[GreenKeys] an interesting oddity M21-A

[Duncan Brown]

The 21-A is a museum piece from about 1920.  It has 5 separate selector 
magnets, and they were driven in parallel (although one at a time) from 
a "Receiving Distributor" (similar to the distributor in a "Transmitting 
Distributor" in the M14 tape reader) that did a serial to parallel 
conversion in a multiplex system. The input to the Receiving Distributor 
was serial data of 4 channels (time division multiplex). The receiving 
distributor distributed the bits to each of up to 4 printers and the 
bits were (mechanically) latched into the machine. The distributor then 
generated a "print" signal to the M21 to print the character onto the 
tape.  (Does this operation sound familiar? think of a data bus and a 
chip-select line!)

There is an article in one of the RTTY Journals on building an 
electronic (tubes, of course) distributor to use the M21 on RTTY.

WU had their own specifications and model numbers for equipment. Morkrum 
Co made the M21A and Kleinschmidt Electric Co. made a M20A & M22A. They 
were almost identical and were interchangeable as far as WU was concerned.

There is also the story that the predecessor of these tape printers was 
a page printer:


WU was always wanting to improve their message capacity and realized 
that if they had to repeat part of a message with a page printer, they 
had to resend the whole message. But with a tape printer, they could 
just resend what was needed.  Also they did not need to send LF & CR and 
wait for those operations with a tape printer.  So WU asked KEC & 
Morkrum to redesign their Type Bar Page Printers into Type Bar Tape 
Printers. The page printers were turned onto their backs and the WU M20, 
M21, M22 were born.

(The AWA Museum has one M20A and two M21As, so we don't need anymore.)

Have fun,

Duncan
K2OEQ


On 28-Jan-15 12:36, Lee Mushel wrote:
> Is there an interesting story behind the Western Union Multiplex 
> Printer Model 21A?   One is presently merrily deteriorating away in my 
> shed and I know I will never do anything with it.....
>
> The price is right but you would have to travel to the hinterlands of 
> SW Wisconsin to pick it up some time.
>
> 73
>
> Lee     K9WRU
>


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