The RPC-4000 (like many computers of the same vintage) used a device called a Friden Flexowriter for input and output. This device was a combination typewriter, printer and paper tape reader & writer.
Input to the RPC-4000 could be done either by typing on the keyboard, or reading a paper tape. Output from the RPC-4000 would be printed to the flexowriter.
The character set used by the RPC-4000 is essentially the Flexowriter character set. It uses 6-bit codes, allowing for 64 characters, as opposed the 128 characters in 7-bit ASCII. However the flexowriter had a typewriter mechanism that allowed for shifting between upper and lower case, and there are codes in the character set that select which case to use for printing of subsequent characters.
The Flexowriter character set also has some ‘control codes’, similar to the control codes that make up the first 32 ASCII characters. Nowadays most of these control codes are long since obsolete, since they relate to long dead media like 80 column punch cards, paper tape or magnetic tape reels.
While looking for the definition of some of the Flexowriter control codes referenced in the RPC-4000 documentation, I stumbled across ‘Making Electrons Count‘ – a documentary made in the early 1950s about the MIT “Whirlwind” digital computer facility. It is 20 minutes long, the first 10 minutes or so are a breathless description of the wonders of digital computers and the bright future they would bring us.
The 2nd half I found eye-opening; it tells the story of someone with a complex maths problem to solve, and the journey they go on to program the Whirlwind to solve it, which includes digesting a thick tome on “PROGRAMMING: Statement; Mathematical Analysis; Flow Diagram; Code; Tape Preparation; Computing; Debugging; Interpreting Results.”, attending a 2 week programming course, several interactions with Flexowriters, and a 4:00am visit to the “Night Typist”.
As well as the video itself, I also came across a nice commentary on it that gives a lot of useful context to the technology and techniques captured in the film.