[HCARC] Teletype

[SARA SANDSTROM]

Yes, TTY terminals were very slow and paper tape wasn't much better.  You have to remember that TTY's competitor was manual Morse.  Even 60 wpm was an improvement and eventually the speed went up to 75 and 100 wpm.  
  
In the 70's I did some work with DEC PDP computers.  We used punched tape among other things.  The punched tape was plastic, mylar I think, and could be reused many times.  The process was to enter a very short routine, an address and a couple commands, using the 32 levers (pink and purple!) on the front of the PDP.  This routine would turn on the punched tape reader and allow you to enter a little larger routine on punched paper/mylar tape which would enable a magnetic tape drive which read the main program.  It really worked quite well.  Of course it was all done in machine language! 
  
The two places I saw TTY terminals used were for simple remote terminals and as operators consoles.  In both cases not much was entered.  The operator's console was used to control the computer while the remote terminals were used to enter and run BASIC programs.  FORTRAN programs and the associated data was entered using punched cards on a batch terminal with a card reader.  Typical terminals were Teletypes, DEC Writers, TI Silent 700's, and Selectric typewriter terminals.  Teletypes and DEC Writers were big beasts! 
  
The 60 and 20 ma current loops were "local loops".  They only went a few feet.  To cover larger distances, the signal on the loop drove a modem which converted the TTY signal to the tones we're used to.  The phone company, back when that only meant one thing, had a couple modems often used, a Bell 103 and a Bell 202 (I think). Its been a long time!  These were used on normal telephone circuits.  For RTTY, the modem was often called a TU but it performed the same function although the tones were slightly different. 
  
Radio circuits and leased phone circuits were point to point.  TTY networks, on the other hand, went through a switching center where the messages were routed to the correct terminal.  In the 60's and 70's, the switching centers typically used UNIVAC processors, often a 418, 494, or 1218.  In the 70's, the comm centers and switching centers started using DEC PDP's for their comm processors.  Unique gear that was typically used in comm centers were 7-trk tape drives and drum memories.  normal data processing centers used 9-trk tape drives.  The drum memories were huge, larger than a larger desk and weighed a couple tons. 
  
You need to understand how a comm center worked to really appreciate the punched paper tape equipment.  The message originator would type his message on the appropriate message form.  The completed form was sent to the comm center where a "poker" (Yes, thats what we called them) would type the message on a TTY terminal which included a paper tape punch.  The message was now on a paper tape .  The tape was placed in a queue awaiting its turn to be transmitted.  When its turn came, the tape was read by a paper tape reader and transmitted.  The paper tape, appropriately labeled was saved for 30 days or so in case it needed to be retransmitted.  You could see these tapes in a figure eight roll hanging in the comm centers.  Yes, this was an extremely labor intensive operation.  In the 70's, the "pokers" were replaced by optical character readers (OCR) which would read the message form and translate it to a digital signal for transmission.  We had to use a Selectric typewriter with a special OCR font typeball so the OCR could correctly read the message form.  This worked OK but smudges and corrections on the message form were a problem for the OCR.  
  
The Model 40's mentioned by John in his e-mail a couple days ago were going to be a solution.  It was all electronic and would be part of a system that had terminals throughout an organization and effectively eliminate comm centers entirely.  It would have worked except a much cheaper and more flexible system came along called the internet and PC's.  I only saw a half dozen or so Model 40's which we had for tesing. 
  
The TTY system was pretty well matched to the transmission circuits before the late 70's.  Long distance comm was either HF radio or telephone lines.  About the most you could get for bandwidth on a good quality telephone line was 2400 bits/second (bps).  HF radio was even worse.  At times the usable bandwidth was a 100 Hz or less.  There were no fibre optic lines or satellites.  In the early 70's, we were still getting tracking data from Ascension Island over an HF RTTY circuit.  The overseas tracking stations that had leased lines were limited to 2400 bps.  I was at the NASA Manned Spacecraft Center during the early part of the Apollo progeam.  The spacecraft had three 50 kb/s downlinks at the time.  The remote tracking stations would record the data and after the mission send it back at 2400 bps.  As the days went by, you could see the stacks of data build up!  The only wideband circuits we had were between the large NASA centers and operated at 400 kb/s.  
  
Hams used to use paper tape quite a bit.  I'm sure you old timers remember the RTTY "pictures" of Santa Claus, landscapes and pretty women made up of TTY characters.  These picture were usually stored on punched paper tape and transmitted when appropriate or just printed on the local loop. 
  
Thats probably more than any one wanted to know about RTTY but if you didn't hear it from me now, I'd probably forget it soon! 
  
Kerry