- Example keyboards
The Maxi-Switch Company (later Maxi Switch Inc) was an American switch and keyboard manufacturer, known mostly for switch types that appear to be derivatives of products from other brands, but also for a distinctive type of plunger over rubber dome arrrangement. 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.
Maxi-Switch were at one stage owned by EECO (Electronic Engineering Company of California). They 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.”
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 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.
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 was advertised as “pre-tested” reed switches, suggesting that the reed capsule is enclosed inside the switch. Series 2700 was described as “new” in Electronic Engineers Master catalogue, 1973; it was not depicted.
Series 2900 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.
Series 2900 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 Series 2900 hexadecimal keypad appears to be dated April 1977.
Series 3100 is a mechanical switch type. It is described as having wiping contacts. In Electronic Design Vol. 21 No. 17 from 16th of August 1973, Series 3100 was stated as having “dual bifurcated gold contacts” and a lifetime of 10⁶ (one million) operations under load. Series 3100 encoded keyboards were said to cost as low as $50. The same advertisement was repeated over a year later Electronic Design Vol. 22 No. 26 from the 20th of December 1974 (page 124, PDF page 143), with a single modification: the rated lifetime for Series 3100 was increased to 10⁷ (ten million) cycles.
The unidentified keyboard 3101-090-02 listed below has switches marked “PAT.NO.” and a patent number ending “10,060”. The only US patent for a pushbutton switch from that era that ends with those digits is US 3710060A, filed in July by Bunker Ramo, and granted in January 1973. The patent is contemporary with the Maxi-Switch advertisement and the design fits: it has the side holes as seen in the photos and the dual bifurcated contacts (not for redundancy, but because it is double-break, or bridge contact) and the contact design appears to provide wiping motion. At no point was the patent ever assigned to either Maxi-Switch or EECO, however.
The Bunker Ramo design 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.
Captron 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.
This is a hypothetical designation for what is known as “vintage linear”, a switch that is nearly identical to SMK J-M0404 series. The part numbers begin “2160” instead of the expected “2163”, so this could be Series 6000 instead. No advertisement has been found for these switches and keyboards.
These are identical in principle to Alps elastic contact, while using a shell derived from the “vintage linear” type. They are only known from the oldest TRS-80 Model 4 keyboards — see Maxi-Switch elastic contact illustration.
Known only from a single HP 46086A keypad, these switches are a kind of miniature elastic contact. A rubber dome on a square rubber mat sits directly on the PCB, and then a cover sits over this to hold the keycap. The plunger is integrated into the keycap. The cover is reported to be secured with screws, which pass through four holes in the rubber mat into the cover. This single example is illustrated only with out-of-focus and poorly-framed photographs, and the date of manufacture is not readable (it’s out of the frame of the photo where it would have been readable).
The leading digits of the PCB code (87) do not correspond with the part number, which begins 2185 instead of 2187.
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 aquisition by Silitek, as production was moved at least in part to Silitek’s factories, causing the adoption of Silitek’s own slider-less switch design into the Maxi Switch range.
For now, they are being treated separately but collectively.
Mechanical, reed, capacitive, integrated dome
Note that some series names are hypothetical; see above for more details.
|Keyboard||PCB code||Part number||Date||Country||Serial|
|2900||Block reed||Maxi Switch 2900 Series hexadecimal keyboard||2903-014-02||2129-011||372-507|
|MCM/900 keyboard||293079||2129-069||238 347|
|3100||Discrete mechanical||Unidentified keyboard||3101-090-02||2132-033||1111 456|
|Captron||Block foam pad||Digital Group KEY-1 keyboard||403003-04|
|6300||“Vintage linear”||Cybernetic Data Products keyboard||630010-09||2160029||USA||1488-440|
|Commodore PET add-on keyboard||630011-03||2160010||749-199|
|Billings 6000 keyboard||630067-03||2160148||1983-04-22||USA||1123|
|Xerox 928-900451||630107-02||110S80577 2160132||1984-01-11||USA||016020|
|Geomet 200||KYBD 2160177||2160177||1984-07-24||1228|
|8700||Miniature elastic contact||HP 46086A||870094-01||2185074||42570|
|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|