

The image on the right shows a typical TST 3550 in a black flight case. The device itself is located at the bottom right. A suitable thermal printer is located to the rear of the device, whilst an acoustic coupler is present at the front left. At the left rear is a 9V DC mains power adapter.
  
Flight case with TST 3550, printer and acoustic coupler

At the heart of the 3550 is a eurocard-size digital encryption unit that is also present in other TST encryption devices, such as the 3010 and the 4043 [3]. The card is housed inside a plastic enclosure that is bolted to the bottom of the host computer. In this case the host is a Tandy TRS-80 Model 102, a slightly later version of the TRS-80 Model 100 [1] that was used for the 1530.

Closed flight case TST 3550 mounted inside a flight case Flight case with TST 3550, printer and acoustic coupler Acoustic coupler Thermal printer TST 3550 inside the flight case Power supply for the printer Bare TST 3550 cipher machine

Silent 700
The initial version of the TST-3553 and is predecessor the TST-1550, was based on the Silent 700, a range of portable computers that were introduced by Texas Instruments in 1976. The model that was used by TST was the Silent 745, which had a built-in thermal printer.

The printer was located right behind the keyboard, and at the back of the unit was an acoustic telephone coupler for the built-in modem. The devices were converted by TST into fully featured cipher machines, by adding an internal crypto card. According to TST, the TST-1550 was compatible with the TST-1221 range of pocket cipher machines [2]. The image on the right shows a typical Silent 700 as it was introduced by Texas Instruments (TI) in 1976.

As a result of a problem with the built-in thermal printer, it was not possible to heat all dots when printing graphically at 120 characters per second. TI released an improved version, but eventually the Silent 700 series was discontinued in the mid-1980s. This was a problem for TST, as the TST-3550 was rather popular with its commercial customers. Eventually, they decided to use the Tandy TRS-80 Model 102 instead.
  

The TRS-80 Model 102, had a built-in modem, but not an acoustic coupler. Furthermore, it didn't feature a built-in thermal printer like the Silent 700. The new version of the TST-3550 was then built inside a briefcase, together with a suitable printer and a separate acoustic coupler to make it functionally identical. It seems strange that the model number wasn't changed after the make­over, but this was probably done to avoid confusion with the existing customer base.

Crypto unit
In the late 1970s, TST developed a universal crypto board that was used at the heart of most of their cipher machines at the time. The crypto board typically consisted of a 10 x 16 cm Eurocard with one of more add-on boards. For text-based messages, the crypto board was often used in combination with a host computer from companies such as Tandy, Texas Instruments and EPSON.

In our TST-3550, a Tandy TRS-80 Model 102 was used as the host computer. It was a rather popular home computer during the 1980s which had its operating system in ROM. TST modified the computer by replacing one of the main ROMs by a new one that contained firmware for the crypto board and for entering text messages.

The host computer is used as an input/output device only, and does not contain any specific cryptographic software. It communicates with the crypto board via the RS232 port at the back. It is connected by means of a ribbon cable.
  
Bare TST 3550 cipher machine

The TRS-80 is normally operated from four internal 1.4V AA-size rechargeable batteries that are located in a battery compartment at the bottom. An extra 1.5V rechargeable battery was bolted to the bottom of the computer to keep the clock running when the main batteries were exhausted.

The crypto board itself is located in a plastic enclosure, slightly smaller than the computer itself, that is bolted to the bottom of the TRS-80. The image on the right shows the crypto board of the TST 3550. It has its own Z-80 processor with I/O chips, static RAM and EPROM. At the left are a 25-way D-type socket and 4 LEDs. The LEDs are not visible from the outside.

In order to protect the design from prying eyes and from the competition, the text has been removed from some of the smaller ICs. This would make it more difficult to copy the design.
  
Main crypto card

In the device shown here, the 25-way connector of the crypto board is wired to a 9-way D-type socket at the back of the unit. It is used for connecting the acoustic coupler. Another 25-way D-type socket is als present at the rear of the unit, but it is not wired (unused) in this configuration.

A small daughter card is mounted on top of the crypto board. It is the TST-082 HF FEC Modem, that contains a TST-developed custom chip for Forward Error Correction (FEC) in the bottom right corner: identified as the TST-FEC 992.

TST 3550 seen from the rear. The lower part is the actual cipher unit that is bolted to the bottom of the TRS-80 Model 102. Connection between the cipher unit and the host computer Cipher unit separated from the host computer Close-up of the PCBs inside the cipher unit Daughter card separated from the main crypto board Daughter card, component side. Main crypto card The text has been removed from most of the ICs

Host computer
The host computer of our TST-3550 was a standard Tandy TRS-80 Model 102 that was modified by TST. The standard 6V entry at the side was blocked by isolating the center pin, in order to force the unit to be powered by the internal batteries or via the crypto unit at the bottom.

Furthermore, an extra 1.5V rechargeable battery was bolted to the bottom of the case in order to keep the Real Time Clock (RTC) of the TRS-80 running when the main batteries had run out.

The actual host computer was built by Kyocera in Japan from 1986 onwards for three major customers: Tandy (TRS-80), Olivetti (M-10) and NEC (PC-8201) [2]. The computer is based around a 8085 processor and has its operating system in ROM, together with some additional software, such as a text editor, a communication program and an enhanced version of MS BASIC.
  
TST EPROM replacing the original Microsoft firmware

The standard software was written by Microsoft and some of the programs were even written by Bill Gates himself [2]. The main change when converting the TRS-80 Model 102 into a TST 3550, was the replacement of one of the ROMs for a new one that was developed at TST (MDLTERM). A short red wire was solder onto the motherboard in order to provide 6V power to the crypto board. Finally, the Tandy Radio Shack shield at the top right was swapped for the typical TST logo.

Bottom view of the TST 3550 Interior of the TRS-80 Model 102 Motherboard of the TRS-80 Model 102 Battery compartment (4 x 1.5V AA-size) Extra clock battery TST EPROM replacing the original Microsoft firmware Extra red wire to supply power to the crypto board The TST logo replacing the original TRS-80 logo

TST-3070 Acoustic Coupler
For business men and embassy personnel on the move, the TST-3550 was often used together with the TST-3070 acoustic coupler [4]. In the configuration shown here, it was built inside the flight case, to the left of the main TST-3550.

The image on the right shows how the handset of a standard telephone set was placed in the coupler. Foldable rubber lens caps, of a standard photo camera, were used to keep the handset in place and shield off ambient noise. The same acoustic coupler was used with the TST-3010 cipher machine. More images below.
  
Telephone handset placed in the acoustic coupler

Acoustic coupler Acoustic coupler with the rubber caps out Instructions on how to insert the telephone handset Telephone handset placed in the acoustic coupler

Help required
At present we have no further documentation about the TST 3550. If you have access to any additional information, such as user manuals, circuit diagrams, brochures, photographs, etc., please contact us.

References
  1. Wikipedia, TRS-80 Model 100
    The nearly identical predecessor of the TRS-80 Model 102. Retrieved July 2013.

  2. Old-Computers.com, TRS-80 Model 100 / 102
    Website. Retrieved July 2013.

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

  4. TST Timmann, TST-3070 Akustikkoppler...
    Sales datasheet, 3 pages (German). Date unknown.

  5. Wikipedia, Silent 700
    Retrieved July 2013.
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