STRUCTURE OF A SELECTRIC TYPE ELEMENT
The familiar 'golf ball' element from the Selectric typewriters is a metallic appearing ball that snaps onto the mechanism of the typewriter. Changing elements allows the typewriter to print with different fonts and special characters. This ball is actually constructed of nylon, colored to look metallic as IBM believed that a plastic appearance would be taken as a sign of cheap construction.
The ball is a partial sphere, injected molded as two almost hemispheres then fused into the ball shape. Around the lower 'skirt' are teeth which allow the typewriter mechanism to lock the ball into precisely the right rotary position when typing a character.
The center of the ball is the attachment point to the typewriter mechanism. One hemisphere is the regular or unshifted side, representing the lower character on keycaps, while the other hemisphere is reached by using the Shift key and thus will type the upper character on keycaps where there are two symbols. For letters, the two sides had lower case and upper case letters. Other keys had two symbols, e.g. the key with 1 on the unshifted hemisphere and the ! on the shifted side.
A hemisphere is divided into 11 rotary positions from one edge to the other, these are termed rotations of +5, +4, +3, +2, +1, 0, -1, -2, -3, -4, and -5. The face is also divided into four rings that run left to right. The ball can be tilted to the 0, 1, 2 or 3 tilt level (T-0, T-1, T-2 and T-1,2), in addition to its rotation to one of 11 rotary positions.
This combination of tilt and rotate places one of 44 positions on the hemisphere directly facing the ribbon in front of the paper and platen. You can see the raised letters on the element at each of these 44 positions. Fusing the two sides into a sphere provides the full 88 characters available on a type element.
Note that this is for the original Selectric mechanism, sometimes called Selectric 1 to distinguish it from later versions that had balls with 96 elements in total for the Selectric 2, Selectric 3 and other families that came later. The 1053 is an original Selectric family member and uses the 88 character elements.
TYPEWRITER ELEMENTS USE CORRESPONDENCE CODING
The ordinary Selectric typewriters of that era assigned the characters on the keyboard to various positions on the type element in a fixed way. That is, if you swapped a Letter Gothic element for a Prestige element, the same character was at the same relative location on the ball, although its typography was different to produce the different font type.
On the correspondence elements, some sample characters and their positions were:
- A T2, R -2
- Z T0, R 0
- 0 T0, R +4
- / T3, R -1
MAINFRAME AND TERMINALS HAVE ALTERNATIVE PTTC/BCD CODING
An older encoding system was used to encode characters for typing back in the 1401 computer era, where the 1050 terminal system initially used the Selectric mechanisms. These machines stored characters in Binary Coded Decimal (BCD) and on paper tape in PTTC code. The type elements produced for those older systems had the characters assigned in different locations on the ball.
The 360 and all the Selectric mechanisms in the 1050 series (1052 and 1053) used the BCD encoding. IBM produced some Selectric based terminals, for example the 2741, that could be wired to either coding scheme. Famously, users running the APL language on a 2741 had to know which encoding the terminal was wired for, in order to install the proper version of the type element. These were the 987 and the 988 elements, for correspondence or BCD encoding respectively.
The same characters in the short list for correspondence are on the BCD ball at:
- A T3, R -5
- Z T1, R +4
- 0 T0, R 0
- / T1, R -5
THE TYPEBALL ON AN 1130 AND A S/360 CONSOLE
IBM Selectric Type Elements have a three digit code stamped on the nylon ball just under the lever on the top which identifies the font on the ball as well as whether it is correspondence or BCD. The ball on the 1130 is a 969 - "1131 & 1800 SYSTEM UC~UC" and the 360 console uses a 952 - "360 EXTENDED BCD"
I'm confused. Can you explain further why a different code used in the memory of the controlling machine, required a different physical mapping of letters on the surface of the balls? There is always going to be a translation from bits in memory, to mechanical tilt/rotate movement. So why did the BCD machines require different ball layouts from the EBCDIC machines? (Or was it possibly, just animosity between OPD and DPD engineers?)
ReplyDeleteHi David
DeleteFirst I need to highlight something rather unique with the 1130 compared to other systems. I will use the S/360 to compare, which is useful because the 360 console was the same two mechanisms as the 1130 console - 029 keypunch and 1053 printer. Although IBM called the console a 1052 it was not a typewriter, it was a print only selectric plus the keypunch keyboard.
On the S/360 and most computers, characters are encoded in memory by a scheme that is the same for all peripherals that handle the characters. The letter A will be a specific byte value that the device controller of each peripheral is responsible for converting to and from the actual physical implementation.
The letter A on a keypunch is a set of 12 row values - if you go from top of the card to bottom, the bit value is 100100000000 but in EBCDIC the A is x41 or 01000001 binary. The 1053 that is the console printer of a S/360 will neet T1, T2 and R5 selected to rotate the BCD ball to the letter A and it will be on the unshifted hemisphere. You might consider that a bit value of 1100010 for T1, T2, R1, R2, R2A and R5 while shifting is off (0).
The 1130 does not have a uniform encoding for characters. Each device will put the letter A into memory in its own way. Since the keyboard is 12 rows, value 100100000000 the word produced by the keyboard is 1001000000000000 (padding on the right). The letter A to be printed on the typeball is 11000100 plus padding to the right of 8 more 0 bits. This is the tilt and rotate codes, the hemisphere, and a last bit that, when 1, means the preceeding seven bits select a function rather than typing a character.
The burden is on the programmer to convert continually between character encodings as you read keyboard characters, type then on the console, punch them in cards, or print them on the line printer. Rarely is the code the same for a character from device to device. On the S/360, on the other hand, an A is always 01000001
Now to the second part of the issue - why the type ball is the PTTC/BCD arrangement. Since IBM simply used the physical device details to encode the letter A for the typewriter, if they put a correspondence (office typewriter) ball on it, the code could have been listed as 01110110 for T2, R1, R2, R5 and shifted (1). There is nothing yet that would force the choice of BCD or correspondence.
We need to look at the history of use of the selectric mechanism with computer products. IBM used the selectric with the 1050 Data Communications System product that was connected to the prior generation 14xx and 70xx computers. These systems encoded characters in memory using BCD. They had character requirements that were disjoint from what is appropriate in an office typewriter. They had characters like record mark and field mark. They did NOT use lower case letters. Thus the typeballs used for them would have wasted many positions if they chose the upper/lower typewriter convention.
The 1050 system had the 1053 printer, the 1052 which was a 1053 and an 029 keyboard, the 1054 and 1055 paper tape reader/punch and even the 1056 and 1057 card reader and punch. The card reader from the 1050 system became the 1442 reader on the IBM 1460 computer and carried forward as the 1442 reader/punch for the 1130.
S/360 and 1130 did not use the record mark or other characters from the BCD generation, but they did have characters such as the logical NOT symbol that were not present on typewriters. The typewriter for its part did have a degree, a 1/4 and a 1/2 character that the computers didn't use. Thus it makes some sense for IBM to continue to use the BCD layout of characters around a typeball, but produce versions where they put the new 360 symbols instead of the 14xx/70xx symbols.