[GreenKeys] [External] Local vs. Line mode tape reading: 33 ASR

[Jones, Douglas W]

From: Andy KN4UCL [kn4ucl at gmail.com] -- Sunday, February 7, 2021 12:01 PM

> Gents, my tape reader behaves as expected in Local mode ... but in Line mode ... it does not "activate" 

As Wayne suggested, this is the expected behavior if your 33 was equipped with a reader-run relay -- this was the normal configuration of 33 ASR Teletypes sold for use with DEC minicomputers through the 1960s into the 1970s, and some other minicomputer manufacturers made the same modifications.  The cable from minicomputer to Teletype had 6 wires, 2 for the keyboard current loop, 2 for the printer current loop, and two for the tape-reader enable loop.  All of DEC's Teletype interface boards for the PDP-8 through PDP-11 supported this model.  Receipt of a character by the serial to parallel circuitry on the board disabled the paper-tape reader until the computer software read the character out of the interface reigster.  This eliminated the need for more complex flow control protocols like XON - XOFF -- "stunt box" features that were not available in the early part of the 33 ASR production run.

After UART chips became available with internal FIFO buffering, the interface boards only disabled the reader when the FIFO was full.  If the software was interrupt driven with a decent operating system, the flow control became an irrelevant fossil of an earlier era, but you have to remember that early code for the PDP-11 and most code for the PDP-8 had only the crudest conception of an I/O driver.

          Doug Jones
          jones at cs.uiowa.edu