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Maxi-Switch

Contents

Overview

The Maxi-Switch Company (later Maxi Switch Inc) was an American switch and keyboard manufacturer. Barely any literature has survived, but a few details have been recovered. Maxi-Switch are also known for manufacturing a subset of IBM Model M keyboards in Mexico.

One has to wonder if the name “Maxi-Switch” was a play on Micro Switch.

History

Maxi-Switch was founded in 1968 in Minneapolis. The owner sold the business to EECO (Electronic Engineering Company of California) in 1984. Maxi-Switch had a keyboard plant in Carborca, Mexico and a keyboard and switch business in Scotland, as well as a keypad manufacturing plant in Phoenix, Arizona. In May 1990, EECO filed for bankruptcy protection, and in July that year, the keyboard division was sold to Silitek of Taiwan. Silitek announced they would retain the Tucson, Arizona facility and rename it Maxi Switch Inc.

In December 1995, Maxi Switch “agreed to acquire the manufacturing rights, patents and assets for Lexmark International Inc.’s Select-Ease and Rubber Dome computer keyboards, including a patent for its Buckling Spring technology.”

Series

The only documented correspondence between a series name and PCB code is for the 2900 Series hexadecimal keyboard from 1977, whose PCB is marked 2903-014-02 and whose part number is 2129-011. Other keyboards with block reed switches have PCB codes that begin “29” and part numbers that begin “2129”, suggesting that the PCB code indicates the series name, and that the first two digits of the series name are the third and fourth digits of the part number.

An unidentified keyboard with some form of discrete switch has a PCB code 3101-090-02 and part number 2132-033. As Series 3100 is documented as a mechanical switch type, this would seem to strengthen that PCB code theory. The fact that the part number begins “2132” instead of “2131” cannot be explained.

The rubber dome keyboards however are more confusing. Here, the part numbers typically begin “2186” (PCB) or “2189” (membrane) or “219”. This would suggest series 8600, 8900, 9100, 9200 etc, but it may be that they simply stopped using series names at this point.

Some series are confirmed from the 1973 Electronic Engineers Master advertisement; see Documentation below.

Confirmed

The following series names are confirmed from literature.

Series 1600 and 1800

These are keystrip modules that can use reed capsules (28 V DC at 250 mA), cross-bar palladium contacts (3 A at 28 V DC or 115 V AC) and snap-action power contacts (only in Series 1600, 15 A at 250 V AC or 500 mA at 125 V DC). Series 1600 is 1.5″ high and Series 1800 is 1.1″ high. Key spacing can be 0.625″ or 0.75″.

Series 2700

Series 2700 was advertised as “pre-tested” reed switches; they are comparatively tall, cylindrical switches with fins that appear to attach the switch to a mounting plate. Series 2700 was described as “new” in Electronic Engineers Master catalogue in 1973 but it was also advertised in Electronics in September 1972. Bounce time is 0.4 ms maximum, with 200 mΩ maximum resistance and a current carrying capacity of 500 mA in DC-resistive applications. See Documentation below.

Series 2700 is Amphenol’s 601 Series of reed switches; Maxi-Switch would also offer Amphenol’s mechanical switches as Series 3100.

Series 3100

Just as with Series 2700, Series 3100 is an Amphenol design, seemingly also part of their 601 Series. Series 3100 is a mechanical switch type with gold-plated bifurcated contacts. It is a double-break (bridge contact) design, and thus has two bifurcated contacts per switch, that are described as being of the wiping type. It was depicted in a November 1972 advertisement in Electronic Design magazine, where it is rated for a mere 10⁶ (one million) operations. Series 3100 encoded keyboards were said to cost as low as $50, and the switches were priced at 21¢ each in quantity. The same 10⁶ lifetime was given in Maxi-Switch’s Maxi keyboard family advertisement in Electronic Design in August 1973; when the advertisement was reprinted in December 1974, the rated lifetime was raised to 10⁷ (ten million) cycles, the lifetime quoted by Amphenol for the 601-R switches that appear to be the basis of Maxi-Switch 3100 Series.

The original Amphenol design as patented has additional contacts that press against the PCB to make its electrical connections, and is secured with a nut and screw. Maxi-Switch’s version has standard solder terminals, but these have also been seen on an unbranded version.

Series 3100 switches are Maxi-Switch branded and marked with the Bunker-Ramo patent number, 3710060.

2900 Series

2900 Series comprises ganged reed blocks, with a reported lifetime of over 100 million operations and a 2.5 oz (around 70 g) operating force. The reed capsules can be individually replaced in the field through holes in the PCB. The plastic blocks house only the plungers, not the switch mechanism, which is soldered separately. This is the only confirmed series to date to be examined.

