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RFT TSS

Contents

Overview

TSS—“Tastenschalter mit Schutzrohrkontakt”—is a series of reed keyboard switches from East German brand RFT. The name translates to “pushbutton switch with protective tube contact”, referring to the glass capsule that encloses the metal reeds, rather than the more expected “Reed-kontakt”. These rare switches can be found in the Robotron K7609.51 keyboard used with the PRG600, PRG700 and PRG710 computers. Other use includes the Poly-Computer 880 (or POLY880), Bulgarian ELKA calculators and as front panel buttons in various radios. The Hall effect equivalent is TSH 19 (“Tastenschalter mit Hallelement”). The indicator (lamp-only) types are designated TSS/AE.

Catalogue description

The description in the Passive elektronische Bauelemente ’79|80 catalogue (helpfully uploaded by Holm Tiffe) is as follows:

Der Tastenschalter mit Schutzrohrkontakt ist vorwiegend für den Einsatz in Tastaturen der Datenverarbeitung, Steuer- und Regelungstechnik bestimmt.

Er wird in den Ausführungsformen rastend, nichtrastend, Goldkontakt oder Rhodiumkontakt gefertigt. Das Frontflächentraster beträgt 17,5 × 17,5 mm (Montageplattenbefestigung) bzw. 19 mm × 19 mm (Schienenbefestigung).

Zur Beleuchtung wird die Signalkleinlampe MSKF nach TGL 10 449 eingesetzt.

Das Schutzrohrkontaktsystem zeichnet sich durch hohe Zuverlässigkeit, Schutz vor Umwelteinflüssen, geringe Prellneigung und TTL Kompatibilität aus.

Broadly speaking, this indicates:

The reed pushbutton switch is mainly intended for use in data processing keyboards and control engineering.

It is manufactured in momentary and alternate action configurations, with a choice of gold and rhodium contact surfaces. The switches are designed for a 17.5 × 17.5 mm mounting plate and 19 mm × 19 mm rail mounting.

The switches can be illuminated using an MSKF miniature bulb (per standard TGL 10 449).

The reed capsule system is characterized by high reliability, protection against environmental influences, low bounce time and TTL compatibility.

Characteristics

Size

There are two general sizes of these switches: short and tall.

Short version

The plate-aligned short version is depicted in the 1979–80 catalogue. The upper portion of the shell (above the alignment plate) is 15 mm square and 7.5 mm tall, and the shell is 15 mm tall overall. These switches were available in upright (“stehend”) and horizontal (“liegend”) versions; the latter has a 90° angle to the terminals to allow the switch to lie on the PCB with the terminals facing into the PCB. In practice, the horizontal versions also have a metal jacket with its own solder terminals to secure the switch.

Tall version

The tall version is covered by East German standard TGL 34716 from October 1979. These have a shell that is 22.8 mm tall. These are supplied in two forms: TSS 17,5 and TSS/AE 17,5 are designed for a mounting plate with 17.5 mm spacing (“17,5 mm” with the European decimal comma), and TSS 19 and TSS/AE 19 use a rail alignment system with standard 19 mm spacing. The upper portion of the shell is also enlarged to 17 mm square at the top, tapering down to 16.7 mm square at the bottom of the upper portion of the shell.

The two tall types are illustrated below:

10ko on Deskthority (from Bulgaria) supplied some TSS 17,5 switches which are almost but not quite the same as in TGL 34716. The standard depicts 4.0±0.5 mm of travel, while his are quite clearly 4.5 mm travel, and 0.4 mm taller.

Alignment

TSS switches are all PCB-mounted. Correct spacing and orientation for the upright types is achieved using either of two methods: plate alignment (referred to as “Montageplattenbefestigung”: mounting plate attachment) and rail alignment (“Schienenbefestigung”: rail attachments). Neither the 17.5 or 19 mm grid types physically attach to the plate or rails: they merely sit inside holes or align to slots. This process, if correctly constructed, will still reduce the amount of keystroke shock reaching the solder joints on the PCB. By contrast, top-down alignment plates (as can be used with Cherry M8 and Datanetics DC-50) help protect the solder joints when removing keycaps but do not take up keystroke impact.

The catalogue does not illustrate either the mounting plate or rail mount options. The rail mount system can be seen in a K7609 keyboard although the details are not clearly presented.

