Newsgroups: comp.robotics
Path: brunix!uunet!gatech!hubcap!dawill
From: dawill@hubcap.clemson.edu (david williams)
Subject: Re: Miniboard 2.0 & NMI F68HC11
Message-ID: <1992Aug28.135419.3725@hubcap.clemson.edu>
Keywords: Miniboard
Organization: Clemson University
References: <1992Aug26.192907.10931@aio.jsc.nasa.gov> <1992Aug27.010906.1515@news.media.mit.edu> <1992Aug27.142429.16553@hubcap.clemson.edu> <1992Aug27.205711.23050@news.media.mit.edu>
Date: Fri, 28 Aug 1992 13:54:19 GMT
Lines: 51

fredm@media.mit.edu (Fred G Martin) writes:

  [ Discussion of miniboard's simple serial port deleted ]

>Personally, I would think that the appropriate things to do are (1)
>modify your software to deal with the hardware, or (2) flat out
>replace the Mini Board's hacked-up serial line solution with something
>normal, like the one-chip Maxim MAX233 solution.  This chip has
>built-in charge pumps and voltage inverters for generating legitimate
>plus and minus ten volt supplies from a single +5v supply.  Pretty
>neat!

   As a matter of fact, I've used this critter before, and it works
very well.  I can't say enough about Maxim's products:  they do what 
they're supposted to, first time.

   Note: the MAX233 is an improved version of the original, the MAX232.
the MAX232 required several external capacitors for it's charge pumps.
The MAX233, on the other hand, has no requirements for any external
parts.  (makes using it a real breeze)  The package is a 20-pin DIP.

>It's a tough call; maybe the Mini Board should have used this
>solution.  It would have added board area and a few dollars of parts
>costs, but then people wouldn't be hassled with serial line
>weirdnesses.

   You know, I have a feeling that a few people are going to be bitten
by the Miniboard's serial port:  Since the RxD and TxD lines are tied
together, a pontential point of error is going to be in the timing
of transmitted characters.  Suppose you just received a block that needs
to be responded to.  If the MiniBoard transmits at the same time the
sending computer does, the two bytes will collide at the Miniboard
interface.  This means we're going to have to use a half-duplex
scheme in which the MiniBoard "knows" when the sending computer is
guaranteed not to be transmitting.  A good example of this sort 
of scheme is the kermit protocol.

   Even worse, what if we try to hook the MiniBoard up to another 
serial port that works the same way? (sending side supplies the negative
voltage) - No communications will be possible! 

>Oh well.

   yep.  Oh well is right.  Get out the x-acto knives when you get 
the miniboard.

-- 
      Dave Williams
        dawill@hubcap.clemson.edu
           "Huh?  What?  Could you repeat the question?"