The reed blocks have a base to contain the return springs, and are secured to the PCB with screws and nuts. It appears that two nuts are placed onto each screw: one to hold the switch assembly together, and one to secure the assembly to the PCB.

2900 Series seems to be largely identical to a type of Key Tronic reed switch, leading to the question of who had the design first. The Key Tronic design can be seen in a 1978 Sundstrand keyboard. The leaflet with the 2900 Series hexadecimal keypad appears to be dated April 1977.

2900 Series was advertised in Computer Design magazine in November 1977, with a cutaway illustration of the switch; see Documentation below.

CAP-TRON™

CAP-TRON is a block foam pad capacitive series. The design is very similar to Series 2900. The modules lack the base found in Series 2900 and are open at the bottom so that the foam pad can reach the PCB; consequently the return spring is placed under the keycap instead. The sense pads on the PCB are curiously not protected by solder mask.

CAP-TRON keyboards offered 2-key rollover or N-key rollover, with the switch having a rated lifetime of 100 million cycles. The change in capacitance is given as being seven times higher than required by the electronics, ensuring reliable operation.

The switches were advertised as “CAP-TRON” (see Documentation below), while in Electronic Design magazine in 1976 [ED1976-FOK] they are given as “Captron” without the hyphen. The one known PCB code suggests Series 4000. The only known example is the Digital Group KEY-1 keyboard.

6000 Series

6000 Series (or Series 6000) is a variant of the mechanical portion of SMK JM-0400 series. While not officially confirmed, the fact that 8000 Series is also identical to the corresponding SMK product all but proves that SMK either licensing their tooling or producing the switches on Maxi-Switch’s behalf just as they did for NEC.

The oldest advertisement discovered to date (for “6000 series”) is from Computer Design magazine in April 1979; see Documentation below). The keyboard thus advertised was microprocessor-driven with mechanical switches. The advertisement notes, “Electronic hysteresis eliminates switch tease and multiple char output problems.” There is a photograph showing a keyboard, but there is no way to determine what switch type is used. A later advertisement in Design News magazine in 1983 for Series 6000 and 8000 Series depicts the switch contacts alongside an assembled switch, confirming that 6000 Series is what has until recently been termed “Maxi-Switch vintage linear”.

The scope of 6000 series is unclear. The MAX-II (or MAX-11) keyboard (see Documentation) is documented as being 6000 series, giving us the specifications for the switches. David Given’s Maxi-Switch 2160029 appears to be a MAX-II, and it has “Maxi-Switch vintage linear” switches, as do all other Maxi-Switch keyboards with part numbers in that range. The original MAX advertisement shows the same PCB holes (for unpopulated key stations) as David’s apparent MAX-II, suggesting that the original MAX keyboard was also “vintage linear”. However, the advertisment also notes that the switches have wiping action, which is not a known behaviour of that type of switch.

For 6000 series, the specifications are as follows (taken from the Standard-Tastatur MAX-II product leaflet, given under Documentation):

Total travel 0.120±0.017″ (3±0.4 mm)
Pretravel 0.050″ (1.27 mm)
Overtravel 0.070″ (1.77 mm)
Operating force 3.5±1 oz (100±30 gf) standard
Bounce time Below 3 ms
Contact rating 12 V DC, 100 mA
Initial contact resistance Below 1 Ω
Operating lifetime 10 million cycles

The bounce time, rated lifetime and electrical rating match the specifications of SMK JM-0400 mechanical switches when advertised in 1979. The seemingly later SMK J-M 9031 flyer gave the rated lifetime as 100 million cycles, and the operating force of 90±30 grams (90±20%) and pretravel (at exactly half way) of 0.06″ (1.5 mm) both differ from Maxi-Switch’s specifications given above. However, the 1983 announcement for Series 6000 does cite a “100-million-plus operation live cycle”.

There is also a rare example of these switches with top branding, in an Intelligent Systems Corp Intecolor 101894.

8000 Series

8000 Series is an elastic contact type formerly described as “Maxi Switch integrated dome”. 8000 Series was advertised and depicted in Design News magazine in 1983 (see Documentation below). These switches were at that time reported to have been tested to more than 60 million operations, and were described to “maintain a constant contact resistance over their operational life, and are inert to hostile environments containing fumes, gases and contaminants.”

Thus far, these switches are only known from the oldest TRS-80 Model 4 keyboards — see Maxi-Switch elastic contact illustration. 8000 Series is identical to an SMK switch design, and will also be made by SMK for Maxi-Switch or using tooling licensed from SMK.

Unconfirmed

Miniature elastic contact

These are a very low profile elastic contact type. The keycaps are retained by plastic frames that sit atop rubber domes. These frames are secured to the PCB using screws; at each key position there are two screw holes and two fixing pins in the frame. The frames are branded “Maxi-Switch Co” and come in sizes of one, three, four and five units.