Action

TSS switches are available in single pole form in both momentary and alternate action. Alternate action was only offered for “Variant I” contact material (rhodium).

Indicator modules

There are also two indicator variants defined in TGL 34716 for the tall version: TSS/AE 17,5 and TSS/AE 19, where “AE” denotes “Anzeigeelement” or “display element”. TSS/AE has the same appearance as TSS but lacks the switch components including the switch terminals (only the lamp terminals are present). TSS/AE is 1.5 lighter than TSS, at 5.5 g versus 7 g.

Contact material

The catalogue and standard each list two available contact materials. The catalogue names them RKR 13 and RKG 13, being rhodium and gold respectively (or more likely rhodium and gold alloys). The gold contact types have lower contact resistance and lower minimum switching voltage. The rated lifetimes are not compared, but the rhodium contact type is likely intended for higher switching voltage (see the materials page for more details).

The standard instead gives only “Variant I” and “Variant II”, which appear to correspond to RKR 13 and RKG 13. Again, the rated lifetimes are not comparable, being given as 1.8×10⁶ cycles at 40 V 125 mA for Variant I and 1×10⁷ cycles at 5 V 5 mA for Variant II. No indication is given as to how long Variant I would last if operated at 5 V 5 mA, or Variant II at 40 V 125 mA, although the latter is beyond the acceptable operating range for gold alloy contacts.

The contact material is marked with a paint dot on the base of the switch. A blue dot indicates Variant II (believed to be gold or gold alloy). The absence of such a marking indicates Variant I. Switches have also been found with a white paint dot of unknown meaning.

Part designations

Part numbers follow the pattern shown below:

21050. 0AV-0M000 TGL 34716

“21050”
Drawing number
“TGL 34716”
Applicable East German standard
A
Action: 0 = indicator, 1 = momentary, 2 = alternate action
V
Variant: 1 = plate-aligned switch (TSS 17,5); 2 = rail-aligned switch (TSS 19); 3 = plate-aligned indicator (TSS/AE 17,5); 4 = rail-aligned indicator (TSS/AE 19)
M
Contact material: 0 = no contacts, 5 = Variant I, 6 = Variant II
“0”
Unused position

The “variant” position is officially termed “Befestigungsart” (type of fastening) but this position is non-orthogonal with respect to switches and indicators and therefore functions as a more general variant indication.

Specifications

The 1979–80 RFT catalogue gives the following specifications, where RKR 13 is the rhodium-contact version and RKG 13 is the gold-contact version:

Characteristic Contact type
RKR 13 RKG 13
Switching current, min. 0.01 mA 0.001 mA
Switching current, max. 300 mA 200 mA
Switching voltage, max. 42 V
Switching power with resistive load 5 W 4 W
Average lifespan, momentary 1×10⁶ at 5 V 5 mA
Average lifespan, alternate action 5×10⁵ at 5 V 5 mA
Contact resistance during rated lifetime ≤ 1 Ω
Operating force, momentary ≤ 1.3 N
Operating force, alternate action ≤ 2 N

Some details may be incorrect as the table is badly formatted and incomplete. The catalogue entry describes 17.5 mm grid plate-aligned switches and 19 mm grid rail-aligned switches but depicts only the short type.

TGL 34716 provides a slightly different set of specifications for the tall switches:

Characteristic Contact material
Variant I Variant II
Switching current, min. 0.01 mA 0.001 mA
Switching current, max. 300 mA 200 mA
Switching voltage, min. 1 mV
Switching voltage, max. 80 V 70 V
Switching power with resistive load 5 W 4 W
Average lifespan, momentary 1.8×10⁶ cycles at 40 V 125 mA 1×10⁷ cycles at 5 V 5 mA
Average lifespan, alternate action 2.5×10⁵ cycles at 40 V 125 mA
Contact resistance during rated lifetime ≤ 5 Ω ≤ 1 Ω
Bounce time ≤ 0.5 ms
Operating force, momentary 0.7–1.2 N
Operating force, alternate action ≤ 2 N

The operating force is a little on the high side, being a little heavier than Cherry MX Linear Grey: the latter is rated at 80±25 cN, with a total travel force of around 110 cN, which at the same tolerance (±25 cN) would lie within the range 85–135 cN, a fraction above that of TSS.