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28091Re: Intellec 4 MOD 40 working!

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  • s100doctor
    Nov 13, 2012
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      --- In midatlanticretro@yahoogroups.com, Kyle Owen <kylevowen@...> wrote:
      >
      > I was given two Intellecs, a 4 MOD 40 and an 8 MOD 80, a little over a week
      > ago. Both are in phenomenal condition. They were bought from surplus in the
      > early-to-mid 1980s and stored for the next 30 years, not ever even turned
      > on within that time period.

      This is a fantastic find! I've wanted a MOD 80 for a long time. I have some MOD 40 stuff. When time permits me, I'll chat with you about Intellec stuff. But I hope you make up some Web pages about these two computers to document them and your work.

      > Does anyone have a simple design for a 20mA current loop to RS-232
      > converter they would share? The Intellec unfortunately only offered current
      > loop, since that I guess was the "hip" thing at the time. After all, who
      > wouldn't want to type in their software and get the printout at a blazing
      > 110 baud? :)

      This is like saying "unfortunately cowboys only rode horses, after all, who wouldn't want to travel on an animal and ride at a blazing 15 miles per hour?". Teletypes in 1975 and earlier offered text output, keyboard input, and program storage and entry (paper tape). Terminals generally didn't have ANY mass storage, otherwise - that came later.

      Here's some preserved information about another computer, made not too much after the MOd 80 was available:

      http://www.retrotechnology.com/memship/mem_basys.html

      The manual for it has a current loop to CMOS/TTL converter design, and a description of it, on page 63-65 of the PDF:

      http://www.retrotechnology.com/memship/basys_man1.pdf

      You may have to adjust the components for +5/-5v or +12/-12v operation but you may well get away with +5/0v. Just be sure that a RS-232 negative level (transmit low) doesnt' forward bias that base-emitter junction on Q1 and fry the transistor with too much current. This calls for a little transistor theory...

      Circuits like these, depend a little on the voltage of your current source. I don't know if the Intellec has a current loop supply, or what it expects, see the docs. But in general, it's typical to use optoisolators to keep nasty current loop voltages (and electrical noise from TTY's) away from your microcomputer.

      Herb Johnson
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