In the HP 46086A keypad, each key has a single-unit frame and a single-unit dome. By comparison, there is also a Decision Data keyboard that uses all four of the known frame sizes and large rubber dome mats.

The leading digits of the PCB codes (87) do not correspond with the part numbers, which begin 2185 instead of 2187. Possible series names include 8500 and 8700; 8000 Series is already assigned to the full-travel elastic contact switches, so it’s somewhat unlikely that these are also 8000 Series. Note that the part number prefix of 2185 immediately precedes the prefix of the more common plunger-over-dome families with prefixes 2186 and 2189.

Rubber dome

Clear distinction between rubber dome families is not yet possible. Tentatively it appears that part numbers beginning “2186” indicate conductive domes over a PCB, while part numbers beginning “2189” indicate conductive domes over a single membrane sheet. The PCB for HP 46086A (a keyboard with miniature integrated dome switches) has part number 2185074, which is very close.

As these keyboards are held to be of much less interest to anyone, disassembly photos are rare, which impedes understanding. This is muddied further by the acquisition by Silitek, as production was moved at least in part to Silitek’s factories, causing the adoption of Silitek’s own plunger-less switch design into the Maxi Switch range.

For now, they are being treated separately but collectively.

Keyboards

Encoding

The Electronic Design article Focus on Keyboards (vol. 20 no. 23, 9th November 1972, [ED1972-FOK]) depicts two models of Maxi-Switch keyboard, one MOS-encoded and the other TTL-encoded. The TTL-encoded model is described as using “14 DIPs to provide codes to 16 bits”. By comparison, the MOS-encoded model “uses a single DIP to provide any eight-bit code.” Both models are shown with cylindrical switches, which are reasonably likely to be 2700 series. The MOS-encoded keyboard looks to use two-of-N encoding with optimised PCB tracks, which needs two diodes per switch or double-pole switches, the latter being a standard option for reed keyboard switches.

The only known CAP-TRON keyboard also appears to be TTL-encoded, as seem to be their ganged reed keyboards.

Example keyboards

Mechanical, reed, capacitive, integrated dome

Note that some series names are hypothetical; see above for more details.

Series Description Examples
Keyboard PCB code Part number Date Country Serial
CAP-TRON Block foam pad Digital Group KEY-1 keyboard 403003-04 ca. 1976
2900 Block reed Maxi Switch 2900 Series hexadecimal keyboard 2903-014-02 2129-011 ca. 1977 372-507
MCM/900 keyboard 293079 2129-069 ca. 1977 238 347
Unidentified keyboard 293103-06
3100 Discrete mechanical Unidentified keyboard 3101-090-02 2132-033 1111 456
Unidentified keyboard 3103-009-02 2132-048 ca. 1975 586 425
6000 “Vintage linear” Cybernetic Data Products keyboard 630010-09 2160029 USA 1488-440
Unidentified ASCII keyboard 630015-02 2160014 ca. 1979 436-090
Unidentified keyboard 630010-10 2160029 1982-10-14 USA 5996
Commodore PET add-on keyboard 630011-03 2160010 749-199
Billings 6000 keyboard 630067-03 2160148 1983-04-22 USA 1123
Kaypro II keyboard 630184-02 2160150 1983-07-29 USA 017098
Xerox 928-900451 630107-02 110S80577 2160132 1984-01-11 USA 016020
Geomet 200 KYBD 2160177 2160177 1984-07-24 1228
Maxi-Switch 2160029 (MAX-II or MAX-11?) 630010-11 2160029 1984-10-05 USA 016703
8700? Miniature elastic contact HP 46086A 870094-01 2185074 1981-08-06? 42570
Decision Data XT-layout keyboard 870069-04 2185041 1984-12-11 USA 022366

Rubber dome

Keyboard Part number or model Type Keycaps Date Country
Maxi-Switch ME 101 218603XXX Conductive dome, plungers, PCB Not depicted Unreadable in photo Unreadable in photo
Gateway 2000 AnyKey 2189014-00-211 Conductive dome, plungers, single membrane Double-shot 1992-01-?? Mexico
Unidentified internals 2189XXX-XX Conductive dome, plungers, single membrane 1995 week 23
Maxi Switch 2192004-XX-XXX 2192004-00-001 Silitek rubber dome over membrane 1993 Malaysia
ALR 2196003-XX-XXX 2196003-00-200 Standard plungers; other details unknown Lasered 1997-11-15 Mexico

Equipment

In 1971, Maxi-Switch announced a test unit for encoded keyboards in Electronics magazine. The device exists to verify that the keyboard is producing the correct output signals; the operator would connect the keyboard into the device, and presumably press keys and inspect the output. Few details are given, but it appears that parity checking is included. The device takes the form of a roughly cube-shaped box with some buttons on the front, and it would appear that all testing would be manual.

Documentation