Bells on terminals (was: Re: YouTube videos of ASR33, etc.)
steve at radiorobots.com
Mon Jan 21 15:35:29 CST 2008
Being able to feel the physics without instruments is always fun.
Tom Watson wrote:
>The discussion of bells on terminals reminded me of a weird experience. I had
>built up a terminal using a Qume printing mechanism (Sprint 4 interface) and
>added a keyboard and UART to make it complete. The printer didn't have a
>"bell" so I had to make one. In looking around for a nice tone to use, I found
>the one from the baud rate generator (MC14411). Since I was using an ASCII
>sequence, I had little use for the 134.5 rate (it is used for IBM 2741's) so I
>picked it up and with the use of a (couple?) one-shot, I made it into the beep.
> It was 16x the rate, so the tone was 2153.3Hz (see the data sheet). It worked
>quite nicely, and was pleasant sounding enough. Both control-G's and I were
>Then the terminal was hooked up to an acoustic modem. While this isn't bad in
>itself, the astute among us will note that the tone for the bell is very near
>the center of the passband (2025-2225Hz) of the originate modems detector. So,
>every time someone sent a control-G to the terminal ALL sorts of weird things
>happened. Needless to say, I changed the wire to a different tap on the
>MC14411 baud rate generator chip. Yes, hardware can have unexpected bugs.
>Somewhere I still have this terminal. Daisywheel printers were fun to play
>with. I have the schematics somewhere if anyone needs info.
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